Parallel.For Method
Definition
Important
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Executes a for loop in which iterations may run in parallel.
Overloads
| For(Int32, Int32, Action<Int32,ParallelLoopState>) |
Executes a |
| For(Int32, Int32, Action<Int32>) |
Executes a |
| For(Int64, Int64, Action<Int64,ParallelLoopState>) |
Executes a |
| For(Int64, Int64, Action<Int64>) |
Executes a |
| For(Int32, Int32, ParallelOptions, Action<Int32,ParallelLoopState>) |
Executes a |
| For(Int32, Int32, ParallelOptions, Action<Int32>) |
Executes a |
| For(Int64, Int64, ParallelOptions, Action<Int64,ParallelLoopState>) |
Executes a |
| For(Int64, Int64, ParallelOptions, Action<Int64>) |
Executes a |
| For<TLocal>(Int32, Int32, Func<TLocal>, Func<Int32,ParallelLoopState,TLocal,TLocal>, Action<TLocal>) |
Executes a |
| For<TLocal>(Int64, Int64, Func<TLocal>, Func<Int64,ParallelLoopState,TLocal,TLocal>, Action<TLocal>) |
Executes a |
| For<TLocal>(Int32, Int32, ParallelOptions, Func<TLocal>, Func<Int32,ParallelLoopState,TLocal,TLocal>, Action<TLocal>) |
Executes a |
| For<TLocal>(Int64, Int64, ParallelOptions, Func<TLocal>, Func<Int64,ParallelLoopState,TLocal,TLocal>, Action<TLocal>) |
Executes a |
For(Int32, Int32, Action<Int32,ParallelLoopState>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop in which iterations may run in parallel and the state of the loop can be monitored and manipulated.
public:
static System::Threading::Tasks::ParallelLoopResult For(int fromInclusive, int toExclusive, Action<int, System::Threading::Tasks::ParallelLoopState ^> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (int fromInclusive, int toExclusive, Action<int,System.Threading.Tasks.ParallelLoopState> body);static member For : int * int * Action<int, System.Threading.Tasks.ParallelLoopState> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Integer, toExclusive As Integer, body As Action(Of Integer, ParallelLoopState)) As ParallelLoopResultParameters
- fromInclusive
- Int32
The start index, inclusive.
- toExclusive
- Int32
The end index, exclusive.
- body
- Action<Int32,ParallelLoopState>
The delegate that is invoked once per iteration.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
The exception that contains all the individual exceptions thrown on all threads.
Examples
The following example executes up to 100 iterations of a loop in parallel. Each iteration pauses for a random interval from 1 to 1,000 milliseconds. A randomly generated value determines on which iteration of the loop the ParallelLoopState.Break method is called. As the output from the example shows, no iterations whose index is greater than the ParallelLoopState.LowestBreakIteration property value start after the call to the ParallelLoopState.Break method.
using System;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
var rnd = new Random();
int breakIndex = rnd.Next(1, 11);
Console.WriteLine($"Will call Break at iteration {breakIndex}\n");
var result = Parallel.For(1, 101, (i, state) =>
{
Console.WriteLine($"Beginning iteration {i}");
int delay;
lock (rnd)
delay = rnd.Next(1, 1001);
Thread.Sleep(delay);
if (state.ShouldExitCurrentIteration)
{
if (state.LowestBreakIteration < i)
return;
}
if (i == breakIndex)
{
Console.WriteLine($"Break in iteration {i}");
state.Break();
}
Console.WriteLine($"Completed iteration {i}");
});
if (result.LowestBreakIteration.HasValue)
Console.WriteLine($"\nLowest Break Iteration: {result.LowestBreakIteration}");
else
Console.WriteLine($"\nNo lowest break iteration.");
}
}
// The example displays output like the following:
// Will call Break at iteration 8
//
// Beginning iteration 1
// Beginning iteration 13
// Beginning iteration 97
// Beginning iteration 25
// Beginning iteration 49
// Beginning iteration 37
// Beginning iteration 85
// Beginning iteration 73
// Beginning iteration 61
// Completed iteration 85
// Beginning iteration 86
// Completed iteration 61
// Beginning iteration 62
// Completed iteration 86
// Beginning iteration 87
// Completed iteration 37
// Beginning iteration 38
// Completed iteration 38
// Beginning iteration 39
// Completed iteration 25
// Beginning iteration 26
// Completed iteration 26
// Beginning iteration 27
// Completed iteration 73
// Beginning iteration 74
// Completed iteration 62
// Beginning iteration 63
// Completed iteration 39
// Beginning iteration 40
// Completed iteration 40
// Beginning iteration 41
// Completed iteration 13
// Beginning iteration 14
// Completed iteration 1
// Beginning iteration 2
// Completed iteration 97
// Beginning iteration 98
// Completed iteration 49
// Beginning iteration 50
// Completed iteration 87
// Completed iteration 27
// Beginning iteration 28
// Completed iteration 50
// Beginning iteration 51
// Beginning iteration 88
// Completed iteration 14
// Beginning iteration 15
// Completed iteration 15
// Completed iteration 2
// Beginning iteration 3
// Beginning iteration 16
// Completed iteration 63
// Beginning iteration 64
// Completed iteration 74
// Beginning iteration 75
// Completed iteration 41
// Beginning iteration 42
// Completed iteration 28
// Beginning iteration 29
// Completed iteration 29
// Beginning iteration 30
// Completed iteration 98
// Beginning iteration 99
// Completed iteration 64
// Beginning iteration 65
// Completed iteration 42
// Beginning iteration 43
// Completed iteration 88
// Beginning iteration 89
// Completed iteration 51
// Beginning iteration 52
// Completed iteration 16
// Beginning iteration 17
// Completed iteration 43
// Beginning iteration 44
// Completed iteration 44
// Beginning iteration 45
// Completed iteration 99
// Beginning iteration 4
// Completed iteration 3
// Beginning iteration 8
// Completed iteration 4
// Beginning iteration 5
// Completed iteration 52
// Beginning iteration 53
// Completed iteration 75
// Beginning iteration 76
// Completed iteration 76
// Beginning iteration 77
// Completed iteration 65
// Beginning iteration 66
// Completed iteration 5
// Beginning iteration 6
// Completed iteration 89
// Beginning iteration 90
// Completed iteration 30
// Beginning iteration 31
// Break in iteration 8
// Completed iteration 8
// Completed iteration 6
// Beginning iteration 7
// Completed iteration 7
//
// Lowest Break Iteration: 8
Imports System.Threading
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim rnd As New Random()
Dim breakIndex As Integer = rnd.Next(1, 11)
Dim lowest As New Nullable(Of Long)()
Console.WriteLine("Will call Break at iteration {0}", breakIndex)
Console.WriteLine()
Dim result = Parallel.For(1, 101, Sub(i, state)
Console.WriteLine("Beginning iteration {0}", i)
Dim delay As Integer
Monitor.Enter(rnd)
delay = rnd.Next(1, 1001)
Monitor.Exit(rnd)
Thread.Sleep(delay)
If state.ShouldExitCurrentIteration Then
If state.LowestBreakIteration < i Then
Return
End If
End If
If i = breakIndex Then
Console.WriteLine("Break in iteration {0}", i)
state.Break()
If state.LowestBreakIteration.HasValue Then
If lowest < state.LowestBreakIteration Then
lowest = state.LowestBreakIteration
Else
lowest = state.LowestBreakIteration
End If
End If
End If
Console.WriteLine("Completed iteration {0}", i)
End Sub )
Console.WriteLine()
If lowest.HasValue Then
Console.WriteLine("Lowest Break Iteration: {0}", lowest)
Else
Console.WriteLine("No lowest break iteration.")
