Get or set the number of elements that the ArrayList can contain in C#

The Capacity property of an ArrayList in C# gets or sets the number of elements that the ArrayList can contain without resizing. This is different from the Count property, which represents the actual number of elements currently stored in the ArrayList.

When elements are added and the capacity is exceeded, the ArrayList automatically doubles its capacity to accommodate more elements.

Syntax

Following is the syntax for getting the capacity −

int capacity = arrayList.Capacity;

Following is the syntax for setting the capacity −

arrayList.Capacity = newCapacity;

Understanding Capacity vs Count

Using Default Capacity

When no initial capacity is specified, ArrayList starts with a default capacity and automatically expands as needed −

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList arrList = new ArrayList();
      arrList.Add(25);
      arrList.Add(50);
      arrList.Add(75);
      arrList.Add(100);
      arrList.Add(125);
      arrList.Add(150);
      arrList.Add(175);
      arrList.Add(200);
      arrList.Add(225);
      arrList.Add(250);
      Console.WriteLine("Elements in ArrayList...");
      foreach(int i in arrList) {
         Console.WriteLine(i);
      }
      Console.WriteLine("Count of elements = " + arrList.Count);
      Console.WriteLine("Capacity = " + arrList.Capacity);
   }
}

The output of the above code is −

Elements in ArrayList...
25
50
75
100
125
150
175
200
225
250
Count of elements = 10
Capacity = 16

Setting Initial Capacity

You can specify an initial capacity when creating the ArrayList to optimize memory usage −

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList arrList = new ArrayList(10);
      arrList.Add(25);
      arrList.Add(50);
      arrList.Add(75);
      arrList.Add(100);
      Console.WriteLine("Elements in ArrayList...");
      foreach(int i in arrList) {
         Console.WriteLine(i);
      }
      Console.WriteLine("Count of elements = " + arrList.Count);
      Console.WriteLine("Capacity = " + arrList.Capacity);
   }
}

The output of the above code is −

Elements in ArrayList...
25
50
75
100
Count of elements = 4
Capacity = 10

Modifying Capacity at Runtime

The capacity can be changed dynamically during program execution −

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList arrList = new ArrayList();
      arrList.Add("Apple");
      arrList.Add("Banana");
      Console.WriteLine("Initial Capacity: " + arrList.Capacity);
      Console.WriteLine("Initial Count: " + arrList.Count);
      
      arrList.Capacity = 20;
      Console.WriteLine("After setting Capacity to 20: " + arrList.Capacity);
      Console.WriteLine("Count remains: " + arrList.Count);
   }
}

The output of the above code is −

Initial Capacity: 4
Initial Count: 2
After setting Capacity to 20: 20
Count remains: 2

Conclusion

The Capacity property controls the internal storage size of an ArrayList, while Count represents the actual number of elements. Setting an appropriate initial capacity can improve performance by reducing memory reallocations as elements are added.