Mock returns null when mocking using Moq

Question

I might be missing something obvious here. Using Asp.Net core background service. The simplified setup is as follows:

SUT

public class OrderService: BaseOrderService 
{
    IConsumer<string, string> _consumer;

    public OrderService(IBuilder consumerBuilder, ILogger logger) : base(logger)
    {
        _consumer = consumerBuilder.Build(() => Provider);
    }
}

public abstract class BaseOrderService: BackgroundService
{
    protected BaseOrderService(ILogger logger){}

    ....
    ....
}

IBuilder

public interface IBuilder
{
    IConsumer<string, string> Build(Func<string> providerFunc);
}

Unit test

[Fact]
public async Task TestDescription()
{
    var mockBuilder = new Mock<IBuilder>();
    var mockConsumer = new Mock<IConsumer<string, string>>();
    mockBuilder.Setup(x => x.Build(It.IsAny<string>)).Returns(mockConsumer.Object);

    var sut = new OrderService(mockBuilder.Object, NullLogger.Instance);
    await sut.StartAsync(default);
    ....
    ....
}

Question

• Upon running my test, in the OrderService constructor, calling _consumer = _consumerBuilder.Build(()=> Provider) returns null , although I have provided the mock setup in the test.

What am I missing? Any pointers will be very much appreciated.

Answer 1

The wrong argument matcher was used.

The mocked member expected Func<string>

IConsumer<string, string> Build(Func<string> providerFunc);

But your code setup It.IsAny<string>

mockBuilder
    .Setup(x => x.Build(It.IsAny<string>)) //<-- WRONG
    .Returns(mockConsumer.Object);

It should be It.IsAny<Func<string>>()

 mockBuilder
    .Setup(x => x.Build(It.IsAny<Func<string>>())) //<--
    .Returns(mockConsumer.Object);