如果您返回一个可序列化的对象，WebAPI 将根据您的客户端发送的 Accept 标头自动发送 JSON 或 XML.

If you return a serializable object, WebAPI will automatically send JSON or XML based on the Accept header that your client sends.

如果你返回一个字符串，你会得到一个字符串.

If you return a string, you'll get a string.