Artikel
11/18/2011

Queryable.LongCount<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)

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Returns an Int64 that represents the number of elements in a sequence that satisfy a condition.

Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function LongCount(Of TSource) ( _
    source As IQueryable(Of TSource), _
    predicate As Expression(Of Func(Of TSource, Boolean)) _
) As Long

public static long LongCount<TSource>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, bool>> predicate
)

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> that contains the elements to be counted.

predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>
A function to test each element for a condition.

Return Value

Type: System.Int64
The number of elements in source that satisfy the condition in the predicate function.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception	Condition
ArgumentNullException	source or predicate is nulla null reference (Nothing in Visual Basic).
OverflowException	The number of matching elements exceeds MaxValue.

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The LongCount<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling LongCount<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling LongCount<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it counts the number of items in source that satisfy the condition specified by predicate and returns an Int64.

Examples

The following code example demonstrates how to use LongCount<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) to count the elements in an array that satisfy a condition.

   Structure Pet
      Public Name As String
      Public Age As Integer
   End Structure

   Shared Sub LongCountEx2()
      Dim pets() As Pet = {New Pet With {.Name = "Barley", .Age = 8}, _
                         New Pet With {.Name = "Boots", .Age = 4}, _
                         New Pet With {.Name = "Whiskers", .Age = 1}}

      Const Age As Integer = 3

      ' Count the number of Pet objects where Pet.Age is greater than 3.
      Dim count As Long = pets.AsQueryable().LongCount(Function(Pet) Pet.Age > Age)

      outputBlock.Text &= String.Format("There are {0} animals over age {1}.", count, Age) & vbCrLf
   End Sub

   ' This code produces the following output:

   ' There are 2 animals over age 3.

         class Pet
         {
            public string Name { get; set; }
            public int Age { get; set; }
         }

         public static void LongCountEx2()
         {
            Pet[] pets = { new Pet { Name="Barley", Age=8 },
                               new Pet { Name="Boots", Age=4 },
                               new Pet { Name="Whiskers", Age=1 } };

            const int Age = 3;

            // Count the number of Pet objects where Pet.Age is greater than 3.
            long count = pets.AsQueryable().LongCount(pet => pet.Age > Age);

            outputBlock.Text += String.Format("There are {0} animals over age {1}.", count, Age) + "\n";
         }

         /*
             This code produces the following output:

             There are 2 animals over age 3.
         */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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