ContainsKey() method in C#

The ContainsKey() method in C# is used to check whether a specific key exists in a Hashtable or other collection types like Dictionary. This method returns true if the key is found, otherwise it returns false.

Syntax

Following is the syntax for the ContainsKey() method −

bool ContainsKey(object key)

Parameters

• key − The key to locate in the collection.

Return Value

Returns true if the key exists in the collection; otherwise, false.

Using ContainsKey() with Hashtable

Example

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      Hashtable h = new Hashtable();
      h.Add(1, "Sam");
      h.Add(2, "Jack");
      h.Add(3, "Andy");
      h.Add(4, "Katie");
      h.Add(5, "Beth");
      h.Add(6, "Benjamin");
      
      Console.WriteLine("Keys and Values list:");
      foreach (var key in h.Keys) {
         Console.WriteLine("Key = {0}, Value = {1}", key, h[key]);
      }
      
      Console.WriteLine("Does key 3 exist? " + h.ContainsKey(3));
      Console.WriteLine("Does key 10 exist? " + h.ContainsKey(10));
   }
}

The output of the above code is −

Keys and Values list:
Key = 6, Value = Benjamin
Key = 5, Value = Beth
Key = 4, Value = Katie
Key = 3, Value = Andy
Key = 2, Value = Jack
Key = 1, Value = Sam
Does key 3 exist? True
Does key 10 exist? False

Using ContainsKey() with Dictionary

Example

using System;
using System.Collections.Generic;

public class Program {
   public static void Main() {
      Dictionary<string, int> ages = new Dictionary<string, int>();
      ages.Add("Alice", 25);
      ages.Add("Bob", 30);
      ages.Add("Charlie", 28);
      
      Console.WriteLine("Dictionary contents:");
      foreach (var pair in ages) {
         Console.WriteLine("{0}: {1}", pair.Key, pair.Value);
      }
      
      string searchKey = "Bob";
      if (ages.ContainsKey(searchKey)) {
         Console.WriteLine("{0} is {1} years old", searchKey, ages[searchKey]);
      } else {
         Console.WriteLine("{0} not found", searchKey);
      }
      
      searchKey = "David";
      if (ages.ContainsKey(searchKey)) {
         Console.WriteLine("{0} is {1} years old", searchKey, ages[searchKey]);
      } else {
         Console.WriteLine("{0} not found", searchKey);
      }
   }
}

The output of the above code is −

Dictionary contents:
Alice: 25
Bob: 30
Charlie: 28
Bob is 30 years old
David not found

Practical Use Case

Example

using System;
using System.Collections.Generic;

public class StudentGrades {
   public static void Main() {
      Dictionary<int, string> students = new Dictionary<int, string>();
      students.Add(101, "A");
      students.Add(102, "B+");
      students.Add(103, "A-");
      
      int[] studentIds = {101, 104, 102, 105};
      
      foreach (int id in studentIds) {
         if (students.ContainsKey(id)) {
            Console.WriteLine("Student {0}: Grade {1}", id, students[id]);
         } else {
            Console.WriteLine("Student {0}: No grade found", id);
         }
      }
   }
}

The output of the above code is −

Student 101: Grade A
Student 104: No grade found
Student 102: Grade B+
Student 105: No grade found

Conclusion

The ContainsKey() method is essential for safely checking if a key exists before accessing its value in collections like Hashtable and Dictionary. It prevents KeyNotFoundException errors and enables conditional logic based on key presence, making your code more robust and error-free.