I'm using Moq for mocking a method call and I want to mock a null response. The Login method returns a LoginResult type.

My Login interface:

public async Task<LoginResult> Login(string userPin)

My unit test mock:

var credentialsService = new Mock<ICredentialsService>();
credentialsService.Setup(x => x.Login("1234")).ReturnsAsync((LoginResult)null);

With the above mock, I receive the following warnings:

warning 1:

Warning CS8600 Converting null literal or possible null value to non-nullable type

warning 2:

Argument of type 'ISetup<ICredentialsService, Task>' cannot be used for parameter 'mock' of type 'IReturns<ICredentialsService, Task<LoginResult?>>' in 'IReturnsResult ReturnsExtensions.ReturnsAsync<ICredentialsService, LoginResult?>(IReturns<ICredentialsService, Task<LoginResult?>> mock, LoginResult? value)' due to differences in the nullability of reference types.

How can I resolve these two warnings?

CodePudding user response：

Are you aware of the fact if you don't setup your mock then it will return null by default?

This is due to the fact that the Mock<T>() constructor uses the MockBehavior.Default value.

The MockBehavior can have the following values:

• Strict: an exception is thrown whenever a method or property is invoked without a matching configuration
• Loose: Moq accepts all invocations and attempts to create a valid return value
• Default: same as Loose

So, if you would define your mock with MockBehavior.Strict then you would receive an exception if there were no Setup-Returns statements. But because the default behaviour is Loose that's why you do not need to perform any Setup-Returns calls.

CodePudding user response：

The shown example uses LoginResult but the warning shows the actual type of the expected return type to be LoginResult?. That is the cause of the conversion warning.

credentialsService.Setup(x => x.Login("1234")).ReturnsAsync((LoginResult?)null);

If using ReturnsAsync is causing issues, use Returns with the Task created manually

credentialsService
    .Setup(x => x.Login(It.IsAny<string>()))
    .Returns(s => Task.FromResult<LoginResult?>((LoginResult?)null));