C# | Creating a read-only wrapper for the List

List<T>.AsReadOnly Method is used to get a read-only ReadOnlyCollection<T> wrapper for the current collection.

Syntax:

public System.Collections.ObjectModel.ReadOnlyCollection AsReadOnly ();

Return Value: It returns an object that acts as a read-only wrapper around the current List<T>.

Example:

// C# code to create a read-only 
// wrapper for the List<T> 
using System; 
using System.Collections; 
using System.Collections.Generic; 
  
class GFG { 
  
    // Driver code 
    public static void Main() 
    { 
  
        // Creating an List<T> of Integers 
        List<int> firstlist = new List<int>(); 
  
        // Adding elements to List 
        firstlist.Add(1); 
        firstlist.Add(2); 
        firstlist.Add(3); 
        firstlist.Add(4); 
        firstlist.Add(5); 
        firstlist.Add(6); 
        firstlist.Add(7); 
  
        Console.WriteLine("Before Wrapping: "); 
  
        // Displaying the elements in the List 
        foreach(int i in firstlist) 
        { 
            Console.WriteLine(i); 
        } 
  
        // Creating a Read-Only packing 
        // around the List<T> 
        IList<int> mylist2 = firstlist.AsReadOnly(); 
  
        Console.WriteLine("After Wrapping: "); 
  
        // Displaying the elements 
        foreach(int m in mylist2) 
        { 
            Console.WriteLine(m); 
        } 
  
        Console.WriteLine("Trying to add new element into mylist2:"); 
  
        // it will give error 
        mylist2.Add(8); 
    } 
} 

Output:

Before Wrapping: 
1
2
3
4
5
6
7
After Wrapping: 
1
2
3
4
5
6
7
Trying to add new element into mylist2:

Runtime Error:

Unhandled Exception:
System.NotSupportedException: Collection is read-only.

Note:

A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection. If changes are made to the underlying collection, the read-only collection reflects those changes.
This method is an O(1) operation.

Reference:

https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.asreadonly?view=netframework-4.7.2

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