How to easily match on object or list arguments with NSubstitute

There is a Moq counterpart to this post: Using Moq to verify that an object or list was passed to a method. Click there if you would like to see how to do this using Moq.


Sometimes, when using Arg.Is(), we would like the argument to look like a certain object, or worse, a list of objects. Perhaps we would like it to look like an object, but ignore some of the properties. These things are possible with Arg.Is(), but they are quite ugly, brittle, and usually require many lines of code.

There is a better way, which is to leverage FluentAssertion‘s BeEquivalentTo(). If you’re not familiar with FluentAssertions, it’s merely a set of extension methods made to make asserting more readable. In my opinion, BeEquivalentTo() is the most useful method it ships with. It has a very extensive page on its docs about how that method performs its object graph comparison if you’re curious.

We would normally use BeEquivalentTo() something like this:


Okay, great. So, how does that help us here? Well, please calm down and I’ll show you.


NSubstitute helper methods for object comparison

The long and short of is that we can create an AssertionScope inside of an Arg.Is(), run whatever actions we want inside of the scope, and then check if the scope has any failures.

A demonstration will probably explain it better. Here are the extension methods I use and how I use them:

Extension methods:

public static class ArgIs
    // This is just public in case you want to create custom ones in your tests
    // Or, you could make it private and create new extension methods utilizing it
    public static bool MatchingAssertion(Action assertion)
        using var assertionScope = new AssertionScope();
        return !assertionScope.Discard().Any();

    // This is the method we will be using
    public static T EquivalentTo<T>(T value) where T : class
        return Arg.Is<T>(arg => MatchingAssertion(() => arg.Should().BeEquivalentTo(value, string.Empty)));

Using them would look something like:

myService.Setup(x => x.SomeMethod(ArgIs.EquivalentTo(someObject))).Returns(expectedValue);

Notice that I was a little cute with the class name ArgIs. You can’t extend Arg, since it’s static, so ArgIs still reads pretty similarly.


A note on list types

This is also explained in a comment in the example Github repo in the LotteryServiceTests, if you prefer to look at the code.

Although it isn’t specific to these methods, there is a bit of a gotcha with using any Arg.Is() calls on list types, and it has to do with type inheritance, rather than this Arg.Is() itself. I thought I would mention it since it’s easy to overlook.

Consider that you are passing an IEnumerable into a method in your real code. In your tests, you’ve created a List that you expect the passed in argument to be equivalent to. The compiler lets you do this, since a List is an IEnumerable. However, the assertion will fail. This is because Arg.Is() is really Arg.Is<T>(), where the T is usually inferred by the object you pass in. That means if you pass in a List, it will be looking specifically for a List to match on. An IEnumerable is not a List, so it fails. It would work, however, if you specify the T as IEnumerable (although your IDE may tell you this is redundant), create an IEnumerable instead of a List, or if you cast the object to T inside the extension method. You could also do none of these and instead opt to just be mindful of your testing data.

Take whatever route is easiest for you; I just thought this was something worth noting.

Github Example

You can find a full working example of this at the following Github repository:

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