public async Task GivenADifferentFakeUrlEndpoint_GetSomeFooDataAsync_ThrowsAnException() { // Arrange. const string responseData = "{ \"Id\":69, \"Name\":\"Jane\" }"; var messageResponse = FakeHttpMessageHandler.GetStringHttpResponseMessage(responseData); var options = new HttpMessageOptions { RequestUri = "http://this.is.not.the.correct.endpoint", HttpResponseMessage = messageResponse }; var messageHandler = new FakeHttpMessageHandler(options); var myService = new MyService(messageHandler); // Act. // NOTE: network traffic will not leave your computer because you've faked the response, above. var result = await Should.ThrowAsync <InvalidOperationException>(myService.GetSomeFooDataAsync()); // Assert. result.Message.ShouldStartWith("No HttpResponseMessage found"); }