End If
End Sub
End Module
' The example displays output like the following:
' Will call Break at iteration 8
'
' Beginning iteration 1
' Beginning iteration 13
' Beginning iteration 97
' Beginning iteration 25
' Beginning iteration 49
' Beginning iteration 37
' Beginning iteration 85
' Beginning iteration 73
' Beginning iteration 61
' Completed iteration 85
' Beginning iteration 86
' Completed iteration 61
' Beginning iteration 62
' Completed iteration 86
' Beginning iteration 87
' Completed iteration 37
' Beginning iteration 38
' Completed iteration 38
' Beginning iteration 39
' Completed iteration 25
' Beginning iteration 26
' Completed iteration 26
' Beginning iteration 27
' Completed iteration 73
' Beginning iteration 74
' Completed iteration 62
' Beginning iteration 63
' Completed iteration 39
' Beginning iteration 40
' Completed iteration 40
' Beginning iteration 41
' Completed iteration 13
' Beginning iteration 14
' Completed iteration 1
' Beginning iteration 2
' Completed iteration 97
' Beginning iteration 98
' Completed iteration 49
' Beginning iteration 50
' Completed iteration 87
' Completed iteration 27
' Beginning iteration 28
' Completed iteration 50
' Beginning iteration 51
' Beginning iteration 88
' Completed iteration 14
' Beginning iteration 15
' Completed iteration 15
' Completed iteration 2
' Beginning iteration 3
' Beginning iteration 16
' Completed iteration 63
' Beginning iteration 64
' Completed iteration 74
' Beginning iteration 75
' Completed iteration 41
' Beginning iteration 42
' Completed iteration 28
' Beginning iteration 29
' Completed iteration 29
' Beginning iteration 30
' Completed iteration 98
' Beginning iteration 99
' Completed iteration 64
' Beginning iteration 65
' Completed iteration 42
' Beginning iteration 43
' Completed iteration 88
' Beginning iteration 89
' Completed iteration 51
' Beginning iteration 52
' Completed iteration 16
' Beginning iteration 17
' Completed iteration 43
' Beginning iteration 44
' Completed iteration 44
' Beginning iteration 45
' Completed iteration 99
' Beginning iteration 4
' Completed iteration 3
' Beginning iteration 8
' Completed iteration 4
' Beginning iteration 5
' Completed iteration 52
' Beginning iteration 53
' Completed iteration 75
' Beginning iteration 76
' Completed iteration 76
' Beginning iteration 77
' Completed iteration 65
' Beginning iteration 66
' Completed iteration 5
' Beginning iteration 6
' Completed iteration 89
' Beginning iteration 90
' Completed iteration 30
' Beginning iteration 31
' Break in iteration 8
' Completed iteration 8
' Completed iteration 6
' Beginning iteration 7
' Completed iteration 7
'
' Lowest Break Iteration: 8
Because iterations of the loop are still likely to be executing when the ParallelLoopState.Break method is called, each iteration calls the ParallelLoopState.ShouldExitCurrentIteration property to check whether another iteration has called the ParallelLoopState.Break method. If the property value is true, the iteration checks the value of the ParallelLoopState.LowestBreakIteration property and, if it is greater than the current iteration's index value, returns immediately.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with two arguments:
An Int32 value that represents the iteration count.
A ParallelLoopState instance that can be used to break out of the loop prematurely. The ParallelLoopState object is created by the compiler; it cannot be instantiated in user code.
Calling the Break method informs the for operation that iterations after the current one don't have to execute. However, all iterations before the current one will still have to be executed if they haven't already.
Therefore, calling Break is similar to using a break operation within a conventional for loop in a language like C#, but it is not a perfect substitute: For example, there is no guarantee that iterations after the current one will definitely not execute.
If executing all iterations before the current one is not necessary, use the Stop method instead of using Break. Calling Stop informs the for loop that it may abandon all remaining iterations, regardless of whether they're before or after the current iteration, because all required work will have already been completed. However, as with Break, there are no guarantees regarding which other iterations will not execute.
If a loop is ended prematurely, the ParallelLoopResult structure that is returned will contain relevant information about the loop's completion.
If fromInclusive is greater than or equal to toExclusive, the method returns immediately without performing any iterations.
See also
Applies to
For(Int32, Int32, Action<Int32>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop in which iterations may run in parallel.
public:
static System::Threading::Tasks::ParallelLoopResult For(int fromInclusive, int toExclusive, Action<int> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (int fromInclusive, int toExclusive, Action<int> body);static member For : int * int * Action<int> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Integer, toExclusive As Integer, body As Action(Of Integer)) As ParallelLoopResultParameters
- fromInclusive
- Int32
The start index, inclusive.
- toExclusive
- Int32
The end index, exclusive.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
The exception that contains all the individual exceptions thrown on all threads.
