Question

A typed array implements both the System.Collections.IList and System.Collections.Generic.ICollection<T> interfaces, which both have their own IsReadOnly properties. But what on earth is going on here?

var array = new int[10];
Console.WriteLine(array.IsReadOnly); // prints "False"

var list = (System.Collections.IList)array;
Console.WriteLine(list.IsReadOnly); // prints "False"

var collection = (System.Collections.Generic.ICollection<int>)array;
Console.WriteLine(collection.IsReadOnly); // prints "True"

The IList view of the array behaves as I’d expect, returning the same as the array itself, however the ICollection<T> view of the array returns true.

Is there any rational explanation for this behaviour, or is it a compiler/CLR bug? (I’d be really surprised if it’s the latter as you’d imagine this would have been found before now, but it’s so counter-intuitive I can’t think what the explanation could be…).

I’m using C#3.0/.NET 3.5 SP1.

Answer 1

There was plenty of agony over this decision, as evident in the comments on this feedback article.