Array.AsReadOnly Constructor IList int StringBuilder

`ReadOnlyCollection`

This makes an array or List read-only. With this type from System.Collections.ObjectModel, we provide a collection of elements that cannot be changed.

In programs, an immutable collection can be used with less copying—we do not need to worry about it being modified. This can make programs simpler and easier to reason about.

First example

In this program, a list of ints is created, and then the ReadOnlyCollection constructor is used with that List as an argument.

Detail Inside the ReadOnlyCollection constructor, a reference to the argument is stored as an IList reference field.

Info In the ReadOnlyCollection constructor, no elements are copied. The constructor is fast.

Tip You can loop over the ReadOnlyCollection using the foreach-loop. A for-loop is also possible with the Count property and indexer.

Tip 2 CopyTo provides a way to copy the elements to an array. You must allocate the array yourself.

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;

class Program
{
    static void Main()
    {
        List<int> list = new List<int>();
        list.Add(1);
        list.Add(3);
        list.Add(5);

        // Constructor.
        ReadOnlyCollection<int> read = new ReadOnlyCollection<int>(list);

        // Loop over ReadOnlyCollection.
        foreach (int value in read)
        {
            Console.WriteLine("read: {0}", value);
        }

        // Copy ReadOnlyCollection to an array.
        int[] array = new int[3];
        read.CopyTo(array, 0);

        // Display array.
        foreach (int value in array)
        {
            Console.WriteLine("array: {0}", value);
        }

        // Use methods on ReadOnlyCollection.
        int count = read.Count;
        bool contains = read.Contains(-1);
        int index = read.IndexOf(3);
        Console.WriteLine("{0}, {1}, {2}", count, contains, index);
    }
}read: 1
read: 3
read: 5
array: 1
array: 3
array: 5
3, False, 1

Example 2

With ReadOnlyCollection, the collection itself is read-only. This means that if you have a ReadOnlyCollection of int, nothing can be changed.

Here You can still change the individual StringBuilder objects by accessing their methods.

Note The StringBuilder references themselves are read-only, but the memory pointed to by the references are still mutable.

using System;
using System.Collections.ObjectModel;
using System.Text;

class Program
{
    static void Main()
    {
        // Set up the ReadOnlyCollection.
        StringBuilder[] array = new StringBuilder[1];
        array[0] = new StringBuilder();
        var read = new ReadOnlyCollection<StringBuilder>(array);

        // Now we change an object pointed to in a ReadOnlyCollection.
        read[0].Append("Dot").Append("Net").Append("Perls");
        Console.WriteLine(read[0]);
    }
}
DotNetPerls

A note

Read-only status is not transferred to other references. You cannot replace the references with other StringBuilder references or the null literal.

Summary

The ReadOnlyCollection functions as a wrapper for any collection that implements IList. It adds a level of indirection that reduces possible changes.