TestHttpClientFactory for unit tests

_usage.cs

var testHttpClientFactory = new TestHttpClientFactory()
    .When(r => r.RequestUri.AbsolutePath == "/foobar", r => new HttpResponseMessage(HttpStatusCode.Redirect))
    .When(r => true, r => new HttpResponseMessage(HttpStatusCode.NotFound));

// run some tests

// check results
var logItem = testHttpClientFactory.Log.SingleOrDefault(tpl => tpl.request.RequestUri.Host == "example.com");
Assert.Equal(HttpStatusCode.Whatever, logItem.response.StatusCode);

TestHttpClientFactory.cs

public class TestHttpClientFactory : IHttpClientFactory
{
    private class MockHttpMessageHandler : HttpMessageHandler
    {
        public readonly IList<(Predicate<HttpRequestMessage> predicate, Func<HttpRequestMessage, HttpResponseMessage> factory)> Behaviours =
            new List<(Predicate<HttpRequestMessage> predicate, Func<HttpRequestMessage, HttpResponseMessage> factory)>();

        public readonly IList<(HttpRequestMessage, HttpResponseMessage)> Log = new List<(HttpRequestMessage, HttpResponseMessage)>();

        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            var httpResponseMessage = Behaviours.First(b => b.predicate(request)).factory(request);
            Log.Add((request, httpResponseMessage));
            return Task.FromResult(httpResponseMessage);
        }
    }

    private readonly MockHttpMessageHandler handler;
    private readonly HttpClient client;

    public TestHttpClientFactory()
    {
        handler = new MockHttpMessageHandler();
        client = new HttpClient(handler);
    }

    public TestHttpClientFactory When(Predicate<HttpRequestMessage> predicate, Func<HttpRequestMessage, HttpResponseMessage> factory)
    {
        handler.Behaviours.Add((predicate, factory));
        return this;
    }

    public HttpClient CreateClient(string name) => client;

    public IList<(HttpRequestMessage request, HttpResponseMessage response)> Log => handler.Log.ToImmutableList();
}