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Callback issue again.
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Tamas Karoly via .NET 247 (VIP)
Hi Guys,

This might have come up several times, but I still have problems with calling a managed object's member function from an unmanaged function. Here is what I was trying to do:

-------- unmanaged code -------
typedef void (*CALLBACK_FN)();

static CALLBACK_FN mFn = NULL;

void SetCallbackFn(CALLBACK_FN Fn)
{
    mFn = Fn;
}

void CallCallbackFn()
{
    mFn();
}

-------- managed code -------
public __delegate void ManagedCallback();

public __gc class CManagedWrapper
{
    GCHandle    mFnHandle;
public:
    void SetCallback(ManagedCallback* Fn)
    {
        mFnHandle = GCHandle::Alloc(Fn);
        SetCallbackFn((CALLBACK_FN)Fn);
    }

    void ReleaseCallback()
    {
        mFnHandle.Free();
    }

    void CallFunction()
    {
        CallCallbackFn();
    }
};

When I call CManagedWrapper.CallFunction() I receive an 'Object reference was not set to an instance of an object' error, although I called SetCallback() with param new ManagedCallback(<some function in an object>). I guess the problem is that I simply comvert the delegate to a c-style function pointer. However, the GCHandle.Alloc() should pin the object.

Could you please tell me what's wrong with this code or suggest any other way to achieve the same result?

Thank you,
Tamas Karoly

-----------------------
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Mattias Sjögren
Tamas,

>However, the GCHandle.Alloc() should pin the object.

No, since you're not allocating a pinning handle. And even if you
were, a pointer to a pinned delegate isn't the same as a function
pointer.

>Could you please tell me what's wrong with this code or suggest any other way to achieve the same result?

Have a look at http://www.codeproject.com/managedcpp/cbwijw.asp

Mattias

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Mattias Sjögren [MVP] mattias @ mvps.org
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