Verify evaluates the same object on reuse

[lund76]

Hi

It was really hard to come up with a descriptive title, so right of the bat .. sorry for that.

But I've come across something that I'm not sure is a bug, or by design. But until I broke it down into a smaller example, I was quite sure it was a bug.

I've created a repo with democode:
https://github.com/lund76/MoqMyWords

My mockup has a method that clearly has an error (it's not supposed to update all 3) :

public class PriceService
{
    private readonly IInventoryService _inventoryService;

    public PriceService(IInventoryService inventoryService)
    {
        _inventoryService = inventoryService;
    }

    public async Task<Product> UpdateOnlyNewestPrice(List<Product> products)
    {
        foreach (var p in products.OrderByDescending(o => o.Created).Take(3))
        {
            await _inventoryService.UpdateAsync(p);
        }
        return products.FirstOrDefault() ?? new Product();
    }
}

And my test (verify a little funny formed to showcase this issue)

[Fact]
public async Task MoqMyWord_UpdateService_OnlyCalledOnce()
{

    var productList = new List<Product>
    {
        new() { Price = 1, Created = DateTime.UtcNow.AddDays(-1) },
        new() { Price = 2, Created = DateTime.UtcNow.AddDays(-2) },
        new() { Price = 3, Created = DateTime.UtcNow.AddDays(-3) }
    };

    Mock<IInventoryService> service = new Mock<IInventoryService>();

    var priceService = new PriceService(service.Object);

    await priceService.UpdateOnlyNewestPrice(productList);

    //For the sake of the argument a silly way to evaluate any price less than 3 is valid
    //This is only to make sure the test fails.
    service.Verify(u => u.UpdateAsync(It.Is<Product>(p => EvaluatePrice(p))),Times.Once);          
}
private bool EvaluatePrice(Product product)
{
    _outputHelper.WriteLine($"product price is {product.Price}");
    return product.Price < 3;
}

This test fails as expected, since the service is called 2 times:

If I then make a shitty change to the implementation, and only change a property of an existing object:

I would still expect it to fail with 2 calls to the service, but it fails with no calls to the service. Moq doesn't recognize the change in the object

[stakx]

Hi @lund76, no worries about the issue title, it's actually quite acurate. This question gets asked a lot (recently e.g. in #1309 and #1187). When Moq records invocations (for later verification), it simply captures the invocation arguments in an object[] array – it does not deep-clone every argument. This means that the usual .NET rules apply: reference types (such as your Product class) are captured by reference, and value types are captured by value (which essentially amounts to a shallow clone). Therefore if the same Product is captured on three invocations, and you change that Product instance over time, this will be reflected in the recorded invocations.

I'm a little early, but... have a happy new year! 🎆

[lund76]

Thanks for the reply. It what pretty sure it was something along those lines.
I've tried searching a bit for the issue, but must have given up to quickly - sorry for that :)

Have a happy new year as well :)

[stakx]

FYI @lund76, I'm likely going to merge #1319 which will provide a new way to solve this issue; see the last code example in the PR's description for an explanation of how this might help.