Examples
The following example uses the For method for 100 invocations of a delegate that generates random byte values and computes their sum.
using System;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
ParallelLoopResult result = Parallel.For(0, 100, ctr => { Random rnd = new Random(ctr * 100000);
Byte[] bytes = new Byte[100];
rnd.NextBytes(bytes);
int sum = 0;
foreach(var byt in bytes)
sum += byt;
Console.WriteLine("Iteration {0,2}: {1:N0}", ctr, sum);
});
Console.WriteLine("Result: {0}", result.IsCompleted ? "Completed Normally" :
String.Format("Completed to {0}", result.LowestBreakIteration));
}
}
// The following is a portion of the output displayed by the example:
// Iteration 0: 12,509
// Iteration 50: 12,823
// Iteration 51: 11,275
// Iteration 52: 12,531
// Iteration 1: 13,007
// Iteration 53: 13,799
// Iteration 4: 12,945
// Iteration 2: 13,246
// Iteration 54: 13,008
// Iteration 55: 12,727
// Iteration 56: 13,223
// Iteration 57: 13,717
// Iteration 5: 12,679
// Iteration 3: 12,455
// Iteration 58: 12,669
// Iteration 59: 11,882
// Iteration 6: 13,167
// ...
// Iteration 92: 12,275
// Iteration 93: 13,282
// Iteration 94: 12,745
// Iteration 95: 11,957
// Iteration 96: 12,455
// Result: Completed Normally
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim result As ParallelLoopResult = Parallel.For(0, 100, Sub(ctr)
Dim rnd As New Random(ctr * 100000)
Dim bytes(99) As Byte
rnd.NextBytes(bytes)
Dim sum As Integer
For Each byt In bytes
sum += byt
Next
Console.WriteLine("Iteration {0,2}: {1:N0}", ctr, sum)
End Sub)
Console.WriteLine("Result: {0}", If(result.IsCompleted, "Completed Normally",
String.Format("Completed to {0}", result.LowestBreakIteration)))
End Sub
End Module
' The following is a portion of the output displayed by the example:
' Iteration 0: 12,509
' Iteration 50: 12,823
' Iteration 51: 11,275
' Iteration 52: 12,531
' Iteration 1: 13,007
' Iteration 53: 13,799
' Iteration 4: 12,945
' Iteration 2: 13,246
' Iteration 54: 13,008
' Iteration 55: 12,727
' Iteration 56: 13,223
' Iteration 57: 13,717
' Iteration 5: 12,679
' Iteration 3: 12,455
' Iteration 58: 12,669
' Iteration 59: 11,882
' Iteration 6: 13,167
' ...
' Iteration 92: 12,275
' Iteration 93: 13,282
' Iteration 94: 12,745
' Iteration 95: 11,957
' Iteration 96: 12,455
' Result: Completed Normally
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the iteration count (Int32) as a parameter.
If fromInclusive is greater than or equal to toExclusive, the method returns immediately without performing any iterations.
See also
Applies to
For(Int64, Int64, Action<Int64,ParallelLoopState>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with 64-bit indexes in which iterations may run in parallel and the state of the loop can be monitored and manipulated.
public:
static System::Threading::Tasks::ParallelLoopResult For(long fromInclusive, long toExclusive, Action<long, System::Threading::Tasks::ParallelLoopState ^> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (long fromInclusive, long toExclusive, Action<long,System.Threading.Tasks.ParallelLoopState> body);static member For : int64 * int64 * Action<int64, System.Threading.Tasks.ParallelLoopState> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Long, toExclusive As Long, body As Action(Of Long, ParallelLoopState)) As ParallelLoopResultParameters
- fromInclusive
- Int64
The start index, inclusive.
- toExclusive
- Int64
The end index, exclusive.
- body
- Action<Int64,ParallelLoopState>
The delegate that is invoked once per iteration.
Returns
A ParallelLoopResult structure that contains information on what portion of the loop completed.
Exceptions
The body argument is null.
The exception that contains all the individual exceptions thrown on all threads.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int64), and a ParallelLoopState instance that may be used to break out of the loop prematurely.
Calling the Break method informs the for operation that iterations after the current one don't have to be executed, but all iterations before the current one do.
Therefore, calling Break is similar to using a break operation within a conventional for loop in a language like C#, but it is not a perfect substitute: For example, there is no guarantee that iterations after the current one will definitely not execute.
If executing all iterations before the current one is not necessary, use the Stop method instead of using Break. Calling Stop informs the for loop that it may abandon all remaining iterations, regardless of whether they're before or after the current iteration, because all required work will have already been completed. However, as with Break, there are no guarantees regarding which other iterations will not execute.
If a loop is ended prematurely, the ParallelLoopResult structure that is returned will contain relevant information about the loop's completion.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
See also
Applies to
For(Int64, Int64, Action<Int64>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with 64-bit indexes in which iterations may run in parallel.
public:
static System::Threading::Tasks::ParallelLoopResult For(long fromInclusive, long toExclusive, Action<long> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (long fromInclusive, long toExclusive, Action<long> body);static member For : int64 * int64 * Action<int64> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Long, toExclusive As Long, body As Action(Of Long)) As ParallelLoopResultParameters
- fromInclusive
- Int64
The start index, inclusive.
- toExclusive
- Int64
The end index, exclusive.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
The exception that contains all the individual exceptions thrown on all threads.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the iteration count (Int64) as a parameter.
If fromInclusive is greater than or equal to toExclusive, the method returns immediately without performing any iterations.
See also
Applies to
For(Int32, Int32, ParallelOptions, Action<Int32,ParallelLoopState>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop in which iterations may run in parallel, loop options can be configured, and the state of the loop can be monitored and manipulated.
public:
static System::Threading::Tasks::ParallelLoopResult For(int fromInclusive, int toExclusive, System::Threading::Tasks::ParallelOptions ^ parallelOptions, Action<int, System::Threading::Tasks::ParallelLoopState ^> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (int fromInclusive, int toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Action<int,System.Threading.Tasks.ParallelLoopState> body);static member For : int * int * System.Threading.Tasks.ParallelOptions * Action<int, System.Threading.Tasks.ParallelLoopState> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Integer, toExclusive As Integer, parallelOptions As ParallelOptions, body As Action(Of Integer, ParallelLoopState)) As ParallelLoopResultParameters
- fromInclusive
- Int32
The start index, inclusive.
