C# | Creating a read-only wrapper for List

Last Updated : 27 Jan, 2019

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

Syntax:

public System.Collections.ObjectModel.ReadOnlyCollection<T> 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.Generic; 
  
class GFG { 
  
    // Driver code 
    public static void Main() 
    { 
  
        // Creating an List<T> of Integers 
        List<int> mylist = new List<int>(); 
  
        // Adding elements to List 
        mylist.Add(17); 
        mylist.Add(19); 
        mylist.Add(21); 
        mylist.Add(9); 
        mylist.Add(75); 
        mylist.Add(19); 
        mylist.Add(73); 
  
        Console.WriteLine("Before Wrapping: "); 
  
        // Displaying the elements 
        // in the mylist 
        foreach(int i in mylist) 
        { 
            Console.WriteLine(i); 
        } 
  
        // Creating a Read-Only packing 
        // around the List 
        IList<int> readlist = mylist.AsReadOnly(); 
  
        Console.WriteLine("After Wrapping: "); 
  
        // Displaying the elements 
        // of Read-Only Collection 
        foreach(int j in readlist) 
        { 
            Console.WriteLine(j); 
        } 
          
        // You can add elements to 
        // the original List i.e. mylist 
        Console.WriteLine("Adding new element to mylist: "); 
        mylist.Add(35); 
          
        // Displaying the elements 
        // in the mylist 
        foreach(int k in mylist) 
        { 
            Console.WriteLine(k); 
        } 
  
        // But you cannot add elements 
        // into the Read-Only Collection 
        Console.WriteLine("Trying to add new element into readlist:"); 
  
        // it will give error 
        readlist.Add(34); 
          
    } 
} 

Output:

Before Wrapping: 
17
19
21
9
75
19
73
After Wrapping: 
17
19
21
9
75
19
73
Adding new element to mylist: 
17
19
21
9
75
19
73
35
Trying to add new element into readlist:

Runtime Error: