How to get keys from a HashTable in C#?

The Hashtable is a non-generic collection in C# that stores key-value pairs, similar to the generic Dictionary collection. It is defined in the System.Collections namespace.

A Hashtable consists of key/value pairs where each key is computed as a hash code and stored in different buckets internally. When accessing the Hashtable, this hash code is matched to locate the corresponding value, which optimizes lookup performance.

Let's explore how to retrieve keys from a Hashtable in C#.

Syntax

Following is the syntax for accessing Hashtable keys using a foreach loop

foreach(DictionaryEntry entry in hashtable) {
   Console.WriteLine(entry.Key);
}

Following is the syntax for getting keys using the Keys collection

ICollection keys = hashtable.Keys;
foreach(string key in keys) {
   Console.WriteLine(key);
}

Using Foreach Loop with DictionaryEntry

In this approach, we iterate through the Hashtable using a foreach loop with DictionaryEntry to access both keys and values

using System;
using System.Collections;

class Program {
   static void Main() {
      // Create a hashtable instance
      Hashtable cityTable = new Hashtable();
      
      // Adding key/value pairs
      cityTable.Add("US", "New York");
      cityTable.Add("FR", "Paris");
      cityTable.Add("UK", "London");
      cityTable.Add("IN", "Mumbai");
      cityTable.Add("GER", "Berlin");
      
      // Print hashtable keys only
      Console.WriteLine("City Table Keys:");
      foreach(DictionaryEntry entry in cityTable) {
         Console.WriteLine(entry.Key);
      }
   }
}

The output of the above code is

City Table Keys:
FR
US
IN
GER
UK

Displaying Both Keys and Values

using System;
using System.Collections;

class Program {
   static void Main() {
      // Create a hashtable instance
      Hashtable cityTable = new Hashtable();
      
      // Adding key/value pairs
      cityTable.Add("US", "New York");
      cityTable.Add("FR", "Paris");
      cityTable.Add("UK", "London");
      cityTable.Add("IN", "Mumbai");
      cityTable.Add("GER", "Berlin");
      
      // Print hashtable key-value pairs
      Console.WriteLine("City Table Key-Value Pairs:");
      foreach(DictionaryEntry entry in cityTable) {
         Console.WriteLine("{0} = {1}", entry.Key, entry.Value);
      }
   }
}

The output of the above code is

City Table Key-Value Pairs:
FR = Paris
US = New York
IN = Mumbai
GER = Berlin
UK = London

Using Keys Collection

In this approach, we first retrieve the collection of keys using the Keys property and then iterate through this collection

using System;
using System.Collections;

class Program {
   static void Main() {
      // Create a hashtable instance
      Hashtable langTable = new Hashtable();
      
      // Adding key/value pairs
      langTable.Add("CPP", "C++");
      langTable.Add("CS", "C#");
      langTable.Add("JAVA", "Java");
      langTable.Add("PL", "Perl");
      langTable.Add("JS", "JavaScript");
      
      // Get collection of keys
      ICollection keys = langTable.Keys;

      Console.WriteLine("Language Keys:");
      foreach(string key in keys) {
         Console.WriteLine(key);
      }
   }
}

The output of the above code is

Language Keys:
PL
JS
CS
JAVA
CPP

Comparison of Approaches

Conclusion

You can retrieve keys from a Hashtable in C# using either a foreach loop with DictionaryEntry or by accessing the Keys collection. The foreach approach is more versatile as it provides access to both keys and values, while the Keys collection approach is more efficient when you only need the keys.