我目前正在尝试将一个C#委托封送到一个C++函数指针中,我查看了Microsoft的示例:
// MarshalDelegate1.cpp
// compile with: /clr
#include <iostream>
using namespace System;
using namespace System::Runtime::InteropServices;
#pragma unmanaged
// Declare an unmanaged function type that takes two int arguments
// Note the use of __stdcall for compatibility with managed code
typedef int (__stdcall *ANSWERCB)(int, int);
int TakesCallback(ANSWERCB fp, int n, int m) {
printf_s("[unmanaged] got callback address, calling it...\n");
return fp(n, m);
}
#pragma managed
public delegate int GetTheAnswerDelegate(int, int);
int GetNumber(int n, int m) {
Console::WriteLine("[managed] callback!");
return n + m;
}
int main() {
GetTheAnswerDelegate^ fp = gcnew GetTheAnswerDelegate(GetNumber);
GCHandle gch = GCHandle::Alloc(fp);
IntPtr ip = Marshal::GetFunctionPointerForDelegate(fp);
ANSWERCB cb = static_cast<ANSWERCB>(ip.ToPointer());
Console::WriteLine("[managed] sending delegate as callback...");
// force garbage collection cycle to prove
// that the delegate doesn't get disposed
GC::Collect();
int answer = TakesCallback(cb, 243, 257);
// release reference to delegate
gch.Free();
}
调用 GCHandle::Alloc() 函数旨在防止垃圾回收器收集委托。但是我理解变量 GetTheAnswerDelegate^ fp 已经使得委托保持了生命,因为它是根对象。实际上即使我删除对 GCHandle 的调用,示例仍然可以工作。只有当我像这样内联委托实例化时:
IntPtr ip = Marshal::GetFunctionPointerForDelegate(gcnew GetTheAnswerDelegate(GetNumber));
然后我看到了一个崩溃。
那么,这个来自微软的例子是错的还是我漏掉了什么?