Odtnhj

I'm doing some unit testing in C# using NUnit and NSubstitute. I have a class called Adapter, which has a method, GetTemplates(), I want to unit test. GetTemplates() uses httpclient, which I have mocked out using an interface.

The call in GetTemplates looks something like:

public async Task<List<Template>> GetTemplates()

 //Code left out for simplificity. 

 var response = await _client.GetAsync($"GetTemplates");

 if (!response.IsSuccessStatusCode)
 
 throw new Exception();
 

I want _client.GetAsync to return a HttpResponseMessage with a HttpStatusCode.BadRequest so that I can test if the exception is being thrown.

The test method looks like:

[Test]
public void GetTemplate_ReturnBadRequestHttpMessage_ThrowException()

 //Arrange.
 var httpMessage = new HttpResponseMessage(HttpStatusCode.BadRequest);
 _client.GetAsync("").Returns(Task.FromResult(httpMessage));

 //Act.
 var ex = Assert.ThrowsAsync<Exception>(async () => await _Adapter.GetSigningTemplates());

 //Assert.
 Assert.IsInstanceOf<Exception>(ex);
 

When the method has run, it returns

System.NullReferenceException: Object reference not set to an instance of an object.

What did I do wrong?

That is because the arrangement of the mocked client does not match what is actually invoked when the test is exercised.

The client expects

var response = await _client.GetAsync($"GetTemplates");

but the setup is for

 _client.GetAsync("")

note the different arguments passed. When mocks do not get exactly what was setup they usually return the default value of their return type, which in this case is null.

Change the test to use the expected parameter

_client.GetAsync($"GetTemplates").Returns(Task.FromResult(httpMessage));

Reference Return for specific args

or use an argument matcher

_client.GetAsync(Arg.Any<string>()).Returns(Task.FromResult(httpMessage));

Reference Argument matchers