- toExclusive
- Int32
The end index, exclusive.
- parallelOptions
- ParallelOptions
An object that configures the behavior of this operation.
- body
- Action<Int32,ParallelLoopState>
The delegate that is invoked once per iteration.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The CancellationToken in the parallelOptions argument is canceled.
The exception that contains all the individual exceptions thrown on all threads.
The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int32), and a ParallelLoopState instance that may be used to break out of the loop prematurely.
If fromInclusive is greater than or equal to toExclusive, the method returns immediately without performing any iterations.
See also
Applies to
For(Int32, Int32, ParallelOptions, Action<Int32>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop in which iterations may run in parallel and loop options can be configured.
public:
static System::Threading::Tasks::ParallelLoopResult For(int fromInclusive, int toExclusive, System::Threading::Tasks::ParallelOptions ^ parallelOptions, Action<int> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (int fromInclusive, int toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Action<int> body);static member For : int * int * System.Threading.Tasks.ParallelOptions * Action<int> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Integer, toExclusive As Integer, parallelOptions As ParallelOptions, body As Action(Of Integer)) As ParallelLoopResultParameters
- fromInclusive
- Int32
The start index, inclusive.
- toExclusive
- Int32
The end index, exclusive.
- parallelOptions
- ParallelOptions
An object that configures the behavior of this operation.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The CancellationToken in the parallelOptions argument is canceled.
The exception that contains all the individual exceptions thrown on all threads.
The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.
Examples
The following example shows how to cancel a parallel loop:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
class ParallelForCancellation
{
// Demonstrated features:
// CancellationTokenSource
// Parallel.For()
// ParallelOptions
// ParallelLoopResult
// Expected results:
// An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
// The order of execution of the iterations is undefined.
// The iteration when i=2 cancels the loop.
// Some iterations may bail out or not start at all; because they are temporally executed in unpredictable order,
// it is impossible to say which will start/complete and which won't.
// At the end, an OperationCancelledException is surfaced.
// Documentation:
// http://msdn.microsoft.com/library/system.threading.cancellationtokensource(VS.100).aspx
static void CancelDemo()
{
CancellationTokenSource cancellationSource = new CancellationTokenSource();
ParallelOptions options = new ParallelOptions();
options.CancellationToken = cancellationSource.Token;
try
{
ParallelLoopResult loopResult = Parallel.For(
0,
10,
options,
(i, loopState) =>
{
Console.WriteLine("Start Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);
// Simulate a cancellation of the loop when i=2
if (i == 2)
{
cancellationSource.Cancel();
}
// Simulates a long execution
for (int j = 0; j < 10; j++)
{
Thread.Sleep(1 * 200);
// check to see whether or not to continue
if (loopState.ShouldExitCurrentIteration) return;
}
Console.WriteLine("Finish Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);
}
);
if (loopResult.IsCompleted)
{
Console.WriteLine("All iterations completed successfully. THIS WAS NOT EXPECTED.");
}
}
// No exception is expected in this example, but if one is still thrown from a task,
// it will be wrapped in AggregateException and propagated to the main thread.
catch (AggregateException e)
{
Console.WriteLine("Parallel.For has thrown an AggregateException. THIS WAS NOT EXPECTED.\n{0}", e);
}
// Catching the cancellation exception
catch (OperationCanceledException e)
{
Console.WriteLine("An iteration has triggered a cancellation. THIS WAS EXPECTED.\n{0}", e.ToString());
}
finally
{
cancellationSource.Dispose();
}
}
}
Imports System.Threading
Imports System.Threading.Tasks
Module LoopCancellation
' Demonstrated features:
' CancellationTokenSource
' Parallel.For()
' ParallelOptions
' ParallelLoopResult
' Expected results:
' An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
' The order of execution of the iterations is undefined.
' The iteration when i=2 cancels the loop.
' Some iterations may bail out or not start at all; because they are temporally executed in unpredictable order,
' it is impossible to say which will start/complete and which won't.
' At the end, an OperationCancelledException is surfaced.
' Documentation:
' http://msdn.microsoft.com/library/system.threading.cancellationtokensource(VS.100).aspx
Private Sub Main()
Dim cancellationSource As New CancellationTokenSource()
Dim options As New ParallelOptions()
options.CancellationToken = cancellationSource.Token
Try
Dim loopResult As ParallelLoopResult = _
Parallel.For(0, 10, options, Sub(i, loopState)
Console.WriteLine("Start Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i)
' Simulate a cancellation of the loop when i=2
If i = 2 Then
cancellationSource.Cancel()
End If
' Simulates a long execution
For j As Integer = 0 To 9
Thread.Sleep(1 * 200)
' check to see whether or not to continue
If loopState.ShouldExitCurrentIteration Then
Exit Sub
End If
Next
Console.WriteLine("Finish Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i)
End Sub)
If loopResult.IsCompleted Then
Console.WriteLine("All iterations completed successfully. THIS WAS NOT EXPECTED.")
End If
Catch e As AggregateException
' No exception is expected in this example, but if one is still thrown from a task,
' it will be wrapped in AggregateException and propagated to the main thread.
Console.WriteLine("An action has thrown an AggregateException. THIS WAS NOT EXPECTED." & vbLf & "{0}", e)
Catch e As OperationCanceledException
' Catching the cancellation exception
Console.WriteLine("An iteration has triggered a cancellation. THIS WAS EXPECTED." & vbLf & "{0}", e)
Finally
cancellationSource.Dispose()
End Try
End Sub
End Module
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the iteration count (Int32) as a parameter.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
See also
Applies to
For(Int64, Int64, ParallelOptions, Action<Int64,ParallelLoopState>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with 64-bit indexes in which iterations may run in parallel, loop options can be configured, and the state of the loop can be monitored and manipulated.
public:
static System::Threading::Tasks::ParallelLoopResult For(long fromInclusive, long toExclusive, System::Threading::Tasks::ParallelOptions ^ parallelOptions, Action<long, System::Threading::Tasks::ParallelLoopState ^> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (long fromInclusive, long toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Action<long,System.Threading.Tasks.ParallelLoopState> body);static member For : int64 * int64 * System.Threading.Tasks.ParallelOptions * Action<int64, System.Threading.Tasks.ParallelLoopState> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Long, toExclusive As Long, parallelOptions As ParallelOptions, body As Action(Of Long, ParallelLoopState)) As ParallelLoopResultParameters
- fromInclusive
- Int64
The start index, inclusive.
