problem w/ returning char array from an unmanaged dll

[rkpatil]

have an unmanaged C++ DLL with following method:

the DLL interface for unmanaged C++ is:
SendCommand(void *h, char *c, char *d, char *b);

the declaration in C# is:
[DllImport("test.dll", CharSet = CharSet.Ansi, EntryPoint = "SendCommand")]
unsafe public static extern char* SendCommand(void* h, [MarshalAs(UnmanagedType.LPStr)] string cmd, [MarshalAs(UnmanagedType.LPStr)] string data, [MarshalAs(UnmanagedType.LPArray)] ref byte[] buffer);

when i call the function
byte buf = new byte[25];
SendCommand(r, "test string", "", buf);

The problem is that buf should have a value in it when the call is completed. However buf is empty. I have another function which only accepts a byte array as the input and it is a char * on the c++ side and that returns valid values in the byte array. So I am guessing the problem could be with the passing of the strings as the SendCommand method will return correct values when it understands the strings correctly. The SendCommand method works correctly if I make the call using C++ code but does not work correctly from C#. Could be LPSTR not be working correctly as it is supposed to?

THANKS