多執行緒程序的參數和傳回值 (C# 和 Visual Basic)

發行項
02/28/2013

因為必須將執行緒類別的建構函式 (Constructor) 傳遞給不使用引數且不傳回值的程序參考，所以在多執行緒應用程式 (Multithreaded Application) 中提供和傳回值並不容易。以下章節將說明一些簡單的方法，來提供參數以及從個別執行緒上的程序傳回值。

提供多執行緒程式的參數

為多執行緒方法呼叫提供參數的最佳做法是，將目標方法包裝在類別之中，接著為此類別定義將當做新執行緒參數的欄位。這種做法的好處是，每次您想要啟動新執行緒時，就可以建立類別的新執行個體以及其本身的參數。例如，假設您有個函式是用來計算三角形的面積，如下列程式碼所示：

Function CalcArea(ByVal Base As Double, ByVal Height As Double) As Double
    CalcArea = 0.5 * Base * Height
End Function

double CalcArea(double Base, double Height)
{
    return 0.5 * Base * Height;
}

您可以寫入類別來包裝 CalcArea 函式，並建立欄位以儲存輸入參數，如以下所示：

Class AreaClass
    Public Base As Double
    Public Height As Double
    Public Area As Double
    Sub CalcArea()
        Area = 0.5 * Base * Height
        MessageBox.Show("The area is: " & Area.ToString)
    End Sub
End Class

class AreaClass
{
    public double Base;
    public double Height;
    public double Area;
    public void CalcArea()
    {
        Area = 0.5 * Base * Height;
        MessageBox.Show("The area is: " + Area.ToString());
    }
}

若要使用 AreaClass，您可以建立 AreaClass 物件，並設定 Base 和 Height 屬性，如下列程式碼所示：

Protected Sub TestArea()
    Dim AreaObject As New AreaClass
    Dim Thread As New System.Threading.Thread(
                        AddressOf AreaObject.CalcArea)
    AreaObject.Base = 30
    AreaObject.Height = 40
    Thread.Start()
End Sub

protected void TestArea()
{
    AreaClass AreaObject = new AreaClass();

    System.Threading.Thread Thread =
        new System.Threading.Thread(AreaObject.CalcArea);
    AreaObject.Base = 30;
    AreaObject.Height = 40;
    Thread.Start();
}

請注意，TestArea 程序在呼叫 CalcArea 方法之後，不會檢查 Area 欄位的值。由於 CalcArea 是在個別的執行緒上執行，因此如果您在呼叫 Thread.Start 之後立即檢查 Area 欄位，則不保證它已設定好。下一章節將討論從多執行緒程序傳回值的較佳做法。

從多執行緒程序傳回值

從個別執行緒上執行的程序傳回值之所以複雜的原因是，程序不能是函式且無法使用 ByRef 引數。傳回值的最簡單方式是使用 BackgroundWorker 元件管理執行緒，並在工作完成時引發事件，且使用事件處理常式處理結果。

以下範例將藉由引發事件來從個別執行緒上執行的程序傳回值：

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

class AreaClass2
{
    public double Base;
    public double Height;
    public double CalcArea()
    {
        // Calculate the area of a triangle.
        return 0.5 * Base * Height;
    }
}

private System.ComponentModel.BackgroundWorker BackgroundWorker1
    = new System.ComponentModel.BackgroundWorker();

private void TestArea2()
{
    InitializeBackgroundWorker();

    AreaClass2 AreaObject2 = new AreaClass2();
    AreaObject2.Base = 30;
    AreaObject2.Height = 40;

    // Start the asynchronous operation.
    BackgroundWorker1.RunWorkerAsync(AreaObject2);
}

private void InitializeBackgroundWorker()
{
    // Attach event handlers to the BackgroundWorker object.
    BackgroundWorker1.DoWork +=
        new System.ComponentModel.DoWorkEventHandler(BackgroundWorker1_DoWork);
    BackgroundWorker1.RunWorkerCompleted +=
        new System.ComponentModel.RunWorkerCompletedEventHandler(BackgroundWorker1_RunWorkerCompleted);
}

private void BackgroundWorker1_DoWork(
    object sender,
    System.ComponentModel.DoWorkEventArgs e)
{
    AreaClass2 AreaObject2 = (AreaClass2)e.Argument;
    // Return the value through the Result property.
    e.Result = AreaObject2.CalcArea();
}

private void BackgroundWorker1_RunWorkerCompleted(
    object sender,
    System.ComponentModel.RunWorkerCompletedEventArgs e)
{
    // Access the result through the Result property.
    double Area = (double)e.Result;
    MessageBox.Show("The area is: " + Area.ToString());
}

使用 QueueUserWorkItem 方法的選擇性 ByVal 狀態物件變數，即可提供參數並將值傳至執行緒集區執行緒。執行緒計時器執行緒也支援狀態物件，以達成這個目的。如需執行緒集區和執行緒計時器的詳細資訊，請參閱執行緒集區 (C# 和 Visual Basic)和執行緒計時器 (C# 和 Visual Basic)。

請參閱

共用方式為

多執行緒程序的參數和傳回值 (C# 和 Visual Basic)

提供多執行緒程式的參數

從多執行緒程序傳回值

請參閱

工作

參考

概念

其他資源

其他資源