public static extern void CoTaskMemFree(IntPtr ptr);

         CoTaskMemFree(outCredBuffer);

As explained [here], memory for the returned unmanaged string must be freed by calling LocalFree. Therefore, you must define the string as an IntPtr in managed code and call LocalFree yourself. Otherwise, the marshaler would call CoTaskMemFree which is not correct in this case.

    // ppwszTip will be freed by marshler using CoTaskMemFree()

static extern void CoTaskMemFree(IntPtr pv);