What is content negotiation in Web API?

Question

Ravi Vishwakarma · Answer

Content Negotiation (or conneg) in ASP.NET Web API is the process of
determining the best format (media type) to return in the response, based on what the client can accept.
It's how the server chooses JSON, XML, or other formats based on:
The client's Accept headerAvailable formatters on the server
How It Works
When a client sends a request to the API, it may include an Accept header:
GET /api/products
Accept: application/json
ASP.NET Web API:
Looks at the Accept header.Matches it with the available media type formatters (e.g., JSON, XML).Serializes the response in the best matching format.
🧪 Example
Request:
GET /api/products
Accept: application/xml
Response:
andlt;Productandgt;andlt;Idandgt;1andlt;/Idandgt;andlt;Nameandgt;Phoneandlt;/Nameandgt;andlt;/Productandgt;
If the client had sent:
Accept: application/json
Then the server responds with:
{ "Id": 1, "Name": "Phone" }
Built-in Formatters in Web API
Formatter
Media Type(s)
JsonMediaTypeFormatter
application/json, text/json
XmlMediaTypeFormatter
application/xml, text/xml
You can also create custom formatters for CSV, 
YAML, etc.
Server Configuration (Optional)
You can control or extend formatters in WebApiConfig.cs:
config.Formatters.JsonFormatter.SupportedMediaTypes
      .Add(new MediaTypeHeaderValue("text/html"));
To remove XML support:
config.Formatters.Remove(config.Formatters.XmlFormatter);
Use Cases
Use Case
Benefit
Mobile apps prefer JSON
Lightweight, faster parsing
Legacy systems require XML
Compatibility
Browser default is HTML
Can force JSON via custom formatter
Force Format without Accept Header
You can use URL query strings or route extensions:
/api/products?format=json
/api/products.json
Requires custom logic or third-party packages (e.g., WebApiContrib.Formatting).
Summary
Feature
Description
Definition
Process of selecting response format based on client preference
Header Used
Accept
Formats Supported
JSON, XML (by default)
Customizable
Yes, you can add/remove/override formatters
and

Formatter	Media Type(s)
JsonMediaTypeFormatter	`application/json`, `text/json`
XmlMediaTypeFormatter	`application/xml`, `text/xml`

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What is content negotiation in Web API?

Ravi Vishwakarma

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1 Answers

How It Works

🧪 Example

Request:

Response:

Built-in Formatters in Web API

Server Configuration (Optional)

Use Cases

Force Format without Accept Header

Summary

Liked By

Use Case	Benefit
Mobile apps prefer JSON	Lightweight, faster parsing
Legacy systems require XML	Compatibility
Browser default is HTML	Can force JSON via custom formatter

Feature	Description
Definition	Process of selecting response format based on client preference
Header Used	`Accept`
Formats Supported	JSON, XML (by default)
Customizable	Yes, you can add/remove/override formatters