Understanding Media Type Formatters in ASP.NET Web API

Media type formatters are responsible for serializing and deserializing data between the client and server in ASP.NET Web API. They help in formatting the data in the requested media type, which could be JSON, XML, or any other custom media type.

In ASP.NET Web API, there are four in-built media type formatters, which are:

  1. JSON media type formatter
  2. XML media type formatter
  3. Form URL-encoded media type formatter
  4. Multipart form data media type formatter

Let’s take a look at a simple code snippet that demonstrates the use of the JSON media type formatter:

// GET api/values
public IEnumerable<string> Get()
    return new string[] { "value1", "value2" };

// POST api/values
public HttpResponseMessage Post([FromBody] string value)
    // Code to add the value to the database
    return new HttpResponseMessage(HttpStatusCode.Created);

In the above code, we have a simple Web API controller with two methods, one to retrieve data and another to add data to the database. The first method returns a collection of strings, which is serialized to JSON by the JSON media type formatted before being sent to the client.

The second method accepts a string parameter from the client, which is deserialized by the JSON media type formatter before being passed to the method. The [FromBody] attribute specifies that the value should be taken from the request body.

Similarly, you can use the other media type formatters to format data in the requested media type. You can also create custom media type formatters by implementing the MediaTypeFormatter class.

Configuring different types of media type formatters

To configure different types of media type formatters in ASP.NET Web API, you can add or remove formatters from the GlobalConfiguration object. Here’s an example of how to configure the JSON and XML media type formatters:

public static class WebApiConfig
    public static void Register(HttpConfiguration config)
        // Remove the default XML formatter

        // Add the JSON formatter
        config.Formatters.Add(new JsonMediaTypeFormatter());

In the above code, we first remove the default XML media type formatter by calling config.Formatters.Remove() and passing in the XmlMediaTypeFormatter object.

Next, we add the JSON media type formatter by creating a new instance of the JsonMediaTypeFormatter class and calling config.Formatters.Add() to add it to the list of available formatters.

You can also create custom media type formatters by inheriting from the MediaTypeFormatter class and implementing the CanReadType() and CanWriteType() methods. Here’s an example of a custom media type formatter for CSV data:

public class CsvMediaTypeFormatter : MediaTypeFormatter
    public CsvMediaTypeFormatter()
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/csv"));

    public override bool CanReadType(Type type)
        return false;

    public override bool CanWriteType(Type type)
        return typeof(IEnumerable).IsAssignableFrom(type);

    public override async Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content, TransportContext transportContext)
        var csv = new CsvWriter(new StreamWriter(writeStream));
        await csv.FlushAsync();

In this example, we create a new class called CsvMediaTypeFormatter that inherits from the MediaTypeFormatter class. We add the supported media type (text/csv) to the SupportedMediaTypes collection in the constructor.

We then implement the CanReadType() and CanWriteType() methods to indicate whether this formatter can read or write the specified type. In this case, we only support writing CSV data for types that implement the IEnumerable interface.

Finally, we override the WriteToStreamAsync() method to write the CSV data to the output stream using the CsvHelper library. This method is called by Web API to serialize the data when this formatter is used.

To use this custom media type formatter, you would add it to the GlobalConfiguration object in the same way as the built-in formatters:

config.Formatters.Add(new CsvMediaTypeFormatter());

configuring different types of media type formatters in ASP.NET Web API is a simple process that allows you to support a wide range of media types and data formats for your API.

Overall, media type formatters play an essential role in ASP.NET Web API by allowing clients to consume data in the format they require, while also allowing the server to handle requests in the format it expects.

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