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C# | Check if an array is read-only or not

Last Updated : 11 Jul, 2025

is used to get a value that indicates whether the

is read-only or not.

public bool IsReadOnly { get; }

This property always returns

for all arrays. Below programs illustrate the use of above-discussed property:

// C# program to illustrate
// IsReadOnly Property of
// Array class
using System;
namespace geeksforgeeks {

class GFG {

    // Main Method
    public static void Main()
    {

        // declares an 1D Array of string
        string[] topic;

        // assigning null to array
        topic = new string[] { null };

        // Here we check whether is
        // array of fixed size or not
        Console.WriteLine("Result: " + topic.IsReadOnly);
    }
}
}

// C# program to illustrate
// IsReadOnly Property of
// Array class
using System;
namespace geeksforgeeks {

class GFG {

    // Main Method
    public static void Main()
    {

        // Two-dimensional array 
        int[, ] arr = new int[, ] {{1, 2}, 
                                   {3, 4},  
                                   {5, 6},  
                                   {7, 8}}; 


        // Here we check whether is
        // array of fixed size or not
        Console.WriteLine("Result: " + arr.IsReadOnly);
    }
}
}

• Array implements the IsReadOnly property because it is needed by the System.Collections.IList interface.
• An array which is read-only does not allow the addition, removal, or modification of elements after the creation of an array.
• If a user requires a read-only collection, then he or she must use a System.Collections class which implements the System.Collections.IList interface.
• If a user cast or convert an array to an IList interface object, the IList.IsReadOnly property returns false. However, if the user cast or convert an array to an IList<T> interface, the IsReadOnly property returns true.
• Retrieving the value of this property is an O(1) operation.

• https://learn.microsoft.com/en-us/dotnet/api/system.array.isreadonly?view=netframework-4.7.2

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