Labview call a dll generated by labwindows

I'm trying to call a dll that generated by CVI

CVI use  LoadLibrary to call another dll compiled by C.

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point = LoadLibrary("MXIO.dll");
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While I call the dll in labview,  always popup a window that show "point null " and labview break down.

If I use CVI call the dll, no error showed and running ok.

many thanks your help.