Check whether an element is contained in the ArrayList in C#

The ArrayList.Contains() method in C# checks whether a specific element exists in the ArrayList. This method returns a boolean value − true if the element is found, false otherwise.

Syntax

Following is the syntax for the Contains() method −

public virtual bool Contains(object item)

Parameters

• item − The object to locate in the ArrayList. The value can be null.

Return Value

Returns true if the item is found in the ArrayList; otherwise, false.

Using Contains() with String Elements

The following example demonstrates checking for string elements in an ArrayList −

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList list = new ArrayList();
      list.Add("One");
      list.Add("Two");
      list.Add("Three");
      list.Add("Four");
      list.Add("Five");
      list.Add("Six");
      list.Add("Seven");
      list.Add("Eight");

      Console.WriteLine("ArrayList elements...");
      foreach(string str in list) {
         Console.WriteLine(str);
      }

      Console.WriteLine("ArrayList is read-only? = " + list.IsReadOnly);
      Console.WriteLine("Does the element 'Six' exist in the ArrayList? = " + list.Contains("Six"));
      Console.WriteLine("Does the element 'Nine' exist in the ArrayList? = " + list.Contains("Nine"));
   }
}

The output of the above code is −

ArrayList elements...
One
Two
Three
Four
Five
Six
Seven
Eight
ArrayList is read-only? = False
Does the element 'Six' exist in the ArrayList? = True
Does the element 'Nine' exist in the ArrayList? = False

Using Contains() with Numeric Elements

The following example shows how to check for numeric elements in an ArrayList −

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList arrList = new ArrayList();
      arrList.Add(100);
      arrList.Add(200);
      arrList.Add(300);
      arrList.Add(400);
      arrList.Add(500);

      Console.WriteLine("Display elements in a range...");
      IEnumerator demoEnum = arrList.GetEnumerator(1, 3);
      while (demoEnum.MoveNext()) {
         Object ob = demoEnum.Current;
         Console.WriteLine(ob);
      }

      Console.WriteLine("Does the element 300 exist in the ArrayList? = " + arrList.Contains(300));
      Console.WriteLine("Does the element 1000 exist in the ArrayList? = " + arrList.Contains(1000));
   }
}

The output of the above code is −

Display elements in a range...
200
300
400
Does the element 300 exist in the ArrayList? = True
Does the element 1000 exist in the ArrayList? = False

How It Works

The Contains() method uses the Equals() method to compare each element in the ArrayList with the specified object. It performs a linear search through the collection, checking each element sequentially until a match is found or all elements have been examined.

Key Rules

• The method performs case-sensitive comparison for string elements.

• It can search for null values if the ArrayList contains null elements.

• The search operation has O(n) time complexity where n is the number of elements.

• For custom objects, ensure the Equals() method is properly overridden for accurate comparison.

Conclusion

The ArrayList.Contains() method provides a simple way to check if an element exists in an ArrayList. It returns a boolean value and works with any object type, making it useful for validation and conditional operations in your C# applications.