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

In C#, arrays are typically mutable by default, meaning their elements can be modified after creation. However, you can check if an array is read-only using the IsReadOnly property from the ICollection interface.

The IsReadOnly property returns true if the array cannot be modified, and false if elements can be changed. Regular arrays in C# always return false for this property.

Syntax

Following is the syntax to check if an array is read-only −

bool isReadOnly = arrayName.IsReadOnly;

Using IsReadOnly Property

Example

The following example demonstrates how to check if an array is read-only −

using System;

public class Demo {
    public static void Main() {
        string[] products = new string[] { "Laptop", "Mobile", "Tablet" };
        
        Console.WriteLine("Products array:");
        foreach(string product in products) {
            Console.WriteLine("- " + product);
        }
        
        Console.WriteLine("\nArray Properties:");
        Console.WriteLine("Is the array having fixed size? = " + products.IsFixedSize);
        Console.WriteLine("Is the array read only? = " + products.IsReadOnly);
        
        // Demonstrating that we can modify the array
        products[0] = "Desktop";
        Console.WriteLine("After modification, first element: " + products[0]);
    }
}

The output of the above code is −

Products array:
- Laptop
- Mobile
- Tablet

Array Properties:
Is the array having fixed size? = True
Is the array read only? = False
After modification, first element: Desktop

Checking Empty Array

Example

Even empty arrays are not read-only by default −

using System;

public class Demo {
    public static void Main() {
        string[] emptyArray = new string[] { };
        int[] numbers = { 10, 20, 30, 40, 50 };
        
        Console.WriteLine("Empty array properties:");
        Console.WriteLine("Length: " + emptyArray.Length);
        Console.WriteLine("Is fixed size? = " + emptyArray.IsFixedSize);
        Console.WriteLine("Is read only? = " + emptyArray.IsReadOnly);
        
        Console.WriteLine("\nNumbers array properties:");
        Console.WriteLine("Length: " + numbers.Length);
        Console.WriteLine("Is fixed size? = " + numbers.IsFixedSize);
        Console.WriteLine("Is read only? = " + numbers.IsReadOnly);
        
        // Modifying numbers array
        numbers[2] = 99;
        Console.WriteLine("Modified element at index 2: " + numbers[2]);
    }
}

The output of the above code is −

Empty array properties:
Length: 0
Is fixed size? = True
Is read only? = False

Numbers array properties:
Length: 5
Is fixed size? = True
Is read only? = False
Modified element at index 2: 99

Array Properties Comparison

Understanding Read-Only vs Fixed Size

It's important to distinguish between read-only and fixed size:

• IsReadOnly = False: You can modify existing elements

• IsFixedSize = True: You cannot add or remove elements

Example

using System;

public class Demo {
    public static void Main() {
        char[] letters = { 'A', 'B', 'C', 'D' };
        
        Console.WriteLine("Original array:");
        Console.WriteLine(string.Join(", ", letters));
        
        Console.WriteLine("\nArray characteristics:");
        Console.WriteLine("Can modify elements (IsReadOnly): " + !letters.IsReadOnly);
        Console.WriteLine("Cannot resize (IsFixedSize): " + letters.IsFixedSize);
        
        // This works - modifying existing element
        letters[1] = 'X';
        Console.WriteLine("\nAfter modifying index 1:");
        Console.WriteLine(string.Join(", ", letters));
    }
}

The output of the above code is −

Original array:
A, B, C, D

Array characteristics:
Can modify elements (IsReadOnly): True
Cannot resize (IsFixedSize): True

After modifying index 1:
A, X, C, D

Conclusion

In C#, regular arrays are never read-only (IsReadOnly always returns false), meaning you can modify their elements. However, arrays are fixed-size (IsFixedSize returns true), so you cannot add or remove elements after creation. Use the IsReadOnly property to programmatically check mutability in generic collection scenarios.