AL developers can mock HttpClient Get, Put, Delete, and Post calls in unit tests. By adding a handler function, you can intercept HttpClient calls and easily substitute the responses for HttpClient calls in unit tests.

When you add an HttpClientHandler to a test method, the handler intercepts every HTTP request that occurs during the execution of that test. The handler receives a TestHttpRequestMessage that contains information about the HTTP request. It also receives a TestHttpResponseMessage that contains the mocked HTTP response values that the handler updates. The handler returns a Boolean value that indicates whether to fall through and issue the original HTTP request (true) or to use the mocked response (false).

The TestHttpRequestPolicy property on test codeunits determines how outbound HTTP requests are treated during test execution. It supports the following values:

BlockOutboundRequests: Any HTTP request issued during the test execution that an HTTP client handler doesn't catch and handle raises an exception.
AllowOutboundFromHandler: An HTTP client handler catches all HTTP requests issued during the test execution. The handler can allow the request to go through and issue the original request to the external endpoint.
AllowAllOutboundRequests: The test execution allows all outbound HTTP requests.

The following example shows a codeunit that makes an external web service call and a test codeunit that intercepts and mocks the call. Notice the use of the HttpClientHandler in the test codeunit.