TestInterface.cs

[Fact]
public async Task Test()
{
    var sku = Guid.NewGuid().ToString();

    var mockProductService = new Mock<IProductService>();
    mockProductService
        .Setup(x => x.GetProductBySku(sku))
        .Returns(Task.FromResult(new Product()
        {
            Sku = sku
        }));

    var sut = new GetProductBySkuHandler(mockProductService.Object);
    var result = await sut.Handle(new GetProductBySkuRequest(sku), CancellationToken.None);

    result.Sku.ShouldBe(sku);
}