How to get key value from HashTable in C#?

Hash tables are non-−generic collections in C#. It stores key − value pairs, similar to a general "dictionary" collection. HashTable is defined in System.Collections.namespace.

HashTable consists of key/value pairs, where each key is calculated as a hash code and stored internally in a different bucket. Whenever the HashTable is accessed, the hash code is matched against the hash code for the specified key, thereby accessing the corresponding value. This mechanism optimizes lookups in hash tables.

Now let us discuss how to get keys from HashTable in C#.

How to get keys from hash table?

We can use two methods to get the keys from the hash table.

• Use for− for each loop and access the keys by iterating through the hash table

• Retrieve the set of keys from the hash table and display them one by one.

Let's demonstrate both methods using examples.

Use a For-Each loop to get keys from the hash table

In this approach, we declare a hash table with key/value pairs. Then using a for-each loop we display the keys in the hash table.

To achieve this programmatically, we will use the following hash table.

{“US", "New York"}
{"FR", "Paris"}
{"UK", "London"}
{"IN", "Mumbai"}
{"GER", "Berlin"}

The above hash table uses the country code as the key and the value as its corresponding important city.

If the above hash table is called Citytable, then we use DictionaryEntry iterator to access its keys as follows -

foreach(DictionaryEntry ele1 in Citytable){
        Console.WriteLine("{0}", ele1.Key);

This code will display each key in the hash table.

Example 1

The complete work plan of this method is as follows -

using System;
using System.Collections;
class MyHashTable {
   // Main Method
   static public void Main() {
    
      // Create a hashtable instance
      Hashtable Citytable = new Hashtable();
      
      // Adding key/value pair in the hashtable using Add() method
      Citytable.Add("US", "New York");
      Citytable.Add("FR", "Paris");
      Citytable.Add("UK", "London");
      Citytable.Add("IN", "Mumbai");
      Citytable.Add("GER", "Berlin");
      
      //Print hashtable keys
      Console.WriteLine("Citytable Keys:");
      foreach(DictionaryEntry ele1 in Citytable){
         Console.WriteLine("{0}", ele1.Key);
      }      
   }
}

In this program, we declare and define a hash table named Citytable, which contains the country code and its important cities as key/value pairs respectively. After filling the hash table with values ​​using the Add() method, we start a for-each loop that iterates the Citytable using the iterator "ele1" of type DictionaryEntry. Then for each iteration, the key value is printed.

Output

Citytable Keys:
FR
US
IN
GER
UK

As we can see, only the keys for Citytable are shown in the output. This is the most common method as we mainly use for-each loop to display the contents of any collection or array.

We can slightly modify the above program so that it displays the value corresponding to a given key.

To do this we have to make the following changes in the iteration loop.

foreach(DictionaryEntry ele1 in Citytable){
   Console.WriteLine("{0} = {1}", ele1.Key, ele1.Value);

In the above code, we now also print the value part of the iteration counter.

Example 2

The complete procedure for this modification is as follows -

using System;
using System.Collections;
class MyHashTable {
   // Main Method
   static public void Main() {
    
      // Create a hashtable instance
      Hashtable Citytable = new Hashtable();
      
      // Adding key/value pair in the hashtable using Add() method
      Citytable.Add("US", "New York");
      Citytable.Add("FR", "Paris");
      Citytable.Add("UK", "London");
      Citytable.Add("IN", "Mumbai");
      Citytable.Add("GER", "Berlin");
      
      //Print hashtable keys
      Console.WriteLine("Citytable Keys:");
      foreach(DictionaryEntry ele1 in Citytable){
         Console.WriteLine("{0} = {1}", ele1.Key, ele1.Value);
      }      
   }
}

Output

Citytable Keys:
FR = Paris
US = New York
IN = Mumbai
GER = Berlin
UK = London

So, Here we show the key/value pairs of Citytable.

Let’s move on to the next method.

Use collections

In this method, we first get the collection of keys from the Hashtable using collection and then print the keys. Keys are printed as string values, which are nothing more than elements of the collection.

Let’s take a hash table in a programming language as an example.

{"CPP", "C++"}
{"CS", "C#"}
{"JAVA", "JAVA"}
{"PL", "PERL"}
{"JS", "JAVASCRIPT"}

We define the above table as a hash table, Langtable, in the program. We then declare an ICollection object "keys" and collect the hash table keys in that object.

ICollection keys = Langtable.Keys;

Once we have the collection of keys, we can easily iterate over the collection and display each key as a string.

foreach (String k in keys){
   Console.WriteLine(k);
}

This way we can display each key in the hash table individually.

Example

The complete procedure for this method is given here.

using System;
using System.Collections;
class MyHashTable {
   // Main Method
   static public void Main() {
    
      // Create a hashtable instance
      Hashtable Langtable = new Hashtable();
      
      // Adding key/value pair in the hashtable using Add() method
      Langtable.Add("CPP", "C++");
      Langtable.Add("CS", "C#");
      Langtable.Add("JAVA", "JAVA");
      Langtable.Add("PL", "PERL");
      Langtable.Add("JS", "JAVASCRIPT");
      
      //Collection of Hashtable keys
      ICollection keys = Langtable.Keys;

      Console.WriteLine("Language Keys:");
      foreach (String k in keys){
         Console.WriteLine(k);
      }
      Console.ReadKey();
   }
}

As already discussed, the above program contains a hash table of programming languages, with language codes as keys and their names as values. We first use an ICollection object to retrieve the collection of keys from the hash table. Then we iterate over this object and display the keys one by one.

Output

Language Keys:
PL
JS
CS
JAVA
CPP

So the program displays the language code (key) of the hash table. Likewise, if we want the value corresponding to each key, we can print it by accessing the hash table with the given key. We leave this modification to the reader.

Therefore, we can obtain (access) the hash table key from the hash table using either of the two methods discussed above. Although both methods are simple, we mainly use the first method, which uses a for-each loop to iterate the hash table for most collections.

In subsequent articles, we will discuss more Hashtable operations that can be performed in C#.

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