You need to look up interop services. You can use tlbimp.exe to generate your interop if it is COM (which from the post above seems not to be the case). Otherwise, you can use the DLLImport attribute and reference the functions this way. You will find it tricky to figure out the proper types to use in the function signature, so you need to research a little. But, this is your start.
[DllImport("TheCplusplus.dll")]
static extern uint BTSeparateItems([In, MarshalAs(UnmanagedType.BStr)] string filePath, out object obj);