Pin a delegate when using a member-function for a C-Style callback

Question

When using a member function for a C-Style callback, is it necessary to pin the delegate so that it is not collected or moved by the GC? I have read several posts on the subject but have not been able to determine if it's necessary or the correct way of implementing.

public delegate void FeedCallbackDelegate(int type,void* data,void *param);

public ref class CStyleApiWrapper
{

    public:

    void StartFeed();

    private:

    FeedCallbackDelegate^ managedFeedCallback;
    IntPtr unmanagedFeedCallbackPtr;

    void FeedCallback(int code,int type,void* data,void *param);

};


void CStyleApiWrapper::StartFeed()
{

    managedFeedCallback = gcnew managedFeedCallback(this, &CStyleApiWrapper::FeedCallback);
    unmanagedFeedCallbackPtr = Marshal::GetFunctionPointerForDelegate(managedFeedCallback);

    //Start Feed
    StartFeed((NOTIFY_FUNC)(void*)unmanagedFeedCallbackPtr,0,NULL);

}

void CStyleApiWrapper::FeedCallback(int type,void* data,void *param)
{
    //Process Feed
    ...
}

edtheprogrammerguy · Accepted Answer · 2013-05-17 00:51:38Z

1

Just calling GetFunctionPointerForDelegate is not enough to keep the GC from reclaiming your delegate. You have to Alloc a handle with GCHandle.Alloc on the delegate. Alloc adds a reference to the delegate, which prevents disposal. Then you have to Free that handle when you are done with it.

Here is a good example page: http://msdn.microsoft.com/en-us/library/367eeye0%28v=VS.100%29.aspx.

answered May 17, 2013 at 0:51

edtheprogrammerguy

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