Check whether a Hashtable contains a specific key or not in C#

The Hashtable class in C# provides the Contains() method to check whether a specific key exists in the collection. This method returns true if the key is found, and false otherwise.

Syntax

Following is the syntax for checking if a Hashtable contains a specific key −

bool result = hashtable.Contains(key);

Parameters

• key − The key to search for in the Hashtable. This parameter is of type Object.

Return Value

The Contains() method returns a bool value −

• true if the Hashtable contains the specified key

• false if the key is not found

Using Contains() Method

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", "");
      hash.Add("Five","Harry");
      hash.Add("Six", "F");
      hash.Add("Seven", "Tom");
      hash.Add("Eight","Andy");
      hash.Add("Nine", "I");
      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 the Hashtable having fixed size? = "+hash.IsFixedSize);
      Console.WriteLine("If Hashtable read-only? = "+hash.IsReadOnly);
      
      Hashtable hash2 = Hashtable.Synchronized(hash);
      Console.WriteLine("Is Hash synchronized = "+hash2.IsSynchronized);
      Console.WriteLine("Does Hashtable contains the key Ten = "+hash.Contains("Ten"));
   }
}

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
Eight and Andy
Nine and I
Six and F
Is the Hashtable having fixed size? = False
If Hashtable read-only? = False
Is Hash synchronized = True
Does Hashtable contains the key Ten = True

Checking Multiple Keys

Example

using System;
using System.Collections;

public class Demo {
   public static void Main(){
      Hashtable hash = new Hashtable();
      hash.Add("1", "AB");
      hash.Add("2", "BC");
      hash.Add("3", "DE");
      hash.Add("4", "EF");
      hash.Add("5", "GH");
      hash.Add("6", "IJ");
      hash.Add("7", "KL");
      hash.Add("8", "MN");
      hash.Add("9", "OP");
      hash.Add("10", "QR");
      
      Console.WriteLine("Value at key 3 = "+hash["3"]);
      Console.WriteLine("Does Hashtable contains the key 12 = "+hash.Contains("12"));
      Console.WriteLine("Does Hashtable contains the key 5 = "+hash.Contains("5"));
      Console.WriteLine("Does Hashtable contains the key 15 = "+hash.Contains("15"));
   }
}

The output of the above code is −

Value at key 3 = DE
Does Hashtable contains the key 12 = False
Does Hashtable contains the key 5 = True
Does Hashtable contains the key 15 = False

Key Points

• The Contains() method only checks for keys, not values

• Key comparison is case-sensitive for string keys

• The method returns false if the key is null and the Hashtable doesn't allow null keys

• Use ContainsValue() method if you need to check for values instead of keys

Conclusion

The Contains() method provides an efficient way to check if a specific key exists in a Hashtable before performing operations like retrieval or modification. This helps prevent exceptions and ensures your code handles missing keys gracefully.