Array.AsReadOnly<T>(T[]) Method

Definition

Returns a read-only wrapper for the specified array.

public static System.Collections.ObjectModel.ReadOnlyCollection<T> AsReadOnly<T> (T[] array);

Type Parameters

T

The type of the elements of the array.

Parameters

array
T[]

The one-dimensional, zero-based array to wrap in a read-only ReadOnlyCollection<T> wrapper.

Returns

A read-only ReadOnlyCollection<T> wrapper for the specified array.

Exceptions

array is null.

Examples

The following example wraps an array in a read-only ReadOnlyCollection<T>.

using System;
using System.Collections.Generic;

public class SamplesArray  {

   public static void Main()  {

      // Create and initialize a new string array.
      String[] myArr = { "The", "quick", "brown", "fox" };

      // Display the values of the array.
      Console.WriteLine( "The string array initially contains the following values:" );
      PrintIndexAndValues( myArr );

      // Create a read-only IList wrapper around the array.
      IList<string> myList = Array.AsReadOnly( myArr );

      // Display the values of the read-only IList.
      Console.WriteLine( "The read-only IList contains the following values:" );
      PrintIndexAndValues( myList );

      // Attempt to change a value through the wrapper.
      try  {
         myList[3] = "CAT";
      }
      catch ( NotSupportedException e )  {
         Console.WriteLine( "{0} - {1}", e.GetType(), e.Message );
         Console.WriteLine();
      }

      // Change a value in the original array.
      myArr[2] = "RED";

      // Display the values of the array.
      Console.WriteLine( "After changing the third element, the string array contains the following values:" );
      PrintIndexAndValues( myArr );

      // Display the values of the read-only IList.
      Console.WriteLine( "After changing the third element, the read-only IList contains the following values:" );
      PrintIndexAndValues( myList );
   }

   public static void PrintIndexAndValues( String[] myArr )  {
      for ( int i = 0; i < myArr.Length; i++ )  {
         Console.WriteLine( "   [{0}] : {1}", i, myArr[i] );
      }
      Console.WriteLine();
   }

   public static void PrintIndexAndValues( IList<string> myList )  {
      for ( int i = 0; i < myList.Count; i++ )  {
         Console.WriteLine( "   [{0}] : {1}", i, myList[i] );
      }
      Console.WriteLine();
   }
}


/*
This code produces the following output.

The string array initially contains the following values:
   [0] : The
   [1] : quick
   [2] : brown
   [3] : fox

The read-only IList contains the following values:
   [0] : The
   [1] : quick
   [2] : brown
   [3] : fox

System.NotSupportedException - Collection is read-only.

After changing the third element, the string array contains the following values:
   [0] : The
   [1] : quick
   [2] : RED
   [3] : fox

After changing the third element, the read-only IList contains the following values:
   [0] : The
   [1] : quick
   [2] : RED
   [3] : fox

*/

Remarks

To prevent any modifications to the array, expose the array only through this wrapper.

A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection; therefore, if changes are made to the underlying collection, the read-only collection reflects those changes.

This method is an O(1) operation.

Applies to

Product Versions
.NET Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 2.0, 2.1