- toExclusive
- Int64
The end index, exclusive.
- parallelOptions
- ParallelOptions
An object that configures the behavior of this operation.
- body
- Action<Int64,ParallelLoopState>
The delegate that is invoked once per iteration.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The CancellationToken in the parallelOptions argument is canceled.
The exception that contains all the individual exceptions thrown on all threads.
The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.
Examples
The following example shows how to use the Parallel.For method with a ParallelOptions object:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
class ParallelOptionsDemo
{
// Demonstrated features:
// Parallel.For()
// ParallelOptions
// Expected results:
// An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
// The order of execution of the iterations is undefined.
// Verify that no more than two threads have been used for the iterations.
// Documentation:
// http://msdn.microsoft.com/library/system.threading.tasks.parallel.for(VS.100).aspx
static void Main()
{
ParallelOptions options = new ParallelOptions();
options.MaxDegreeOfParallelism = 2; // -1 is for unlimited. 1 is for sequential.
try
{
Parallel.For(
0,
9,
options,
(i) =>
{
Console.WriteLine("Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);
}
);
}
// No exception is expected in this example, but if one is still thrown from a task,
// it will be wrapped in AggregateException and propagated to the main thread.
catch (AggregateException e)
{
Console.WriteLine("Parallel.For has thrown the following (unexpected) exception:\n{0}", e);
}
}
}
Imports System.Threading
Imports System.Threading.Tasks
Module ParallelForDemo
' Demonstrated features:
' Parallel.For()
' ParallelOptions
' Expected results:
' An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
' The order of execution of the iterations is undefined.
' Verify that no more than two threads have been used for the iterations.
' Documentation:
' http://msdn.microsoft.com/library/system.threading.tasks.parallel.for(VS.100).aspx
Sub Main()
Dim options As New ParallelOptions()
options.MaxDegreeOfParallelism = 2 ' -1 is for unlimited. 1 is for sequential.
Try
Parallel.For(0, 9, options, Sub(i)
Console.WriteLine("Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i)
End Sub)
Catch e As AggregateException
' No exception is expected in this example, but if one is still thrown from a task,
' it will be wrapped in AggregateException and propagated to the main thread.
Console.WriteLine("Parallel.For has thrown the following (unexpected) exception:" & vbLf & "{0}", e)
End Try
End Sub
End Module
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int64), and a ParallelLoopState instance that may be used to break out of the loop prematurely.
If fromInclusive is greater than or equal to toExclusive, the method returns immediately without performing any iterations.
See also
Applies to
For(Int64, Int64, ParallelOptions, Action<Int64>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with 64-bit indexes in which iterations may run in parallel and loop options can be configured.
public:
static System::Threading::Tasks::ParallelLoopResult For(long fromInclusive, long toExclusive, System::Threading::Tasks::ParallelOptions ^ parallelOptions, Action<long> ^ body);public static System.Threading.Tasks.ParallelLoopResult For (long fromInclusive, long toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Action<long> body);static member For : int64 * int64 * System.Threading.Tasks.ParallelOptions * Action<int64> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For (fromInclusive As Long, toExclusive As Long, parallelOptions As ParallelOptions, body As Action(Of Long)) As ParallelLoopResultParameters
- fromInclusive
- Int64
The start index, inclusive.
- toExclusive
- Int64
The end index, exclusive.
- parallelOptions
- ParallelOptions
An object that configures the behavior of this operation.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The CancellationToken in the parallelOptions argument is canceled.
The exception that contains all the individual exceptions thrown on all threads.
The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.
Examples
The following example shows how to use ParallelOptions to specify a custom task scheduler:
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
class ParallelSchedulerDemo2
{
// Demonstrated features:
// TaskScheduler
// BlockingCollection
// Parallel.For()
// ParallelOptions
// Expected results:
// An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
// The TwoThreadTaskScheduler employs 2 threads on which iterations may be executed in a random order.
// Thus a scheduler thread may execute multiple iterations.
// Documentation:
// http://msdn.microsoft.com/library/system.threading.tasks.taskscheduler(VS.100).aspx
// http://msdn.microsoft.com/library/dd997413(VS.100).aspx
// More information:
// http://blogs.msdn.com/pfxteam/archive/2009/09/22/9898090.aspx
static void Main()
{
ParallelOptions options = new ParallelOptions();
// Construct and associate a custom task scheduler
options.TaskScheduler = new TwoThreadTaskScheduler();
try
{
Parallel.For(
0,
10,
options,
(i, localState) =>
{
Console.WriteLine("i={0}, Task={1}, Thread={2}", i, Task.CurrentId, Thread.CurrentThread.ManagedThreadId);
}
);
}
// No exception is expected in this example, but if one is still thrown from a task,
// it will be wrapped in AggregateException and propagated to the main thread.
catch (AggregateException e)
{
Console.WriteLine("An iteration has thrown an exception. THIS WAS NOT EXPECTED.\n{0}", e);
}
}
// This scheduler schedules all tasks on (at most) two threads
sealed class TwoThreadTaskScheduler : TaskScheduler, IDisposable
{
// The runtime decides how many tasks to create for the given set of iterations, loop options, and scheduler's max concurrency level.
// Tasks will be queued in this collection
private BlockingCollection<Task> _tasks = new BlockingCollection<Task>();
// Maintain an array of threads. (Feel free to bump up _n.)
private readonly int _n = 2;
private Thread[] _threads;
public TwoThreadTaskScheduler()
{
_threads = new Thread[_n];
// Create unstarted threads based on the same inline delegate
for (int i = 0; i < _n; i++)
{
_threads[i] = new Thread(() =>
{
// The following loop blocks until items become available in the blocking collection.
// Then one thread is unblocked to consume that item.
foreach (var task in _tasks.GetConsumingEnumerable())
{
TryExecuteTask(task);
}
});
// Start each thread
_threads[i].IsBackground = true;
_threads[i].Start();
}
}
// This method is invoked by the runtime to schedule a task
protected override void QueueTask(Task task)
{
_tasks.Add(task);
}
// The runtime will probe if a task can be executed in the current thread.
// By returning false, we direct all tasks to be queued up.
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
{
return false;
}
public override int MaximumConcurrencyLevel { get { return _n; } }
protected override IEnumerable<Task> GetScheduledTasks()
{
return _tasks.ToArray();
}
// Dispose is not thread-safe with other members.
// It may only be used when no more tasks will be queued
// to the scheduler. This implementation will block
// until all previously queued tasks have completed.
public void Dispose()
{
if (_threads != null)
{
_tasks.CompleteAdding();
for (int i = 0; i < _n; i++)
{
_threads[i].Join();
_threads[i] = null;
}
_threads = null;
_tasks.Dispose();
_tasks = null;
}
}
}
}
Imports System.Collections.Concurrent
Imports System.Threading
Imports System.Threading.Tasks
Module SchedulerDemo
' Demonstrated features:
' TaskScheduler
' BlockingCollection
' Parallel.For()
' ParallelOptions
' Expected results:
' An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
' The TwoThreadTaskScheduler employs 2 threads on which iterations may be executed in a random order.
' A task is internally created for each thread of the task scheduler (plus an aditional internal task).
' Thus a scheduler thread may execute multiple iterations.
' Documentation:
' http://msdn.microsoft.com/library/system.threading.tasks.taskscheduler(VS.100).aspx
' http://msdn.microsoft.com/library/dd997413(VS.100).aspx
' More information:
' http://blogs.msdn.com/pfxteam/archive/2009/09/22/9898090.aspx
Sub Main()
Dim options As New ParallelOptions()
' Construct and associate a custom task scheduler
options.TaskScheduler = New TwoThreadTaskScheduler()
Try
Parallel.For(0, 10, options, Sub(i, localState)
Console.WriteLine("i={0}, Task={1}, Thread={2}", i, Task.CurrentId, Thread.CurrentThread.ManagedThreadId)
End Sub)
Catch e As AggregateException
' No exception is expected in this example, but if one is still thrown from a task,
' it will be wrapped in AggregateException and propagated to the main thread.
Console.WriteLine("An iteration has thrown an exception. THIS WAS NOT EXPECTED." & vbLf & "{0}", e)
End Try
End Sub
' This scheduler schedules all tasks on (at most) two threads
Private NotInheritable Class TwoThreadTaskScheduler
Inherits TaskScheduler
Implements IDisposable
' The runtime decides how many tasks to create for the given set of iterations, loop options, and scheduler's max concurrency level.
' Tasks will be queued in this collection
Private _tasks As New BlockingCollection(Of Task)()
' Maintain an array of threads. (Feel free to bump up _n.)
Private ReadOnly _n As Integer = 2
Private _threads As Thread()
Public Sub New()
_threads = New Thread(_n - 1) {}
' Create unstarted threads based on the same inline delegate
For i As Integer = 0 To _n - 1
_threads(i) = New Thread(Sub()
' The following loop blocks until items become available in the blocking collection.
' Then one thread is unblocked to consume that item.
For Each task In _tasks.GetConsumingEnumerable()
TryExecuteTask(task)
Next
End Sub)
' Start each thread
_threads(i).IsBackground = True
_threads(i).Start()
Next
End Sub
' This method is invoked by the runtime to schedule a task
Protected Overloads Overrides Sub QueueTask(ByVal task As Task)
_tasks.Add(task)
End Sub
' The runtime will probe if a task can be executed in the current thread.
' By returning false, we direct all tasks to be queued up.
Protected Overloads Overrides Function TryExecuteTaskInline(ByVal task As Task, ByVal taskWasPreviouslyQueued As Boolean) As Boolean
Return False
End Function
Public Overloads Overrides ReadOnly Property MaximumConcurrencyLevel() As Integer
Get
Return _n
End Get
End Property
Protected Overloads Overrides Function GetScheduledTasks() As IEnumerable(Of Task)
Return _tasks.ToArray()
End Function
' Dispose is not thread-safe with other members.
' It may only be used when no more tasks will be queued
' to the scheduler. This implementation will block
' until all previously queued tasks have completed.
Public Sub Dispose() Implements IDisposable.Dispose
If _threads IsNot Nothing Then
_tasks.CompleteAdding()
For i As Integer = 0 To _n - 1
_threads(i).Join()
_threads(i) = Nothing
Next
_threads = Nothing
_tasks.Dispose()
_tasks = Nothing
End If
End Sub
End Class
End Module
Remarks
Supports 64-bit indexes. The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the iteration count (Int64) as a parameter.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
See also
Applies to
For<TLocal>(Int32, Int32, Func<TLocal>, Func<Int32,ParallelLoopState,TLocal,TLocal>, Action<TLocal>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with thread-local data in which iterations may run in parallel, and the state of the loop can be monitored and manipulated.
public:
generic <typename TLocal>
static System::Threading::Tasks::ParallelLoopResult For(int fromInclusive, int toExclusive, Func<TLocal> ^ localInit, Func<int, System::Threading::Tasks::ParallelLoopState ^, TLocal, TLocal> ^ body, Action<TLocal> ^ localFinally);public static System.Threading.Tasks.ParallelLoopResult For<TLocal> (int fromInclusive, int toExclusive, Func<TLocal> localInit, Func<int,System.Threading.Tasks.ParallelLoopState,TLocal,TLocal> body, Action<TLocal> localFinally);static member For : int * int * Func<'Local> * Func<int, System.Threading.Tasks.ParallelLoopState, 'Local, 'Local> * Action<'Local> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For(Of TLocal) (fromInclusive As Integer, toExclusive As Integer, localInit As Func(Of TLocal), body As Func(Of Integer, ParallelLoopState, TLocal, TLocal), localFinally As Action(Of TLocal)) As ParallelLoopResultType Parameters
- TLocal
The type of the thread-local data.
Parameters
- fromInclusive
- Int32
The start index, inclusive.
- toExclusive
- Int32
The end index, exclusive.
- localInit
- Func<TLocal>
The function delegate that returns the initial state of the local data for each task.
