在主控台應用程式中輪詢

ADO.NET 中的非同步作業可讓您在一個執行緒上起始耗時的資料庫作業，同時在另一個執行緒上執行其他工作。 不過，在大部分情況下，您最後將會到達應用程式不應該繼續執行的時機點，直到資料庫作業完成為止。 在這種情況下，輪詢非同步作業以判斷作業是否已經完成會很有用。

您可以使用 IsCompleted 屬性來知道作業是否已經完成。

範例

下列主控台應用程式會更新 AdventureWorks 範例資料庫中的資料，且會非同步地執行其工作。 為了模擬長時間執行的程序，此範例會在命令文字中插入 WAITFOR 陳述式。 一般來說，您不會嘗試讓命令執行速度變慢，但在此案例中，這樣做可讓您更輕鬆地示範非同步行為。

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";
    }
}

另請參閱

• 非同步作業
• ADO.NET 概觀 \(部分機器翻譯\)