How to Test a Custom DelegatingHandler in .NET

What’s the Problem?

When writing unit tests for a custom DelegatingHandler, you might try calling:

var response = await handler.SendAsync(request, cancellationToken);

But this will cause a compiler error. Why? Because SendAsync in DelegatingHandler is protected, meaning you can't call it directly from your test project.

The Simple Solution

Use HttpMessageInvoker, which is designed to work with any HttpMessageHandler (including DelegatingHandler). It provides a public SendAsync method, so you can easily test your handler:

var handler = new YourCustomHandler
{
    InnerHandler = new DummyHandler() // Replace with mock/stub as needed
};

var invoker = new HttpMessageInvoker(handler);
var response = await invoker.SendAsync(request, cancellationToken);

This allows you to simulate HTTP requests through your custom handler, without using HttpClient.

Why This Works

• DelegatingHandler is a subclass of HttpMessageHandler.
• HttpMessageInvoker takes any HttpMessageHandler and exposes a public way to send HTTP requests.
• This bypasses the visibility issue with protected SendAsync.

Tip for Better Testing

Use a mock InnerHandler to control the behavior of the response. This helps you test how your DelegatingHandler reacts to different scenarios.