I can not access Count property of the array but through casting to ICollection !!

Question

        int[] arr = new int[5];
        Console.WriteLine(arr.Count.ToString());//Compiler Error
        Console.WriteLine(((ICollection)arr).Count.ToString());//works print 5
        Console.WriteLine(arr.Length.ToString());//print 5

Do you have an explanation for that?

Answer 1

Arrays have .Length, not .Count.

But this is available (as an explicit interface implementation) on ICollection etc.

Essentially, the same as:

interface IFoo
{
    int Foo { get; }
}
class Bar : IFoo
{
    public int Value { get { return 12; } }
    int IFoo.Foo { get { return Value; } } // explicit interface implementation
}

Bar doesn't have public a Foo property - but it is available if you cast to IFoo:

    Bar bar = new Bar();
    Console.WriteLine(bar.Value); // but no Foo
    IFoo foo = bar;
    Console.WriteLine(foo.Foo); // but no Value

Answer 2

While System.Array implement the ICollection interface it doesn't directly expose the Count property. You can see the explicit implementation of ICollection.Count in the MSDN documentation here.

The same applies to IList.Item.

Take look at this Blog entry for more details on explicit and implicit interface implementation: Implicit and Explicit Interface Implementations

Answer 3

Whilst this doesn't answer your question directly, if you are using .NET 3.5 you can include the namespace;

using System.Linq;

which will then allow you to use a Count() method, similar to when casting your int array as an ICollection.

using System.Linq;

int[] arr = new int[5];
int int_count = arr.Count();

You also then have a whole host of nice functions you can use in Linq too :)