- body
- Func<Int32,ParallelLoopState,TLocal,TLocal>
The delegate that is invoked once per iteration.
- localFinally
- Action<TLocal>
The delegate that performs a final action on the local state of each task.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
-or-
The localInit argument is null.
-or-
The localFinally argument is null.
The exception that contains all the individual exceptions thrown on all threads.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int32), a ParallelLoopState instance that may be used to break out of the loop prematurely, and some local state that may be shared amongst iterations that execute on the same thread.
The localInit delegate is invoked once for each task that participates in the loop's execution and returns the initial local state for each of those tasks. These initial states are passed to the first body invocations on each task. Then, every subsequent body invocation returns a possibly modified state value that is passed to the next body invocation. Finally, the last body invocation on each task returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per task to perform a final action on each task's local state. This delegate might be invoked concurrently on multiple tasks; therefore, you must synchronize access to any shared variables.
The Parallel.For method may use more tasks than threads over the lifetime of its execution, as existing tasks complete and are replaced by new tasks. This gives the underlying TaskScheduler object the chance to add, change, or remove threads that service the loop.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
For an example that uses this method, see How to: Write a Parallel.For Loop with Thread-Local Variables.
See also
Applies to
For<TLocal>(Int64, Int64, Func<TLocal>, Func<Int64,ParallelLoopState,TLocal,TLocal>, Action<TLocal>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with 64-bit indexes and thread-local data in which iterations may run in parallel, and the state of the loop can be monitored and manipulated.
public:
generic <typename TLocal>
static System::Threading::Tasks::ParallelLoopResult For(long fromInclusive, long toExclusive, Func<TLocal> ^ localInit, Func<long, System::Threading::Tasks::ParallelLoopState ^, TLocal, TLocal> ^ body, Action<TLocal> ^ localFinally);public static System.Threading.Tasks.ParallelLoopResult For<TLocal> (long fromInclusive, long toExclusive, Func<TLocal> localInit, Func<long,System.Threading.Tasks.ParallelLoopState,TLocal,TLocal> body, Action<TLocal> localFinally);static member For : int64 * int64 * Func<'Local> * Func<int64, System.Threading.Tasks.ParallelLoopState, 'Local, 'Local> * Action<'Local> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For(Of TLocal) (fromInclusive As Long, toExclusive As Long, localInit As Func(Of TLocal), body As Func(Of Long, ParallelLoopState, TLocal, TLocal), localFinally As Action(Of TLocal)) As ParallelLoopResultType Parameters
- TLocal
The type of the thread-local data.
Parameters
- fromInclusive
- Int64
The start index, inclusive.
- toExclusive
- Int64
The end index, exclusive.
- localInit
- Func<TLocal>
The function delegate that returns the initial state of the local data for each task.
- body
- Func<Int64,ParallelLoopState,TLocal,TLocal>
The delegate that is invoked once per iteration.
- localFinally
- Action<TLocal>
The delegate that performs a final action on the local state of each task.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
-or-
The localInit argument is null.
-or-
The localFinally argument is null.
The exception that contains all the individual exceptions thrown on all threads.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int64), a ParallelLoopState instance that may be used to break out of the loop prematurely, and some local state that may be shared amongst iterations that execute on the same task.
The localInit delegate is invoked once for each task that participates in the loop's execution and returns the initial local state for each of those tasks. These initial states are passed to the first body invocations on each task. Then, every subsequent body invocation returns a possibly modified state value that is passed to the next body invocation. Finally, the last body invocation on each task returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per task to perform a final action on each task's local state. This delegate might be invoked concurrently on multiple tasks; therefore, you must synchronize access to any shared variables.
The Parallel.For method may use more tasks than threads over the lifetime of its execution, as existing tasks complete and are replaced by new tasks. This gives the underlying TaskScheduler object the chance to add, change, or remove threads that service the loop.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
For an example that uses this method, see How to: Write a Parallel.For Loop with Thread-Local Variables.
See also
Applies to
For<TLocal>(Int32, Int32, ParallelOptions, Func<TLocal>, Func<Int32,ParallelLoopState,TLocal,TLocal>, Action<TLocal>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with thread-local data in which iterations may run in parallel, loop options can be configured, and the state of the loop can be monitored and manipulated.
public:
generic <typename TLocal>
static System::Threading::Tasks::ParallelLoopResult For(int fromInclusive, int toExclusive, System::Threading::Tasks::ParallelOptions ^ parallelOptions, Func<TLocal> ^ localInit, Func<int, System::Threading::Tasks::ParallelLoopState ^, TLocal, TLocal> ^ body, Action<TLocal> ^ localFinally);public static System.Threading.Tasks.ParallelLoopResult For<TLocal> (int fromInclusive, int toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Func<TLocal> localInit, Func<int,System.Threading.Tasks.ParallelLoopState,TLocal,TLocal> body, Action<TLocal> localFinally);static member For : int * int * System.Threading.Tasks.ParallelOptions * Func<'Local> * Func<int, System.Threading.Tasks.ParallelLoopState, 'Local, 'Local> * Action<'Local> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For(Of TLocal) (fromInclusive As Integer, toExclusive As Integer, parallelOptions As ParallelOptions, localInit As Func(Of TLocal), body As Func(Of Integer, ParallelLoopState, TLocal, TLocal), localFinally As Action(Of TLocal)) As ParallelLoopResultType Parameters
- TLocal
The type of the thread-local data.
Parameters
- fromInclusive
- Int32
The start index, inclusive.
- toExclusive
- Int32
The end index, exclusive.
- parallelOptions
- ParallelOptions
An object that configures the behavior of this operation.
- localInit
- Func<TLocal>
The function delegate that returns the initial state of the local data for each task.
- body
- Func<Int32,ParallelLoopState,TLocal,TLocal>
The delegate that is invoked once per iteration.
- localFinally
- Action<TLocal>
The delegate that performs a final action on the local state of each task.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
-or-
The localInit argument is null.
-or-
The localFinally argument is null.
-or-
The parallelOptions argument is null.
The CancellationToken in the parallelOptions argument is canceled.
The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.
The exception that contains all the individual exceptions thrown on all threads.
