HTTP Client Message Handler in Web API

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Using an HTTP Client Message Handler in ASP.NET Web API allows you to intercept and manipulate outgoing requests and incoming responses for HTTP clients. This comprehensive guide will walk you through creating a custom HTTP Client Message Handler, providing code examples, step-by-step instructions, and best practices.

Prerequisites

Before you start, ensure you have:

1. A good understanding of ASP.NET Web API.
2. Visual Studio or a code editor of your choice.
3. Familiarity with C# and HTTP protocols.

Step 1: Create a Custom HTTP Client Message Handler

Start by creating a custom HTTP Client Message Handler by inheriting from DelegatingHandler. This handler will intercept outgoing requests and incoming responses.

using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

public class CustomHttpClientHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // You can manipulate the outgoing request here before it is sent

        // Call the inner handler to send the request and receive the response
        var response = await base.SendAsync(request, cancellationToken);

        // You can manipulate the incoming response here before it is returned

        return response;
    }
}

Step 2: Register the Custom HTTP Client Message Handler

2. Register your custom HTTP Client Message Handler in your Web API configuration. This can typically be done in the WebApiConfig.cs file.

public static void Register(HttpConfiguration config)
{
    // Register the custom HTTP Client Message Handler
    config.MessageHandlers.Add(new CustomHttpClientHandler());

    // Other configuration settings
}

Step 3: Use the HTTP Client Message Handler

Now, you can use the custom HTTP Client Message Handler in your API clients. When creating an instance of HttpClient, you can pass an instance of your custom handler as an argument.

HttpClient client = new HttpClient(new CustomHttpClientHandler());

Step 4: Implement Request and Response Handling

4. Within your CustomHttpClientHandler, you can implement logic to modify outgoing requests and process incoming responses. For example, you can add headers, perform authentication, or log requests and responses.

Here’s an example that adds an API key header to outgoing requests:

protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
    // Add an API key header to the outgoing request
    request.Headers.Add("X-API-Key", "YourApiKeyHere");

    // Call the inner handler to send the request and receive the response
    var response = await base.SendAsync(request, cancellationToken);

    // You can process the incoming response here

    return response;
}

Conclusion

By following this comprehensive guide, you’ve created a custom HTTP Client Message Handler in ASP.NET Web API. These handlers provide a powerful way to intercept and manipulate outgoing requests and incoming responses, enabling you to implement cross-cutting concerns such as authentication, logging, and custom header management in your API clients. Customize your CustomHttpClientHandler to meet your specific requirements for request and response handling.