你的函数ConvertRtbToBytes（ string richTextBox）请求一个字符串但是你不需要那里的RichtTextBox对象而不是字符串吗？该Object包含.Document属性。字符串没有

Your function ConvertRtbToBytes(string richTextBox) asks for a string but ain't you needing the RichtTextBox object there in stead of the string? That Object holds the .Document property. A string does not have that