Dotazování v konzolových aplikacích
Asynchronní operace v ADO.NET umožňují zahájit časově náročné databázové operace na jednom vlákně při provádění jiných úloh v jiném vlákně. Ve většině scénářů ale nakonec dosáhnete bodu, kdy by vaše aplikace neměla pokračovat, dokud se nedokončí operace databáze. V takových případech je užitečné se dotazovat asynchronní operace, abyste zjistili, jestli se operace dokončila nebo ne.
Pomocí vlastnosti můžete IsCompleted zjistit, jestli se operace dokončila nebo ne.
Příklad
Následující konzolová aplikace aktualizuje data v ukázkové databázi AdventureWorks a provádí svou práci asynchronně. Aby bylo možné emulovat dlouhotrvající proces, vloží tento příklad příkaz WAITFOR do textu příkazu. Za normálních okolností byste se nepokoušli, aby příkazy běžely pomaleji, ale v tomto případě usnadňují předvedení asynchronního chování.
Imports System
Imports System.Data.SqlClient
Module Module1
Sub Main()
' The WAITFOR statement simply adds enough time to prove the
' asynchronous nature of the command.
Dim commandText As String = _
"UPDATE Production.Product " & _
"SET ReorderPoint = ReorderPoint + 1 " & _
"WHERE ReorderPoint Is Not Null;" & _
"WAITFOR DELAY '0:0:3';" & _
"UPDATE Production.Product " & _
"SET ReorderPoint = ReorderPoint - 1 " & _
"WHERE ReorderPoint Is Not Null"
RunCommandAsynchronously(commandText, GetConnectionString())
Console.WriteLine("Press Enter to continue.")
Console.ReadLine()
End Sub
Private Sub RunCommandAsynchronously( _
ByVal commandText As String, ByVal connectionString As String)
' Given command text and connection string, asynchronously
' execute the specified command against the connection. For
' this example, the code displays an indicator as it's working,
' verifying the asynchronous behavior.
Using connection As New SqlConnection(connectionString)
Try
Dim count As Integer = 0
Dim command As New SqlCommand(commandText, connection)
connection.Open()
Dim result As IAsyncResult = _
command.BeginExecuteNonQuery()
While Not result.IsCompleted
Console.WriteLine("Waiting ({0})", count)
' Wait for 1/10 second, so the counter
' doesn't consume all available resources
' on the main thread.
Threading.Thread.Sleep(100)
count += 1
End While
Console.WriteLine( _
"Command complete. Affected {0} rows.", _
command.EndExecuteNonQuery(result))
Catch ex As SqlException
Console.WriteLine("Error ({0}): {1}", _
ex.Number, ex.Message)
Catch ex As InvalidOperationException
Console.WriteLine("Error: {0}", ex.Message)
Catch ex As Exception
' You might want to pass these errors
' back out to the caller.
Console.WriteLine("Error: {0}", ex.Message)
End Try
End Using
End Sub
Private Function GetConnectionString() As String
' To avoid storing the connection string in your code,
' you can retrieve it from a configuration file.
' If you have not included "Asynchronous Processing=true"
' in the connection string, the command will not be able
' to execute asynchronously.
Return "..." & _
"Asynchronous Processing=true"
End Function
End Module
using System;
using System.Data;
using System.Data.SqlClient;
class Class1
{
[STAThread]
static void Main()
{
// The WAITFOR statement simply adds enough time to
// prove the asynchronous nature of the command.
string commandText =
"UPDATE Production.Product SET ReorderPoint = " +
"ReorderPoint + 1 " +
"WHERE ReorderPoint Is Not Null;" +
"WAITFOR DELAY '0:0:3';" +
"UPDATE Production.Product SET ReorderPoint = " +
"ReorderPoint - 1 " +
"WHERE ReorderPoint Is Not Null";
RunCommandAsynchronously(
commandText, GetConnectionString());
Console.WriteLine("Press Enter to continue.");
Console.ReadLine();
}
private static void RunCommandAsynchronously(
string commandText, string connectionString)
{
// Given command text and connection string, asynchronously
// execute the specified command against the connection.
// For this example, the code displays an indicator as it's
// working, verifying the asynchronous behavior.
using (SqlConnection connection =
new SqlConnection(connectionString))
{
try
{
int count = 0;
SqlCommand command =
new SqlCommand(commandText, connection);
connection.Open();
IAsyncResult result =
command.BeginExecuteNonQuery();
while (!result.IsCompleted)
{
Console.WriteLine(
"Waiting ({0})", count++);
// Wait for 1/10 second, so the counter
// doesn't consume all available
// resources on the main thread.
System.Threading.Thread.Sleep(100);
}
Console.WriteLine(
"Command complete. Affected {0} rows.",
command.EndExecuteNonQuery(result));
}
catch (SqlException ex)
{
Console.WriteLine("Error ({0}): {1}",
ex.Number, ex.Message);
}
catch (InvalidOperationException ex)
{
Console.WriteLine("Error: {0}", ex.Message);
}
catch (Exception ex)
{
// You might want to pass these errors
// back out to the caller.
Console.WriteLine("Error: {0}", ex.Message);
}
}
}
private static string GetConnectionString()
{
// To avoid storing the connection string in your code,
// you can retrieve it from a configuration file.
// If you have not included "Asynchronous Processing=true"
// in the connection string, the command will not be able
// to execute asynchronously.
return "..." + "Asynchronous Processing=true";
}
}