Correct way of checking if a ICollection is actually read-only

Question

There is my model class

public class Model 
{
    public ICollection<string> MyCollection { get; }
}

I want to test MyCollection getter returns an actual read-only collection. That is clients of the class can neither add nor remove elements to/from it at all. What's the correct way to write such test?

Option 1

Assert.That(modelInstance.MyCollection is IReadonlyCollection<string>);

It's returns true e.g when field of the MyCollection property is an instance of List<string> (it implements IReadonlyCollection<string>).

Option 2

var testDelegate = () => modelInstance.MyCollection.Add("QWERTY");
Assert.That(testDelegate, Throws.TypeOf<NotSupportedException>());

I find it not elegant since probably there are other ways to modificate a returned collection (such as indexers for List<string>)

So is changing of type of the property to ReadOnlyCollection<string> the only solution to achieve needed behavior? At that case I do need any unit tests but it does the type of the MyCollection more specific.

Answer 1

ICollection offers a property called IsReadOnly that lets you check that your collection instance is read-only without doing any tricks:

Assert.That(modelInstance.MyCollection.IsReadonly);