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;
}
};