Declare a const array in C#

In C#, you cannot directly declare a const array because arrays are reference types and const fields must be compile-time constants. However, you can achieve similar behavior using readonly fields or ReadOnlyCollection to create immutable arrays.

Syntax

Following is the syntax for declaring a readonly array −

public static readonly DataType[] arrayName = { value1, value2, value3 };

Following is the syntax for using ReadOnlyCollection −

public static readonly ReadOnlyCollection<DataType> arrayName = 
    new ReadOnlyCollection<DataType>(new DataType[] { value1, value2, value3 });

Using readonly Static Arrays

The readonly keyword allows you to create an array that can only be assigned during declaration or in a constructor. Unlike const, readonly values can be set at runtime −

Example

using System;

public class VehicleTypes {
    public static readonly string[] vehicles = { "Car", "Motorbike", "Cab" };
    public static readonly int[] numbers = { 10, 20, 30, 40, 50 };
    
    public static void Main() {
        Console.WriteLine("Vehicle types:");
        foreach (string vehicle in vehicles) {
            Console.WriteLine(vehicle);
        }
        
        Console.WriteLine("\nNumbers:");
        foreach (int number in numbers) {
            Console.WriteLine(number);
        }
        
        // This would cause compile error:
        // vehicles = new string[] { "Bus" };
    }
}

The output of the above code is −

Vehicle types:
Car
Motorbike
Cab

Numbers:
10
20
30
40
50

Using ReadOnlyCollection

For complete immutability, use ReadOnlyCollection which prevents both reassignment and modification of array elements −

Example with ReadOnlyCollection Property

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;

public class ImmutableArrays {
    public static ReadOnlyCollection<string> Colors { get; } = 
        new ReadOnlyCollection<string>(new string[] { "Red", "Green", "Blue" });
    
    public static void Main() {
        Console.WriteLine("Available colors:");
        foreach (string color in Colors) {
            Console.WriteLine(color);
        }
        
        Console.WriteLine("\nTotal colors: " + Colors.Count);
        
        // This would cause compile error:
        // Colors[0] = "Yellow";
    }
}

The output of the above code is −

Available colors:
Red
Green
Blue

Total colors: 3

Comparison of Approaches

Runtime Assignment with readonly

Unlike const, readonly fields can be assigned values at runtime in constructors −

Example

using System;

public class DynamicArray {
    public readonly string[] items;
    
    public DynamicArray(int size) {
        items = new string[size];
        for (int i = 0; i < size; i++) {
            items[i] = "Item " + (i + 1);
        }
    }
    
    public static void Main() {
        DynamicArray arr = new DynamicArray(3);
        
        Console.WriteLine("Dynamic readonly array:");
        foreach (string item in arr.items) {
            Console.WriteLine(item);
        }
    }
}

The output of the above code is −

Dynamic readonly array:
Item 1
Item 2
Item 3

Conclusion

While you cannot declare a true const array in C#, readonly arrays provide compile-time assignment protection, and ReadOnlyCollection offers complete immutability. Use readonly for performance-critical scenarios and ReadOnlyCollection when you need full protection against modifications.