NSubstitute: Checking received methods with array arguments

I assume that your Commander object will take the arguments and puts them in an array which it then uses to call the Processor mock.

Your foos variable is another array which you create on your setup. Arrays don't compare equal to each other even if they have the same elements. So NSubstitute will complain that it didn't receive the expected value (it received another array which happened to contain the same elements).

Edit: Try this version:

//prepare
var processor = Substitute.For<IProcessor>;
var commander = new Commander(processor);
var foo1 = new Foo("alpha");
var foo2 = new Foo("bravo");
var foos = new [] {foo1, foo2};

//act
commander.DoSomething(foo1, foo2);

//verify
processor.Received().ProcessSomething(Arg.Is<Foo[]>(foos2 => foos.SequenceEqual(foos2));

This requires importing the System.Linq Namespace