Examples
The following example uses thread-local variables to compute the sum of the results of many lengthy operations. This example limits the degree of parallelism to four.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
class ThreadLocalForWithOptions
{
// The number of parallel iterations to perform.
const int N = 1000000;
static void Main()
{
// The result of all thread-local computations.
int result = 0;
// This example limits the degree of parallelism to four.
// You might limit the degree of parallelism when your algorithm
// does not scale beyond a certain number of cores or when you
// enforce a particular quality of service in your application.
Parallel.For(0, N, new ParallelOptions { MaxDegreeOfParallelism = 4 },
// Initialize the local states
() => 0,
// Accumulate the thread-local computations in the loop body
(i, loop, localState) =>
{
return localState + Compute(i);
},
// Combine all local states
localState => Interlocked.Add(ref result, localState)
);
// Print the actual and expected results.
Console.WriteLine("Actual result: {0}. Expected 1000000.", result);
}
// Simulates a lengthy operation.
private static int Compute(int n)
{
for (int i = 0; i < 10000; i++) ;
return 1;
}
}
Imports System.Threading
Imports System.Threading.Tasks
Module ThreadLocalForWithOptions
' The number of parallel iterations to perform.
Const N As Integer = 1000000
Sub Main()
' The result of all thread-local computations.
Dim result As Integer = 0
' This example limits the degree of parallelism to four.
' You might limit the degree of parallelism when your algorithm
' does not scale beyond a certain number of cores or when you
' enforce a particular quality of service in your application.
Parallel.For(0, N, New ParallelOptions With {.MaxDegreeOfParallelism = 4},
Function()
' Initialize the local states
Return 0
End Function,
Function(i, loopState, localState)
' Accumulate the thread-local computations in the loop body
Return localState + Compute(i)
End Function,
Sub(localState)
' Combine all local states
Interlocked.Add(result, localState)
End Sub
)
' Print the actual and expected results.
Console.WriteLine("Actual result: {0}. Expected 1000000.", result)
End Sub
' Simulates a lengthy operation.
Function Compute(ByVal n As Integer) As Integer
For i As Integer = 0 To 10000
Next
Return 1
End Function
End Module
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int32), a ParallelLoopState instance that may be used to break out of the loop prematurely, and some local state that may be shared amongst iterations that execute on the same task.
The localInit delegate is invoked once for each task that participates in the loop's execution and returns the initial local state for each of those tasks. These initial states are passed to the first body invocations on each task. Then, every subsequent body invocation returns a possibly modified state value that is passed to the next body invocation. Finally, the last body invocation on each task returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per task to perform a final action on each task's local state. This delegate might be invoked concurrently on multiple threads; therefore, you must synchronize access to any shared variables.
The Parallel.For method may use more tasks than threads over the lifetime of its execution, as existing tasks complete and are replaced by new tasks. This gives the underlying TaskScheduler object the chance to add, change, or remove threads that service the loop.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
See also
Applies to
For<TLocal>(Int64, Int64, ParallelOptions, Func<TLocal>, Func<Int64,ParallelLoopState,TLocal,TLocal>, Action<TLocal>)
- Source:
- Parallel.cs
- Source:
- Parallel.cs
- Source:
- Parallel.cs
Executes a for loop with 64-bit indexes and thread-local data in which iterations may run in parallel, loop options can be configured, and the state of the loop can be monitored and manipulated.
public:
generic <typename TLocal>
static System::Threading::Tasks::ParallelLoopResult For(long fromInclusive, long toExclusive, System::Threading::Tasks::ParallelOptions ^ parallelOptions, Func<TLocal> ^ localInit, Func<long, System::Threading::Tasks::ParallelLoopState ^, TLocal, TLocal> ^ body, Action<TLocal> ^ localFinally);public static System.Threading.Tasks.ParallelLoopResult For<TLocal> (long fromInclusive, long toExclusive, System.Threading.Tasks.ParallelOptions parallelOptions, Func<TLocal> localInit, Func<long,System.Threading.Tasks.ParallelLoopState,TLocal,TLocal> body, Action<TLocal> localFinally);static member For : int64 * int64 * System.Threading.Tasks.ParallelOptions * Func<'Local> * Func<int64, System.Threading.Tasks.ParallelLoopState, 'Local, 'Local> * Action<'Local> -> System.Threading.Tasks.ParallelLoopResultPublic Shared Function For(Of TLocal) (fromInclusive As Long, toExclusive As Long, parallelOptions As ParallelOptions, localInit As Func(Of TLocal), body As Func(Of Long, ParallelLoopState, TLocal, TLocal), localFinally As Action(Of TLocal)) As ParallelLoopResultType Parameters
- TLocal
The type of the thread-local data.
Parameters
- fromInclusive
- Int64
The start index, inclusive.
- toExclusive
- Int64
The end index, exclusive.
- parallelOptions
- ParallelOptions
An object that configures the behavior of this operation.
- localInit
- Func<TLocal>
The function delegate that returns the initial state of the local data for each thread.
- body
- Func<Int64,ParallelLoopState,TLocal,TLocal>
The delegate that is invoked once per iteration.
- localFinally
- Action<TLocal>
The delegate that performs a final action on the local state of each thread.
Returns
A structure that contains information about which portion of the loop completed.
Exceptions
The body argument is null.
-or-
The localInit argument is null.
-or-
The localFinally argument is null.
-or-
The parallelOptions argument is null.
The CancellationToken in the parallelOptions argument is canceled.
The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.
The exception that contains all the individual exceptions thrown on all threads.
Remarks
The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int64), a ParallelLoopState instance that may be used to break out of the loop prematurely, and some local state that may be shared amongst iterations that execute on the same thread.
The localInit delegate is invoked once for each thread that participates in the loop's execution and returns the initial local state for each of those threads. These initial states are passed to the first body invocations on each thread. Then, every subsequent body invocation returns a possibly modified state value that is passed to the next body invocation. Finally, the last body invocation on each thread returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per thread to perform a final action on each thread's local state. This delegate might be invoked concurrently on multiple threads; therefore, you must synchronize access to any shared variables.
The Parallel.For method may use more tasks than threads over the lifetime of its execution, as existing tasks complete and are replaced by new tasks. This gives the underlying TaskScheduler object the chance to add, change, or remove threads that service the loop.
If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.
