Your example did the trick - thanks very much. One of the key things I couldn't work out was how to define fixed length strings as part of the structure:

// specify the size of the string arrays to 32
public const int SA_SIZE = 32;

// struct (class) members
[MarshalAs(UnmanagedType.ByValTStr, SizeConst=SA_SIZE)]
public string lpzDeviceName;

Thanks again for the example,
Gary.