如何将函数指针从 C# 传递给 C++ Dll? [英] How to pass function pointer from C# to a C++ Dll?

问题描述

C++ dll中定义的函数是:

The function defined in C++ dll is:

static double (*Func1)(double);
EXTERN_C __declspec(dllexport) __stdcall double TestDelegate(double (*fun)(double))
{
    Func1 = fun;
    return Func1(25.0);
}


void My_Real_purpose()
{
    SomeClass a;
    a.SetFunction(Func1);//Define behaviour of a by C# in runtime
    a.DoSomething();//Even I want it runs in another thread!
}

我尝试在 C# 中这样调用它:

And I tried to call it in C# like this:

    class A
    {
        [DllImport("DllName.dll")]
        public extern static double TestDelegate(IntPtr f);

        public delegate double MyFuncDelegate(double x);

        public static double MyFunc(double x)
        {
            return Math.Sqrt(x);
        }

        static MyFuncDelegate ff;
        static GCHandle gch;
        public static double Invoke()
        {
            ff = new MyFuncDelegate(MyFunc);
            gch = GCHandle.Alloc(ff);  
            double c = TestDelegate(Marshal.GetFunctionPointerForDelegate(ff));//Error occurs this line
            gch.Free();
            return c;
        }

    }

编译没有错误.但是运行时VS2012显示Access Violation Exception"的错误.

It is compiled without error.But when it runs,VS2012 display an error of "Access Violation Exception".

我已经搜索并尝试了很多方法，例如传递委托而不是 IntPtr，但所有方法都失败了.

I have searched and tried a lot of ways,such as passing a delegate rather than a IntPtr,but all of them turned out to be failed.

那么，在包含函数指针的dll中使用API​​函数的正确方法是什么?或者如何实现My_Real_purpose"函数?

So,what is the correct way to use an API function in a dll which contains function pointer?Or how to realize "My_Real_purpose" function?

推荐答案

您的委托使用 cdecl 调用约定.因此，在 C# 中，您可以像这样声明委托:

Your delegate uses the cdecl calling convention. In C# you would therefore declare the delegate like this:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate double CallbackDelegate(double x);

作为替代方案，您可以决定将 C++ 中的函数指针声明为 __stdcall，在这种情况下，您将删除 UnmanagedFunctionPointer 属性并依赖默认调用约定是 CallingConvention.StdCall.

As an alternative, you could decide to declare the function pointer in C++ as __stdcall, in which case you would remove the UnmanagedFunctionPointer attribute and rely on the default calling convention being CallingConvention.StdCall.

这样实现:

public static double MyFunc(double x)
{
    return Math.Sqrt(x);
}

为了使非托管函数指针保持活动状态(防止 GC)，您需要在变量中保存委托的实例.

In order to keep the unmanaged function pointer alive (guarding against GC), you need to hold an instance of the delegate in a variable.

private static CallbackDelegate delegateInstance;
....
delegateInstance = MyFunc;

在您在这里的简单示例中，C++ 代码不使用 TestDelegate 之外的非托管函数指针，但在更复杂的示例中，您可以这样做，在这种情况下，您必须保留非托管函数指针活着.

In the simple example that you have here, the C++ code does not use the unmanaged function pointer outside of TestDelegate, but in a more complex example you may do so, in which case you must keep the unmanaged function pointer alive.

你导入的函数是这样声明的:

The function that you import is declared like this:

[DllImport("DllName.dll")]
public extern static double TestDelegate(CallbackDelegate f);

你可以这样称呼它:

double retval = TestDelegate(delegateInstance);

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