How to marshal the type of "Cstring" in .NET Compact Framework(C#)?

DLLname:Test_Cstring.dll(OS is WinCE 5.0),source code: extern "C" __declspec(dllexport) int GetStringLen(CString str) { return str.GetLength(); }

I marshal that in .NET Compact Framework(C#),for example: [DllImport("Test_Cstring.dll", EntryPoint = "GetStringLen", SetLastError = true)] public extern static int GetStringLen(string s);

private void Test_Cstring() { int len=-1; len=GetStringLen("abcd"); MessageBox.Show("Length:"+len.ToString()); //result is -1,so PInvoke is unsuccessful! }

The Method of "GetStringLen" in .NET CF is unsuccessful! How to marshal this type of "Cstring"? Any information about it would be very appreciated!