Passing string buffers to API calls

I recently had to make some API calls that passed a string buffer. Here's what I came up with:

[DllImport("msi", CharSet=CharSet.Auto)]

private static extern int MsiRecordGetString(IntPtr recordhandle,

    int Field,  string  ValueBuffer,  ref int len);   



string buffer = new string(' ', 250);

int bufSize=buf.Length;

rv = MsiRecordGetString(hRec, x, buffer, ref bufSize);

Console.Write(buf.Substring(0, bufSize));

Does it sound about right?

Another question is when to use IntPtr instead of int?