Array.AsReadOnly

In the C# language an array itself cannot be read-only. This is not possible with basic array types. For an immutable collection, we need to call a method like AsReadOnly.

With the ReadOnlyCollection and the AsReadOnly method, we add another layer of indirection to eliminate unwanted writes. This gives us an immutable collection—a ReadOnlyCollection instance.

Example

Here the System.Collections.ObjectModel namespace is included. This is because the ReadOnlyCollection is found there. We call Array.AsReadOnly and pass the int array as the argument.

using System;
using System.Collections.ObjectModel;

class Program
{
    static void Main()
    {
        int[] array = { 1, 5, 3 };
        ReadOnlyCollection<int> result = Array.AsReadOnly(array);

        Console.WriteLine("Count: " + result.Count);
        for (int i = 0; i < result.Count; i++)
        {
            Console.WriteLine(result[i]);
        }
    }
}
Count: 3
1
5
3

Internals

What does Array.AsReadOnly do internally? It calls into the ReadOnlyCollection constructor. In that constructor, the array is received as an IList instance.

We demonstrated Array.AsReadOnly. We explored the System.Collections.ObjectModel namespace to see ReadOnlyCollection. AsReadOnly() restricts access to arrays by client code.