6.64Collaboration with DelegatingHandler

When you create an HttpClient via services.AddHttpClient(), .NET provides you with a HttpClientHandler by default as the "endpoint" of the pipeline. All DelegatingHandler instances added through .AddHttpMessageHandler<T>() are wrapped outside the HttpClientHandler, forming the following call chain:

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CustomHandler3 → CustomHandler2 → CustomHandler1 → HttpClientHandler → Network

That is to say:

• Requests start from the outermost DelegatingHandler and are passed inward layer by layer;
• Responses return from HttpClientHandler and are passed back outward layer by layer;
• HttpClientHandler is the only component that actually issues network requests.