Moq: setting up and verifying method calls on interfaces

Manually writing test doubles for every interface is tedious and brittle. Without verification, tests may pass even if the expected method was never called.

Use Moq to create, configure, and verify mocks:

public class OrderServiceTests
{
    private readonly Mock<IOrderRepository> _repoMock = new();
    private readonly OrderService _sut;

    public OrderServiceTests()
        => _sut = new OrderService(_repoMock.Object);

    [Fact]
    public async Task PlaceOrder_Calls_SaveAsync()
    {
        // Arrange
        var order = new Order { Id = 1 };
        _repoMock.Setup(r => r.SaveAsync(It.IsAny<Order>()))
                 .ReturnsAsync(true);

        // Act
        await _sut.PlaceOrderAsync(order);

        // Assert
        _repoMock.Verify(r => r.SaveAsync(
            It.Is<Order>(o => o.Id == 1)), Times.Once);
    }
}

For async methods always use ReturnsAsync or Returns(Task.FromResult(...)).

Moq uses Castle DynamicProxy to generate runtime proxy types implementing the interface. Setup() records an expectation matcher. Verify() checks the call count after the act phase.