mockhttp VS httpmock

I'd say that without wanting to refactor the code a little bit (maybe moving those HttpClients into a service so that they are easily mocked) your best bet would be to use something like WireMock or MockHttp.

What is the need to create a wrapper around the HttpClient? If it's for mocking, you can use this MockedHttpClientHandler for you tests

Voilà! That's how to write tests with HttpClient and Moq. With some extension methods, we could have a small DSL to write more readable tests. For a more fully-featured alternative to write tests for HttpClient, check mockhttp, "a testing layer for Microsoft's HttpClient library."

So if you need to mock http client you may want to look into a library like https://github.com/richardszalay/mockhttp

We will use httpmock for this purpose. Inside function setupSuite(), where we created useCase and provided nil as a billingClient now we will pass mocked http client:

Alternatively, you could use an http mock library like https://github.com/jarcoal/httpmock, and override the urls the RealClient calls with a mock response in your tests.

To mock an API: https://github.com/jarcoal/httpmock to use when your code calls external APIs so you don't want your tests call the real API. No need to create an interface as you do.