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.
To create a custom DelegatingHandler:
-
Inherit from DelegatingHandler:
-
Register the Handler:
In ASP.NET Core, register the handler usingIHttpClientFactory
:
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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