GCHandle 範例

這個範例示範如何將 Managed 物件傳遞至預期 LPARAM 型別的 Unmanaged 函式。 LPARAM 型別是 Unmanaged 參數的指標。

GCHandle 範例使用下列 Unmanaged 函式，顯示其原始函式宣告：

• EnumWindows exported from User32.dll.

  BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);
  

在這個範例中，LibWrap 類別包含 EnumWindows 方法的 Managed 原型。 如同它的參數，Managed 方法會以 CallBack 委派代替 WNDENUMPROC 函式指標，並以 IntPtr 指標代替 LPARAM 型別。

App 類別會使用 GCHandle.Alloc 方法建立 Managed 物件的控制代碼，這可避免 Managed 物件被收集。 對 EnumWindows 方法的呼叫會傳遞委派和 Managed 物件，將控制代碼轉換為 IntPtr。 Unmanaged 函式將型別當做回呼函式的參數傳遞回呼叫端。

宣告原型

Public Delegate Function CallBack(ByVal handle As Integer, _
    ByVal param As IntPtr) As Boolean

Public Class LibWrap
    ' Passes a managed object instead of an LPARAM.
    ' Declares a managed prototype for the unmanaged function.
    Declare Function EnumWindows Lib "user32.dll" ( _
        ByVal cb As CallBack, ByVal param As IntPtr) As Boolean
End Class

public delegate bool CallBack(int handle, IntPtr param);

public class LibWrap
{
    // Passes a managed object as an LPARAM type.
    // Declares a managed prototype for the unmanaged function.
    [DllImport("user32.dll")]
    public static extern bool EnumWindows(CallBack cb, IntPtr param);
}

public delegate bool CallBack(int handle, IntPtr param);

public ref class LibWrap
{
public:
    // Passes a managed object as an LPARAM type.
    // Declares a managed prototype for the unmanaged function.
    [DllImport("user32.dll")]
    static bool EnumWindows(CallBack^ cb, IntPtr param);
};

呼叫函式

Public Class App
    Public Shared Sub Main()
        Dim tw As TextWriter = System.Console.Out
        Dim gch As GCHandle = GCHandle.Alloc(tw)

        ' Platform invoke prevents the delegate from being garbage collected
        ' before the call ends.
        Dim cewp As CallBack
        cewp = AddressOf App.CaptureEnumWindowsProc
        LibWrap.EnumWindows(cewp, GCHandle.op_Explicit(gch))
        gch.Free()
    End Sub

    Public Shared Function CaptureEnumWindowsProc(ByVal handle  As Integer, _
        ByVal param As IntPtr) As Boolean
        Dim gch As GCHandle = GCHandle.op_Explicit(param)
        Dim tw As TextWriter = CType(gch.Target, TextWriter)
        tw.WriteLine(handle)
        Return True
    End Function
End Class

public class App
{
    public static void Main()
    {
        TextWriter tw = System.Console.Out;
        GCHandle gch = GCHandle.Alloc(tw);
        CallBack cewp = new CallBack(CaptureEnumWindowsProc);

        // Platform invoke prevents the delegate from being garbage
        // collected before the call ends.
        LibWrap.EnumWindows(cewp, (IntPtr)gch);
        gch.Free();
    }

    private static bool CaptureEnumWindowsProc(int handle, IntPtr param)
    {
        GCHandle gch = (GCHandle)param;
        TextWriter tw = (TextWriter)gch.Target;
        tw.WriteLine(handle);
        return true;
    }
}

public ref class App
{
public:
    static void Main()
    {
        TextWriter^ tw = System::Console::Out;
        GCHandle gch = GCHandle::Alloc(tw);
        CallBack^ cewp = gcnew CallBack(&CaptureEnumWindowsProc);

        // Platform invoke prevents the delegate from being garbage
        // collected before the call ends.
        LibWrap::EnumWindows(cewp, (IntPtr)gch);
        gch.Free();
    }

private:
    static bool CaptureEnumWindowsProc(int handle, IntPtr param)
    {
        GCHandle gch = (GCHandle)param;
        TextWriter^ tw = (TextWriter^)gch.Target;
        tw->WriteLine(handle);
        return true;
    }
};

請參閱

概念

其他封送處理範例

平台叫用資料型別

在 Managed 程式碼中建立原型