This tutorial shows how support additional media formats in ASP.NET Web API.
本教程演示如何在ASP.NET Web API中支持额外的媒体格式。

6.1.1 Internet Media Types
6.1.1 Internet的媒体类型

A media type, also called a MIME type, identifies the format of a piece of data. In HTTP, media types describe the format of the message body. A media type consists of two strings, a type and a subtype. For example:
媒体类型，也叫做MIME类型，标识了一片数据的格式。在HTTP中，媒体类型描述了消息体的格式。一个媒体类型由两个字符串组成：类型和子类型。例如：

text/html

image/png

application/json

When an HTTP message contains an entity-body, the Content-Type header specifies the format of the message body. This tells the receiver how to parse the contents of the message body.
当一条HTTP消息含有一个实体时，Content-Type（内容类型）报头指定消息体的格式。这是告诉接收器如何解析消息体的内容。

For example, if an HTTP response contains a PNG image, the response might have the following headers.
例如，如果一个HTTP响应含有一个PNG图片，该响应可能会有以下报头。

HTTP/1.1 200 OK Content-Length: 95267 Content-Type: image/png

When the client sends a request message, it can include an Accept header. The Accept header tells the server which media type(s) the client wants from the server. For example:
当客户端发送一条请求消息时，它可能包括一个Accept报头。Accept报头是告诉服务器，客户端希望从服务器得到哪种媒体类型。例如：

Accept: text/html,application/xhtml+xml,application/xml

This header tells the server that the client wants either HTML, XHTML, or XML.
该报头告诉服务器，客户端希望得到的是HTML、XHTML，或XML。

In Web API, the media type determines how Web API serializes and deserializes the HTTP message body. There is built-in support for XML, JSON, and form-urlencoded data, and you can support additional media types by writing a media formatter.
在Web API中，媒体类型决定了Web API如何对HTTP消息体进行序列化和解序列化。对于XML、JSON，以及URL编码的表单数据，已有了内建的支持。而且，通过编写媒体格式化器（Media Formatter），可以支持额外的媒体类型。

To create a media formatter, derive from one of these classes:
为了创建媒体格式化器，需从以下类进行派生：

MediaTypeFormatter. This class uses asynchronous read and write methods.
MediaTypeFormatter。这个类使用了异步读写方法

BufferedMediaTypeFormatter. This class derives from MediaTypeFormatter but wraps the asynchronous read/write methods inside synchronous methods.
BufferedMediaTypeFormatter。这个类派生于MediaTypeFormatter，但将异步读写方法封装在同步方法之中。

Deriving from BufferedMediaTypeFormatter is simpler, because there is no asynchronous code, but it also means the calling thread can block during I/O. 
从BufferedMediaTypeFormatter派生要更简单些，因为没有异步代码，但它也意味着在I/O期间可能会阻塞线程。

6.1.2 Creating a Media Formatter
6.1.2 创建媒体格式化器

The following example shows a media type formatter that can serialize a Product object to a comma-separated values (CSV) format. This example uses the Product type defined in the tutorial Creating a Web API that Supports CRUD Operations. Here is the definition of the Product object:
以下示例演示了一个媒体类型格式化器，它可以将Product对象序列化成一个逗号分隔的值（CSV）格式。该示例使用了“创建支持CRUD操作的Web API”教程（本系列教程的第2.1小节）中定义的Product类型。以下是Product对象的定义：

namespace ProductStore.Models { public class Product { public int Id { get; set; } public string Name { get; set; } public string Category { get; set; } public decimal Price { get; set; } } }

To implement a CSV formatter, define a class that derives from BufferedMediaTypeFormater:
为了实现CSV格式化器，要定义一个派生于BufferedMediaTypeFormater的类：

namespace ProductStore.Formatters { using System; using System.Collections.Generic; using System.IO; using System.Net.Http.Formatting; using System.Net.Http.Headers; using ProductStore.Models;
public class ProductCsvFormatter : BufferedMediaTypeFormatter { } }

In the constructor, add the media types that the formatter supports. In this example, the formatter supports a single media type, "text/csv":
在其构造器中，要添加一个该格式化器所支持的媒体类型。在这个例子中，该格式化器只支持单一的媒体类型：“text/csv”：

public ProductCsvFormatter() { // Add the supported media type. // 添加所支持的媒体类型 SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/csv")); }

Override the CanWriteType method to indicate which types the formatter can serialize:
重写这个CanWriteType方法，以指示该格式化器可以序列化哪种类型：