private static void AddTypeReader(string readerTypeName, ContentReader contentReader, ContentTypeReader reader)
        {
            Type targetType = reader.TargetType;

            if (targetType != null)
            {
                if (targetTypeToReader.ContainsKey(targetType))
                {
                    throw new ContentLoadException("TypeReader duplicate");
                }
                targetTypeToReader.Add(targetType, reader);
            }
            readerTypeToReader.Add(reader.GetType(), reader);
            nameToReader.Add(readerTypeName, reader);
        }