从多线程过程返回值

从多线程过程返回值

由于这些过程不能为函数也不能使用 ByRef 参数，因而从运行于不同线程的过程返回值是很复杂的。 返回值的最简单方法是：使用 BackgroundWorker 组件来管理线程，在任务完成时引发事件，然后用事件处理程序处理结果。

下面的示例通过从运行于单独线程的某过程引发一个事件来返回值：

Private Class AreaClass2
    Public Base As Double
    Public Height As Double
    Function CalcArea() As Double
        ' Calculate the area of a triangle.
        Return 0.5 * Base * Height
    End Function
End Class

Private WithEvents BackgroundWorker1 As New System.ComponentModel.BackgroundWorker

Private Sub TestArea2()
    Dim AreaObject2 As New AreaClass2
    AreaObject2.Base = 30
    AreaObject2.Height = 40

    ' Start the asynchronous operation.
    BackgroundWorker1.RunWorkerAsync(AreaObject2)
End Sub

' This method runs on the background thread when it starts.
Private Sub BackgroundWorker1_DoWork(
    ByVal sender As Object, 
    ByVal e As System.ComponentModel.DoWorkEventArgs
    ) Handles BackgroundWorker1.DoWork

    Dim AreaObject2 As AreaClass2 = CType(e.Argument, AreaClass2)
    ' Return the value through the Result property.
    e.Result = AreaObject2.CalcArea()
End Sub

' This method runs on the main thread when the background thread finishes.
Private Sub BackgroundWorker1_RunWorkerCompleted(
    ByVal sender As Object,
    ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs
    ) Handles BackgroundWorker1.RunWorkerCompleted

    ' Access the result through the Result property.
    Dim Area As Double = CDbl(e.Result)
    MessageBox.Show("The area is: " & Area.ToString)
End Sub

classAreaClass2
{

publicdoubleBase;

publicdoubleHeight;

publicdoubleCalcArea()
{

//Calculatetheareaofatriangle.


return0.5*Base*Height;
}
}


privateSystem.ComponentModel.BackgroundWorkerBackgroundWorker1
=newSystem.ComponentModel.BackgroundWorker();


privatevoidTestArea2()
{
InitializeBackgroundWorker();

AreaClass2AreaObject2=newAreaClass2();
AreaObject2.Base=30;
AreaObject2.Height=40;


//Starttheasynchronousoperation.

BackgroundWorker1.RunWorkerAsync(AreaObject2);
}


privatevoidInitializeBackgroundWorker()
{

//AttacheventhandlerstotheBackgroundWorkerobject.

BackgroundWorker1.DoWork+=

newSystem.ComponentModel.DoWorkEventHandler(BackgroundWorker1_DoWork);
BackgroundWorker1.RunWorkerCompleted+=

newSystem.ComponentModel.RunWorkerCompletedEventHandler(BackgroundWorker1_RunWorkerCompleted);
}


privatevoidBackgroundWorker1_DoWork(

objectsender,
System.ComponentModel.DoWorkEventArgse)
{
AreaClass2AreaObject2=(AreaClass2)e.Argument;

//ReturnthevaluethroughtheResultproperty.

e.Result=AreaObject2.CalcArea();
}


privatevoidBackgroundWorker1_RunWorkerCompleted(

objectsender,
System.ComponentModel.RunWorkerCompletedEventArgse)
{

//AccesstheresultthroughtheResultproperty.


doubleArea=(double)e.Result;
MessageBox.Show("Theareais:"+Area.ToString());
}

可以通过使用 QueueUserWorkItem 方法的可选 ByVal 状态对象变量为线程池线程提供参数和返回值。 线程计时器线程也支持将状态对象用于此目的。 有关线程池和线程计时器的信息，请参见 线程池（C# 和 Visual Basic） 和 线程计时器（C# 和 Visual Basic）。