Moq ReturnsAsync 在元组时返回 Null<object,bool> 而不是提供的价值</object,bool>

Question

This is my first question so far, excuse me if it is not very well-described but I will try my best.

I am using Moq to mock my service layer in unit test for a post api call, and the _service.Create(...) returns a tuple value: Task<(Model.Receipt Receipt, bool IsIdempotent)>
therefor I created a tuple result and pass as ReturnsAsync like below:

var input = JsonConvert.DeserializeObject<Model.Receipt>(_jsonReceiptString);
var output = (Receipt: input, IsIdempotent: true);
_service.Setup(x => x.CreateAsync(input)).ReturnsAsync(output);

everything works fine till here, but at run time, after calling the service in the Post call, the return value is <null,false> !!!
It is sound like returning a default value instead of expected tuple. As I have logging data after this mocking, this causes a failed unit test.
Do you have any idea if I am missing something here?

Answer 1

This setup

_service.Setup(x => x.CreateAsync(input)).ReturnsAsync(output);

..says that if CreateAsync is called and the argument is input , return output . This only works if the argument is actually the same object, the same class instance as input .

Sometimes that works, but often it doesn't. In this case input is deserialized from a string. If the code you're testing also deserializes a string you'll end up with two instances of Receipt . They might be identical, but they're not the same instance so the Setup doesn't work the way you want.

What you likely want is to set up the mock so that if you call CreateAsync and input has certain property values then the mock returns output .

I don't know what Receipt looks like. For the sake of demonstration let's say it looks like this

internal class Receipt
{
    public int Id { get; set; }
    public string Name { get; set; }
}

...and you want your Setup to return output if the argument passed to CreateAsync has the same Id and Name as input . In that case you could do this:

_service.Setup(x =>
        x.CreateAsync(
            It.Is<Models.Receipt>(receipt =>
                receipt.Id == input.Id
                && receipt.Name == input.Name)))
    .ReturnsAsync(output);

This says that Setup is looking for a Receipt argument, and when it gets that argument it's going to execute this function which returns true if the argument's Id and Name property match input 's Id and Name :

receipt =>
    receipt.Id == input.Id
    && receipt.Name == input.Name

---

What if you plan on writing a lot of these tests and you don't want to write that function over and over? You can also create an IEqualityComparer like this:

public class ReceiptEqualityComparer : IEqualityComparer<Receipt>
{
    public bool Equals(Receipt x, Receipt y)
    {
        return x.Id == y.Id && x.Name == y.Name;
    }

    public int GetHashCode(Receipt obj)
    {
        return HashCode.Combine(obj.Id, obj.Name);
    }
}

Unless you need this in your production code I would define this class in the test project. This class contains the logic to compare two instances of Receipt and decide if they are equal.

Now your Setup can look like this:

_service.Setup(x =>
        x.CreateAsync(
            It.Is<Models.Receipt>(input, new ReceiptEqualityComparer())))
    .ReturnsAsync(output);

Now Setup will take the argument passed to CreateAsync and use ReceiptEqualityComparer to determine whether that argument is "equal" to input . If they have the same Id and Name they are equal.

---

Finally, your original code as posted in the question will work if Receipt implements IEquatable<Receipt> . That means that the class has its own built-in logic for comparing properties to see if two instances are equal. I use ReSharper to auto-generate that for me. It looks like this:

public class Receipt : IEquatable<Receipt>
{
    public int Id { get; set; }
    public string Name { get; set; }

    public bool Equals(Receipt? other)
    {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return Id == other.Id && Name == other.Name;
    }

    public override bool Equals(object? obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((Receipt) obj);
    }

    public override int GetHashCode()
    {
        return HashCode.Combine(Id, Name);
    }
}

That might make sense. But if you only need this for your test then I'd prefer either of the previous two approaches instead of modifying the production class so that the test works.