Verifying calls with NSubstitute

This one came up recently so I thought I would do a post for my future self, so I have something to come back to. Being a conscientious developer ¹ I was creating unit tests and I found a situation where I wanted to verify the call to a method was the expected call, in this case the functionality being tested was creating an object and then calling a repository to persist this, I needed to check the object being persisted was as I expected.

Here is a rather simplified example of what we want to test:

class MyThing {
    readonly IFoo foo;
    public MyThing(IFoo foo) {
        this.foo = foo;
    }

    public async Task<bool> DoTheThing(int howManyTimes) {
        var x = new List<int>();
        for (int i = 1; i <= howManyTimes; i++) {
            x.Add(i);
        }

        foo.DoSomething(x);
        return await Task.FromResult(true);
    }
}

public interface IFoo {
    void DoSomething(IList<int> list);
}

Using NSubstitute we can create a mock for IFoo using NSubstitute.Substitute.For<IFoo>(), and now this can be used to verify calls using Received():

// Arrange
var foo = NSubstitute.Substitute.For<IFoo>();

// Act
var sut = new MyThing(foo);
await sut.DoTheThing(3);

// Assert
foo.Received().DoSomething(Arg.Is<IList<int>>(l => l.Count() == 3));
foo.Received().DoSomething(Arg.Is<IList<int>>(l => l.First() == 1));
foo.Received().DoSomething(Arg.Is<IList<int>>(l => l.Last() == 3));

We could alternatively pass in a function that could do some more complex checks, eg:

Func<IList<int>, bool> expectedResult = (list) => { return false;};
foo.Received().DoSomething(Arg.Is<IList<int>>(l => expectedResult(l)));

or we could even capture the arguments used using Do by setting this up during our arrangement:

IList<int> captured = null!;
foo.When(substitute => substitute.DoSomething(Arg.Any<IList<int>>())).Do(info => captured = info.ArgAt<IList<int>>(0));

NSubstitute can do a whole lot more and you could really go to town with some of the functionality, if you are not familiar with it I would encourage you to check it out - https://nsubstitute.github.io/