When using the otherwise superb NSubstitute, one thing has been bothering me for a long time: you cannot match arguments by equivalency.

To illustrate what I mean, let’s use an example. Say you got the following interface

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public interface ISomeInterface
{
    void Use(Person person);
}

public class Person
{
    public DateTime Birthday { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

and a method that does something with it (for the sake of the example):

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static void DoSomethingWith(ISomeInterface service)
{
    service.Use(new Person
    {
        FirstName = "John",
        LastName = "Doe",
        Birthday = new DateTime(1968, 6, 1)
    });
}

Now how would you test that method? You’d start with the following:

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var service = Substitute.For<ISomeInterface>();
DoSomethingWith(service);

But now you’d like to validate that service.Use() was called with the right Person. Because NSubstitute has no built-in equivalency argument matcher, you would have to do this:

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service.Received().Use(Arg.Is<Person>(p => p.FirstName=="John" && p.LastName=="Doe" && p.Birthday==new DateTime(1968, 6, 1));

but obviously that is rather tedious and becomes unmanageable with more properties in Person.

Alternatively, you can do:

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var service = Substitute.For<ISomeInterface>();
Person received = null;
service.WhenForAnyArgs(s => s.Use(null)).Do(ci => received = ci.Arg<Person>());

DoSomethingWith(service);

received.Should()
        .BeEquivalentTo(new Person
        {
            FirstName = "John",
            LastName = "Doe",
            Birthday = new DateTime(1968, 6, 1)
        });

This approach uses FluentAssertions and is a lot more robust, but it doesn’t make for easy reading.

Solution

Ideally, one would like to just write:

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var service = Substitute.For<ISomeInterface>();
DoSomethingWith(service);
service.Received()
    .Use(ArgEx.IsEquivalentTo(new Person
    {
        FirstName = "John",
        LastName = "Doe",
        Birthday = new DateTime(1968, 6, 1)
    }));

And if you install NSubstitute.Equivalency, a little helper library I wrote for this purpose, you can do just that.

I called the type for accessing this matcher ArgEx to avoid creating import conflicts with the standard NSubstitute Arg type.