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The non-generic Hashtable collection class stores a collection of key-value pairs, where key and value are objects.

The Hashtable uses a mechanism of hashing to store each key-value pair and to perform an association between each key-value pair. In hashing, a unique hash code of each key is computed internally and this value of hash code is used as an index at which its associated value is stored in the Hashtable.

Hashtable collection class implements IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, and ICloneable interfaces and it is defined in the System.Collections namespace.

Some important features of Hashtable

• A null cannot be stored as a key.
• A null can be stored as a value.
• Hashtable elements are ordered on the basis of hash code value of each key(which is computed internally) & hence the order of key-value pairs may appear different as compared to their insertion order.
• Hashtable allows the execution time of retrieving an element, lookup and set operations to remain near constant, even for large sets.

Some commonly used constructors of Hashtable

Constructor	Description
Hashtable()	This constructor creates an empty instance of the Hashtable class by using the default initial capacity, hash code provider, and comparer. The capacity of a Hashtable is the number of elements it can hold, as the new elements are added to the Hashtable, its capacity is automatically increased.
Hashtable(int capacity)	This constructor initializes creates a Hashtable that is empty and has the specified initial capacity.

Properties of Hashtable:

Properties	Description
Capacity	This property is used to get or set the total number of elements a Hashtable can hold.
Count	This property gives total number of elements in a Hashtable.
IsFixedSize	This property determines whether a Hashtable has a fixed size or not.
IsReadOnly	This property determines whether a Hashtable is read-only or not.
IsSynchronzied	This property determines whether a Hashtable is thread-safe or not.
Item[Object key]	This property gets or sets a value associated with the specified key in a Hashtable. To use this property, you should not type Item, but you only need to specify the specific key within a pair of square brackets [ ]
Keys	This property gets an ICollection containing the keys of a Hashtable.
Values	This property gets an ICollection containing the values of a Hashtable.
comparer	This property gets or sets IComparer to use for a Hashtable.

Some important methods of Hashtable.

Methods	Description
int Add(Object key, Object value)	This method adds the specified key-value pair in Hashtable.
bool Contains(Object key)	This method checks if Hashtable contains a specific key.
bool ContainsKey(Object key)	This method checks if Hashtable contains a specific key.
bool ContainsValue(Object key)	This method checks if Hashtable contains a specific value.
void Clear()	This method removes all of the elements from the Hashtable.
int GetHash(Object key)	This method returns the hash code for the specified key.
IDictionaryEnumerator GetEnumerator()	This method returns to enumerate the entire Hashtable.
void Remove(Object key)	This method removes the element with the specified key in the Hashtable.

Hashtable example

In the upcoming example, we will create a Hashtable to store objects in the form of key-value pair of objects, where:
• Each key is a String object.
• Each key's associated value is an int and because C# has a unified type system, so any primitive values like int, char, float, etc are implicitly boxed into an Object references.

We will iterate over all the key-value pairs and we will also obtain a value on the basis its key.

//C# Example of Hashtable

using System;
using System.Collections;

class HashtableDemo
{
public static void Main(String[] ar)
{
	//Creating a Hashtable
	Hashtable hm = new Hashtable();

	//Adding key-value pairs by the Add() method
	hm.Add("Max", 1000);
	hm.Add("John", 4000);
	hm.Add("Tom", 2000);
	hm.Add("Ana", 6000);
	hm.Add("Rick", 5000);

	//Getting a collection of the keys using property Keys
	ICollection c = hm.Keys;
	
	// Use the keys to obtain the values.
	foreach(String str in c)
		Console.WriteLine(str + ": " + hm[str]);

	//Using the property Item to retrieve the value of key "Tom"
	//Remember the property Item is not used with its name
	Console.WriteLine("Value at the key, Tom is "+ hm["Tom"]);

	//Using the property Item to retrieve the value of key "Ana"
	Console.WriteLine("Value at the key, Ana is "+ hm["Ana"]);
}
}

Output is :

John: 4000
Max: 1000
Tom: 2000
Ana: 6000
Rick: 5000
Value at the key, Tom is 2000
Value at the key, Ana is 6000

Program Analysis

• In the code above, we have created a non-generic Hashtable to store objects, where each key is a String and its associated value is an int and because C# has a unified type system, so any primitive values like int, char, float are implicitly boxed into an Object references.
• The method Add() is called to add the values of Key-Value pair by specifying the Key and its Value.
• Next, we have retrieved a collection of the keys stored in the Hashtable, by using its property Keys, and using which, we have iterated over each value stored in the Hashtable.
• Next we have used the Item[key] property of the Hashtable(which is used without specifying the word Item), to obtain a value associated with a particular key in a Hashtable.

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Checking if a Hashtable contains a specific key or a specific value

In the next example of Hashtable, we will perform two operations:
• We will determine if the Hashtable contains a specific key by using the methods, Contains(Object), ContainsKey(Object).
• We will determine if the Hashtable contains a specific value by using the method, ContainsValue(Object).




