/// <summary>
/// Adds a new field to the end of the structure stream. This method
/// is only valid for write access raw data.
/// </summary>
/// <param name="rawStruct">The field to add</param>
/// <returns>The index of the addes field, it is always added to the
/// end of the list</returns>
public uint AddField(RawGffField rawField)
{
if (FileAccess.Write != access) throw new InvalidOperationException();
// Seek to the end of the stream and add the struct.
fieldsStream.Seek(0, SeekOrigin.End);
rawField.Serialize(fieldsStream);
// Count the number of structs in the stream and return the index of
// the last one, which is the one we just added.
int count = (int) fieldsStream.Length / Marshal.SizeOf(typeof(RawGffField));
return (uint) (count - 1);
}