C# ReadOnlyCollectionUse 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.
First example. In this program, a list of ints is created, and then the ReadOnlyCollection constructor is used with that List as an argument.
Constructor 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.
Loops You can loop over the ReadOnlyCollection using the foreach-loop. A for-loop is also possible with the Count property and indexer.
CopyTo CopyTo provides a way to copy the elements to an array. You must allocate the array yourself.
Example 2. With ReadOnlyCollection, the collection itself is read-only. This means that if you have a ReadOnlyCollection of int, nothing can be changed.
Int, uint
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.
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>(); 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
A note. Read-only status is not transferred to other references. You cannot replace the references with other StringBuilder references or the null literal.
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. read[0].Append("Dot").Append("Net").Append("Perls"); Console.WriteLine(read[0]); } }
Summary. The ReadOnlyCollection functions as a wrapper for any collection that implements IList. It adds a level of indirection that reduces possible changes.
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