Check if the Hashtable contains a specific Key in C#

To check if a Hashtable contains a specific key in C#, use the ContainsKey() method. This method returns true if the specified key exists in the Hashtable, and false otherwise.

Syntax

Following is the syntax for the ContainsKey() method −

public virtual bool ContainsKey(object key)

Parameters

• key − The key to locate in the Hashtable.

Return Value

Returns true if the Hashtable contains an element with the specified key; otherwise, false.

Using ContainsKey() with String Keys

Example

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      Hashtable hash = new Hashtable();
      hash.Add("One", "Katie");
      hash.Add("Two", "John");
      hash.Add("Three", "Barry");
      hash.Add("Four", "Mark");
      hash.Add("Five", "Harry");
      hash.Add("Six", "Nathan");
      hash.Add("Seven", "Tom");
      hash.Add("Eight", "Andy");
      hash.Add("Nine", "Illeana");
      hash.Add("Ten", "Tim");
      
      Console.WriteLine("Hashtable Key and Value pairs...");
      foreach(DictionaryEntry entry in hash) {
         Console.WriteLine("{0} and {1}", entry.Key, entry.Value);
      }
      
      Console.WriteLine("Is Hashtable having fixed size? = " + hash.IsFixedSize);
      Console.WriteLine("If Hashtable read-only? = " + hash.IsReadOnly);
      Console.WriteLine("The Hashtable consists of the key 'Seven'? = " + hash.ContainsKey("Seven"));
      Console.WriteLine("The Hashtable consists of the key 'Eleven'? = " + hash.ContainsKey("Eleven"));
   }
}

The output of the above code is −

Hashtable Key and Value pairs...
One and Katie
Ten and Tim
Five and Harry
Three and Barry
Seven and Tom
Two and John
Four and Mark
Eight and Andy
Nine and Illeana
Six and Nathan
Is Hashtable having fixed size? = False
If Hashtable read-only? = False
The Hashtable consists of the key 'Seven'? = True
The Hashtable consists of the key 'Eleven'? = False

Using ContainsKey() with Numeric Keys

Example

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      Hashtable hash = new Hashtable();
      hash.Add("1", "A");
      hash.Add("2", "B");
      hash.Add("3", "C");
      hash.Add("4", "D");
      hash.Add("5", "E");
      hash.Add("6", "F");
      hash.Add("7", "G");
      hash.Add("8", "H");
      hash.Add("9", "I");
      hash.Add("10", "J");
      
      Console.WriteLine("Hashtable Key and Value pairs...");
      foreach(DictionaryEntry entry in hash) {
         Console.WriteLine("{0} and {1}", entry.Key, entry.Value);
      }
      
      Console.WriteLine("Is Hashtable having fixed size? = " + hash.IsFixedSize);
      Console.WriteLine("If Hashtable read-only? = " + hash.IsReadOnly);
      Console.WriteLine("The Hashtable consists of the key '5'? = " + hash.ContainsKey("5"));
      Console.WriteLine("The Hashtable consists of the key '15'? = " + hash.ContainsKey("15"));
   }
}

The output of the above code is −

Hashtable Key and Value pairs...
10 and J
1 and A
2 and B
3 and C
4 and D
5 and E
6 and F
7 and G
8 and H
9 and I
Is Hashtable having fixed size? = False
If Hashtable read-only? = False
The Hashtable consists of the key '5'? = True
The Hashtable consists of the key '15'? = False

Checking Multiple Keys

Example

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      Hashtable hash = new Hashtable();
      hash.Add("apple", 5);
      hash.Add("banana", 3);
      hash.Add("orange", 7);
      hash.Add("grape", 12);
      
      string[] keysToCheck = {"apple", "mango", "orange", "peach"};
      
      Console.WriteLine("Checking for keys in Hashtable:");
      foreach(string key in keysToCheck) {
         if(hash.ContainsKey(key)) {
            Console.WriteLine("Key '{0}' found with value: {1}", key, hash[key]);
         } else {
            Console.WriteLine("Key '{0}' not found", key);
         }
      }
   }
}

The output of the above code is −

Checking for keys in Hashtable:
Key 'apple' found with value: 5
Key 'mango' not found
Key 'orange' found with value: 7
Key 'peach' not found

Conclusion

The ContainsKey() method provides an efficient way to check if a Hashtable contains a specific key before attempting to access or modify its value. This helps prevent exceptions and enables conditional operations based on key existence in the collection.