Problems with an managed callback from unmanaged c++ dll

hi. i'll shor possible, since english not first language. have unmanaged dll call following method:

[dllimport("gpixfs.dll", callingconvention = callingconvention.cdecl)] public static extern int gpixfs_asyncexecute(int hgpixfs, int lcomand, intptr lpcmddata, int dwtimeout, int hwnd, ref uint ulrequest);  

asyncexecute method calls callback function signature

typedef (callback * gpixfs_fn_notif ) (lpgpixfs_notif);  

callback function must return int signalize action taken:

0 processed
1 non processed
3 maintenance
4 normal

i declared delegate follows

 

[unmanagedfunctionpointer(callingconvention.cdecl)] public delegate int32 gpixfsnotification(ref gpixfs_notif lpnotif);

public event gpixfsnotification onnotifdeleg; 

and body of delegate function:

public cdnadapter() { this.onnotifdeleg += new gpixfsnotification(gpi_onnotifdeleg); open(); }  public int32 gpi_onnotifdeleg(ref gpixfs_notif lpnotif) { return 0; } 

the problem is, every time run code, got access violation error , app crashes after line 'return 0' (at '}' more specific.) 

i have googled lot , have found out problem evolves using garbage collector, new world me. give me direction?

please, don't ask me why using ref parameters, wasnt me person coded c dll.

thanks in advance

the garbage collector releases objects which are no longer accessible managed code.

passing function pointer unmanaged code involves creating delegate , getting function pointer delegate.

since unmanaged code is, definition, not managed, garbage collector doesn't know that the delegate still needed , release managed code doesn't need it. calling callback after delegate released causes access violation.

to prevent this, have to store the delegate somewhere until no longer required.

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