How to Resize Array in C#: Syntax and Examples

In C#, you can resize an array using the Array.Resize<T> method, which changes the size of the array while preserving existing elements. This method creates a new array internally and assigns it back to your array variable.

Syntax

The syntax to resize an array in C# uses the Array.Resize<T> method where T is the type of the array elements.

• ref T[] array: The array variable to resize, passed by reference.
• int newSize: The new size for the array.

This method modifies the original array variable to point to a new array with the specified size.

csharp

Array.Resize<T>(ref T[] array, int newSize);

Example

This example shows how to resize an integer array from size 3 to size 5, preserving the existing elements and adding default values for new positions.

csharp

using System;

class Program
{
    static void Main()
    {
        int[] numbers = { 1, 2, 3 };
        Console.WriteLine("Original array length: " + numbers.Length);

        // Resize the array to length 5
        Array.Resize(ref numbers, 5);

        Console.WriteLine("Resized array length: " + numbers.Length);
        Console.WriteLine("Array contents:");
        foreach (int num in numbers)
        {
            Console.WriteLine(num);
        }
    }
}

Output

Original array length: 3
Resized array length: 5
Array contents:
1
2
3
0
0

Common Pitfalls

Common mistakes when resizing arrays include:

• Not passing the array variable by ref, which means the original array won't be updated.
• Assuming the array resizes in place; actually, a new array is created and assigned.
• Forgetting that new elements are initialized with default values (e.g., 0 for int, null for reference types).

csharp

using System;

class Program
{
    static void Main()
    {
        int[] numbers = { 1, 2, 3 };

        // Wrong: missing ref keyword - this won't resize the original array
        // Array.Resize(numbers, 5); // This causes a compile error

        // Correct way:
        Array.Resize(ref numbers, 5);

        Console.WriteLine("Resized length: " + numbers.Length);
    }
}

Output

Resized length: 5

Quick Reference

Array.Resize<T> Cheat Sheet:

Action	Syntax	Notes
Resize array	Array.Resize(ref array, newSize);	Resizes array preserving elements, new elements get default values.
Increase size	Array.Resize(ref array, largerSize);	Adds default values at the end.
Decrease size	Array.Resize(ref array, smallerSize);	Truncates elements beyond new size.
Pass by reference	ref keyword is required	Without ref, array won't be updated.

Key Takeaways

Use Array.Resize(ref array, newSize) to resize arrays in C# safely.

Always pass the array variable with the ref keyword to update it.

Resizing creates a new array; existing elements are copied, new ones get default values.

You can both increase and decrease the array size with Array.Resize.

For dynamic collections, consider using List instead of arrays.