//C# Example of Hashtable

using System;
using System.Collections;

class HashtableDemo
{
public static void Main(String[] ar)
{
	//Creating a Hashtable
	Hashtable hm = new Hashtable();


	//Adding key-value pairs by the Add() method
	hm.Add("Max", 1000);
	hm.Add("John", 4000);
	hm.Add("Tom", 2000);
	hm.Add("Ana", 6000);
	hm.Add("Rick", 5000);


	//Getting a collection of the keys using property Keys
	ICollection c = hm.Keys;
	

	// Use the keys to obtain the values.
	Console.WriteLine("The contents of Hashtable:");
	foreach(String str in c)
		Console.WriteLine(str + ": " + hm[str]);


	//Calling the method Contains() to see if Hashtable contains key "Tom"
	Console.WriteLine("Does our Hashtable contains the key, Tom: "+ hm.Contains("Tom"));


	//Calling the method ContainsKey() to see if Hashtable contains key "Ana"
	Console.WriteLine("Does our Hashtable contains the key, Tom: "+ hm.ContainsKey("Ana"));


	//Calling the method ContainsKey() to see if Hashtable contains key "Jack"
	Console.WriteLine("Does our Hashtable contains the key, Jack: "+ hm.ContainsKey("Jack"));


	//Calling the method ContainsValue() to see if Hashtable contains value 4000
	Console.WriteLine("Does our Hashtable contains the value, 4000: "+ hm.ContainsValue(4000));


	//Calling the method ContainsValue() to see if Hashtable contains value 10000
	Console.WriteLine("Does our Hashtable contains the value, 10000: "+ hm.ContainsValue(10000));
}
}





Output is :

The contents of Hashtable:
John: 4000
Max: 1000
Tom: 2000
Ana: 6000
Rick: 5000
Does our Hashtable contains the key, Tom: True
Does our Hashtable contains the key, Tom: True
Does our Hashtable contains the key, Jack: False
Does our Hashtable contains the value, 4000: True
Does our Hashtable contains the value, 10000: False








Removing elements from a Hashtable



In the upcoming example of Hashtable, we will call the Remove(Object) method to remove the elements of a Hashtable. This method is passed a specific key of Hashtable, and if its exits, the method removes the key and its associated value i.e. removes the whole key-value pair from the Hashtable.

For this, we will create a Hashtable to store objects in the form of key-value pair of objects, where:
• Each key is a char value, and because C# has a unified type system, so any primitive values like int, char, float, etc are implicitly boxed into an Object references.
• Each value is a String object.




//C# Example of Hashtable

using System;
using System.Collections;

class HashtableDemo
{
public static void Main(String[] ar)
{
	//Creating a Hashtable
	Hashtable hm = new Hashtable();


	//Adding key-value pairs by the Add() method
	//Where, each key is a character and each value is a String.
	hm.Add('B', "Banana");
	hm.Add('M', "Mango");
	hm.Add('G', "Grapes");
	hm.Add('A', "Apple");
	hm.Add('O',"Orange");


	//Getting a collection of the keys using property Keys
	ICollection c = hm.Keys;
	

	// Use the keys to obtain the values.
	Console.WriteLine("The contents of Hashtable:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + hm[ch]);


	//Calling the method Remove() to remove a key 'A' and its associated value
	hm.Remove('A');

	//Calling the method Remove() to remove a key 'M' and its associated value
	hm.Remove('M');

	//Printing all elements of Hashtable
	Console.WriteLine("The updated contents of Hashtable after removing keys 'A' and 'M' and their values:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + hm[ch]);
}
}





Output is :

The contents of Hashtable:
A: Apple
G: Grapes
B: Banana
O: Orange
M: Mango
The updated contents of Hashtable after removing keys 'A' and 'M' and their values:
G: Grapes
B: Banana
O: Orange








A key in Hashtable cannot be null



In this example of Hashtable, we will show you how inserting a null value for a key, throws an exception.



//C# Key cannot be null

using System;
using System.Collections;

class HashtableDemo
{
public static void Main(String[] ar)
{
	//Creating a Hashtable
	Hashtable hm = new Hashtable();


	//Adding key-value pairs by the Add() method
	//Where, each key is a character and each value is a String.
	hm.Add('B', "Banana");
	hm.Add('M', "Mango");
	hm.Add('G', "Grapes");
	hm.Add('A', "Apple");
	hm.Add('O', "Orange");
	hm.Add(null, "no value");	//Trying to store a null as a key
}
}





Output

Unhandled Exception: System.ArgumentNullException: Key cannot be null.
Parameter name: key
   at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)
   at HashtableDemo.Main(String[] ar)


As you can see, the program has thrown a ArgumentNullException because Key cannot be null.






A value in a Hashtable can be null



In this example of Hashtable, we will show you how inserting a null for a value is completely allowed and doesn't throws an exception.



//C# A value can  be null in a Hashtable

using System;
using System.Collections;

class HashtableDemo
{
public static void Main(String[] ar)
{
	//Creating a Hashtable
	Hashtable hm = new Hashtable();


	//Adding key-value pairs by the Add() method
	//Where, each key is a character and each value is a String.
	hm.Add('B', "Banana");
	hm.Add('M', "Mango");
	hm.Add('G', "Grapes");
	hm.Add('A', "Apple");
	hm.Add('O', "Orange");
	hm.Add('N', null);	//Trying to store a null as a key


	//Getting a collection of the keys using property Keys
	ICollection c = hm.Keys;
	

	//Printing the key-value pairs by using the collection of keys to obtain their values.
	Console.WriteLine("The contents of Hashtable:");
	foreach(char ch in c)
		Console.WriteLine(ch + ": " + hm[ch]);

}
}





Output

The contents of Hashtable:
B: Banana
A: Apple
G: Grapes
O: Orange
N:
M: Mango


As you can see, the program has not thown any exception even when we have specified a null as a value for a key N.





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