ASP.NET Core by Ken

Introduction to DelegatingHandler in ASP.NET

DelegatingHandler is a powerful mechanism in ASP.NET that enables developers to intercept and manipulate HTTP requests and responses. Functioning similarly to middleware, it allows for the implementation of cross-cutting concerns such as logging, authentication, and error handling in a modular and reusable manner.

Key Features:

Middleware-like Behavior: Processes HTTP requests and responses in a pipeline fashion.
Cross-Cutting Concerns: Ideal for tasks like logging, authentication, and error handling.
Reusability: Handlers can be reused across different HttpClient instances.
Testability: Facilitates unit testing by isolating concerns

Common Use Cases for DelegatingHandler

DelegatingHandler is versatile and can be employed for various purposes:

Authentication: Attach tokens or API keys to outgoing requests.
Logging: Log request and response details for monitoring and debugging.
Error Handling: Implement retry policies or handle specific HTTP status codes.
Caching: Cache responses to reduce redundant network calls.
Header Manipulation: Add or modify headers in requests or responses.

DelegatingHandler vs. Middleware

While both DelegatingHandler and middleware can process HTTP requests and responses, they serve different purposes:

Middleware: Operates on incoming HTTP requests to the server.
DelegatingHandler: Operates on outgoing HTTP requests from HttpClient.

Use middleware for server-side concerns and DelegatingHandler for client-side HTTP request processing.

AspNet

DelegatingHandler

HttpClient

Middleware

CrossCuttingConcerns

Implementing a Custom DelegatingHandler

To create a custom DelegatingHandler:

Inherit from DelegatingHandler:

public class CustomHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // Pre-processing logic
        var response = await base.SendAsync(request, cancellationToken);
        // Post-processing logic
        return response;
    }
}

Register the Handler:
In ASP.NET Core, register the handler using IHttpClientFactory:

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Chaining Multiple DelegatingHandlers

Multiple DelegatingHandlers can be chained to create a pipeline:

services.AddTransient<LoggingHandler>();
services.AddTransient<AuthenticationHandler>();
services.AddHttpClient("ChainedClient")
        .AddHttpMessageHandler<LoggingHandler>()
        .AddHttpMessageHandler<AuthenticationHandler>();

In this setup, LoggingHandler processes the request first, followed by AuthenticationHandler. The order of registration determines the sequence of execution.

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Testing DelegatingHandlers

DelegatingHandlers can be tested in isolation:

Mock the Inner Handler:

var mockHandler = new Mock<HttpMessageHandler>();
mockHandler.Protected()
           .Setup<Task<HttpResponseMessage>>("SendAsync", ...)
           .ReturnsAsync(new HttpResponseMessage(HttpStatusCode.OK));

Inject into DelegatingHandler:

Create HttpClient:

This approach allows for unit testing the DelegatingHandler's logic without making actual HTTP calls.

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