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C# ReadOnlyCollection Tips

This C# example program uses the ReadOnlyCollection type from System.Collections.ObjectModel.

ReadOnlyCollection makes an array or List read-only.

With this type from System.Collections.ObjectModel, we provide a collection of elements that cannot be changed. But it may be possible to change some elements themselves.

Example. In this program, a list of ints is created, and then the ReadOnlyCollection constructor is used with that List as an argument. Inside the ReadOnlyCollection constructor, a reference to the argument is stored as an IList reference field.


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


C# program that uses ReadOnlyCollection

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

class Program
    static void Main()
	List<int> list = new List<int>();

	// 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

You can loop over the ReadOnlyCollection using the foreach-loop. A for-loop is also possible with the Count property and indexer. CopyTo provides a way to copy the elements to an array. You must allocate the array yourself.

Finally: We see the Count, Contains, and IndexOf methods. These have the same functionality as the equivalent methods on the List type.

ListList Contains

Example 2. With ReadOnlyCollection, the collection itself is read-only. This means that if you have a ReadOnlyCollection(int), nothing can be changed. Ints are value types. Changing an int in the collection would change the collection itself.


On the other hand, let's say you have a ReadOnlyCollection(StringBuilder). This is kind of a strange thing to have, but it is acceptable. 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.


C# program that changes object in ReadOnlyCollection

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.



Tip: Read-only status is not transferred to other references. The StringBuilder references are completely normal ones.

But: You cannot replace the references with other StringBuilder references or the null literal.

Summary. The ReadOnlyCollection in the System.Collections.ObjectModel namespace functions as a wrapper for any collection that implements IList(T). It does not copy elements. Instead it adds a level of indirection that reduces possible changes.