ContainItemsAssignableTo vs AllBeAssignableTo

Description

Is there any difference between ContainItemsAssignableTo and AllBeAssignableTo? Spending five minutes looking at them, they seem to test the same scenario. (With the exception, that AllBeAssignableTo has special handling for IEnumerable<Type>)

In contrast to other Contain methods, ContainItemsAssignableTo expects all elements to satisfy and not just a subset.

Complete minimal example reproducing the issue

class Base { }
class Derived : Base { }

public void MyTestMethod()
{
    new[] { new Derived(), new Base() }.Should().AllBeAssignableTo<Derived>();  // fails as expected
    
    new[] { new Derived(), new Base() }.Should().ContainItemsAssignableTo<Derived>(); // <-- I would have expected this to pass
}

[eNeRGy164]

These are different assertions.

subject.Should().ContainItemsAssignableTo<T>()
    .Which.Should().ContainSingle();

Collection can contain multiple types, but exactly 1 is matching the specified type

collection.Should().ContainSingle()
    .Which.Should().BeOfType<T>();

Collection can only contain a single item, and it should match the specified type

[siewers]

@eNeRGy164 Yes, you are right of course 😃 I still think that scenario reads a bit messy. I would never talk like that, and reading it as

“subject should contain items assignable to type T, of which there should be only one”

versus

“subject should contain only one item of type T”

, I know which one I would prefer.

However, I am not sure how to express this fluently, without breaking existing convention.

subject.Should().Contain().AtMost(1).OfType<T>();
subject.Should().Contain().AtLeast(1).OfType<T>();