GCHandle 示例

该示例说明如何将托管对象传递给需要 LPARAM 类型的非托管函数。 LPARAM 类型是指向非托管参数的指针。

GCHandle 示例使用以下非托管函数(这里同时显示其原始函数声明):

  • 从 User32.dll 导出的 EnumWindows

    BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);
    

在该示例中,LibWrap 类包含 EnumWindows 方法的托管原型。 作为其参数,该托管方法用 CallBack 委托替换 WNDENUMPROC 函数指针,并用 IntPtr 指针替换 LPARAM 类型。

App 类使用 GCHandle.Alloc 方法创建托管对象的句柄,这样可以防止该托管对象被收集。 对 EnumWindows 方法的调用传递该委托和托管对象,并将句柄强制转换为 IntPtr。 非托管函数将该类型作为回调函数的参数传回到调用方。

声明原型

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);
};

调用函数

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;
    }
};

请参见

概念

其他封送处理示例

平台调用数据类型

在托管代码中创建原型