Mock member-call when parameters do not match exactly

Question

I have a method that I´m trying to mock using NSubstitute:

Foo MyMethod(IEnumerable<string> args);

I want the method to return new Foo { ID = 1 } for a call where the args-collection has exactly one item called "Item1". However when the collection contains only "Item2" I´d like to return another instance of Foo.

So I wrote this code:

var mock = Substitute.For<MyType>();
mock.MyMethod(new[] { "Item1" }).Returns(new Foo { ID = 1 });
mock.MyMethod(new[] { "Item2" }).Returns(new Foo { ID = 2 });

In my calling code I have something like this:

var foo = myType.MyMethod(new[] { "Item1" });

However the mock isn´t executed as arrays are compared using reference-equality. So as I have two instances of string[] sharing the same values they are considered unequal and thus we don´t jump to the mocked return-statement.

So what I´d like to achieve is no matter what exact type of collection we provide to the method the mock should just distinguish on the collections items.

Answer 1

You can use the Arg.Is syntax to do the check yourself:

mock.MyMethod(Arg.Is<string[]>(a => a.Contains("Item1"))).Returns(new Foo { ID = 1 });
mock.MyMethod(Arg.Is<string[]>(a => a.Contains("Item2"))).Returns(new Foo { ID = 2 });

See here for more information: http://nsubstitute.github.io/help/return-for-args/