You can create a DLL where you create a wrapper function for each function from the .lib:
extern "C" __declspec (dllexport) HANDLE OpenFile(char const * filename)
{
return open_file(filename);
}
In C# you use P/Invoke to make the dll function available:
[DllImport("WrapperLib.dll")]
private static extern System.IntPtr OpenFile(string filename);