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DelegatingHandlers in ASP.NET Core allow you to add behavior around outgoing HTTP requests, similar to middleware but for outgoing traffic. This is particularly useful for adding authentication headers, handling retries, or processing responses. Below are practical examples and commands to implement this feature.
Implementing DelegatingHandler in ASP.NET Core
1. Create a Custom DelegatingHandler:
public class AuthHeaderHandler : DelegatingHandler
{
protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
// Add authentication header
request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", "YOUR_ACCESS_TOKEN");
return await base.SendAsync(request, cancellationToken);
}
}
2. Register the DelegatingHandler:
Register the handler as a transient service in `Startup.cs` or Program.cs:
builder.Services.AddTransient<AuthHeaderHandler>();
builder.Services.AddHttpClient("NamedClient").AddHttpMessageHandler<AuthHeaderHandler>();
3. Using Header Propagation:
For propagating headers across services, use the `Microsoft.AspNetCore.HeaderPropagation` NuGet package:
dotnet add package Microsoft.AspNetCore.HeaderPropagation
Configure it in `Startup.cs`:
builder.Services.AddHeaderPropagation(options =>
{
options.Headers.Add("X-Custom-Header");
});
builder.Services.AddHttpClient("NamedClient").AddHeaderPropagation();
4. Example Command to Test HTTP Requests:
Use `curl` to test your API endpoints with custom headers:
curl -H "Authorization: Bearer YOUR_ACCESS_TOKEN" https://yourapi.com/endpoint
Use Cases for DelegatingHandler
- Authentication: Automatically add tokens to outgoing requests.
- Retry Logic: Implement retry mechanisms for failed requests.
- Logging: Log outgoing requests and incoming responses for debugging.
What Undercode Say
DelegatingHandlers in ASP.NET Core provide a powerful way to manage outgoing HTTP requests, enabling developers to encapsulate cross-cutting concerns like authentication, logging, and retries. By using DelegatingHandler, you can ensure that your HTTP clients are consistent and maintainable. The `Microsoft.AspNetCore.HeaderPropagation` package further simplifies header propagation across microservices, reducing boilerplate code. For Linux and Windows users, integrating these practices with tools like `curl` or PowerShell can streamline API testing and debugging. Always remember to register your handlers as transient to avoid state-related issues. For more advanced scenarios, consider exploring Polly for resilience and transient fault handling.
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References:
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