September 8, 2024
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Custom Formatter in.NET Core Web API with Content Negotiation

Craig Johnson07 mins

Content negotiation is the process of selecting the optimal resource for a response when many resource representations are available. For a variety of reasons, content negotiation may not be utilized as frequently as it may be despite being an HTTP function for some time. In short, content negotiation lets you choose—or better yet, “negotiate”—the content that gets returned when you send out a REST API request.

Custom Formatters

Assume that content negotiation for non-boxed types needs to be enabled for our public REST API that we are implementing. ASP.NET Core can be used to generate custom formats. Their intention is to give you the ability to create your own format for any type of media that you need to manage.

The following technique can be used to create the custom formatter.

  • Make a class for output formatting that is derived from TextOutputFormatter.
  • Make a TextInputformatter class-derived input formatted class.
  • As with the XML format, add input and output classes to the InputFormatters and OutputFormatters collections.

As an illustration, let’s put in place a custom CSV output format.

Implementing a Custom Formatter

We only need to implement an output formatter because, for the purposes of this article, formatting responses is all that is relevant. Only in the event that a request body contained a corresponding type would we require an input format.

The goal is to format a response that will yield a CSV file containing a list of blogs and the lists of blog posts that correspond with them.

To our project, let’s add a CsvOutputFormatter class.

public class CsvOutputFormatter : TextOutputFormatter
{
    public CsvOutputFormatter()
    {
        SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("application/csv"));
        SupportedEncodings.Add(Encoding.UTF8);
        SupportedEncodings.Add(Encoding.Unicode);
    }

    protected override bool CanWriteType(Type? type)
    {
        return typeof(Emp).IsAssignableFrom(type)
             || typeof(IEnumerable<Emp>).IsAssignableFrom(type);
    }

    public override async Task WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
    {
        var response = context.HttpContext.Response;
        var buffer = new StringBuilder();

        if (context.Object is IEnumerable<Emp>)
        {
            foreach (var emp in (IEnumerable<Emp>)context.Object)
            {
                FormatCsv(buffer, emp);
            }
        }

        await response.WriteAsync(buffer.ToString(), selectedEncoding);
    }

    private static void FormatCsv(StringBuilder buffer, Emp emp)
    {
        buffer.AppendLine($"{emp.Id},\"{emp.Name},\"{emp.Mno},\"{emp.Salary},\"{emp.Type}\"");
    }
}

Here are some points to consider.

We specify the media type and encodings that this formatter should parse in the constructor.

The overridden CanWriteType method indicates whether or not this serializer can write the Blog type. The response is created using the WriteResponseBodyAsync method. Lastly, the FormatCsv method allows us to format a response exactly how we want it.

Implementing the class is fairly simple, and the FormatCsv method logic should be your primary focus.

All that’s left to do is include the recently created CsvOutputFormatter in the AddMvcOptions() method’s list of OutputFormatters.

builder.Services.AddControllers(options =>
{
    options.RespectBrowserAcceptHeader = true;
    options.ReturnHttpNotAcceptable = true;

}).AddXmlSerializerFormatters()
.AddMvcOptions(options => options.OutputFormatters.Add(new CsvOutputFormatter()));

Let’s try this now and see if it functions as intended. This time, in a Postman request, the application/CSV will be used as the value for the Accept header.


After configuring the custom formatter, let’s try using the JSON API responses and check if that is working or not.

Let’s try using the XML API responses and see if that is working or not.

We learned the new technique and evolved together.

Happy coding!

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