Hashtable in Asp.Net C# and Vb.net

The Asp.Net Hashtable object contains elements in a pair of key and value. Ideally, we use the Hashtable to quickly search for elements. Each key serves as an Index for a value. An inbuilt Hash Function creates a hash code (or index) of the keys stored in the Hashtable.

The Hashtable Constructor

We can create an empty Hashtable by using a constructor. A constructor method starts with the "New" keyword, which initializes an object of a class. In this case, the Hashtable itself is the class and we need to initialize it first.

Code Behind (C#)

Hashtable htBooks = new Hashtable();

Vb.Net

Dim htBooks As New Hashtable

Once initialized, we can now start adding values to the Hashtable, each value attached with a unique key.

Note: You cannot add duplicate keys to the Hashtable. In addition, a key cannot be null. However, it will accept duplicate or null values in the Hashtable.

Method Add()

Use the add() method to add elements with key and value to the Hashtable. As a small demo, we will add a <div> element to our project to display the added elements.

using System;
using System.Collections;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Hashtable htBooks = new Hashtable();

        htBooks.Add(1, "Computer Architecture");
        htBooks.Add(2, "Advanced Composite Materials");
        htBooks.Add(3, "Asp.Net 4 Blue Book");

        divBooks.InnerHtml = "Showing a list of Books <br />";

        foreach (DictionaryEntry DE in htBooks)
        {
            divBooks.InnerHtml = divBooks.InnerText + "<br /> " + DE.Key + " - " + DE.Value;
        }
    }
}

Vb.Net

Partial Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load

        Dim htBooks As New Hashtable

        htBooks.Add(1, "Computer Architecture")
        htBooks.Add(2, "Advanced Composite Materials")
        htBooks.Add(3, "Asp.Net 4 Blue Book")

        divBooks.InnerHtml = "Showing a list of Books <br />"

        For Each DE As DictionaryEntry In htBooks
            divBooks.InnerHtml = divBooks.InnerText & "<br /> " & DE.Key & " - " & DE.Value
        Next
    End Sub
End Class

Output

3 – Asp.Net 4 Blue Book
2 – Advanced Composite Materials
1 – Computer Architecture

It displays the elements using LIFO (Last In First Out) method.

To retrieve the values from the Hashtable, we will use the DictionaryEntry type. We will initialize the type and create an object, which actually stores the pair (key, value). Finally loop through the object using foreach statement (For Each in Vb.Net) to retrieve the key and value pair, and display the list of books in the DIV element.

Method Remove()

Use the Remove() method to remove specified keys from the Hashtable list. It takes a single parameter “Key as Object”. You will need to specify the key inside the brackets. In our example, we will remove the Book with the key BUSI (Business) from the Hashtable and add a new book called the “The Final Frontier” in the “SPA” (Space) category.

Code Behind (C#)

using System;
using System.Collections;

public partial class _Default : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {
        Hashtable htBooks = new Hashtable();

        htBooks.Add("CMP", "Computer Architecture");
        htBooks.Add("MSc", "Advanced Composite Materials");
        htBooks.Add("BUSI", "Challenging Times");

        // SHOW ORIGINAL LIST BEFORE APPLYING Remove().
        foreach (DictionaryEntry DE in htBooks)
        {
            divBooks.InnerHtml = divBooks.InnerText + "<br /> " + DE.Key + " - " + DE.Value;
        }

        htBooks.Remove("BUSI");

        divBooks.InnerHtml = divBooks.InnerText + "<br /> <br /> " + " Book <b>Challenging Times</b> Removed from Hashtable <br /> ";
        htBooks.Add("SPA", "The Final Frontier");
        
        foreach (DictionaryEntry DE in htBooks)
        {
            divBooks.InnerHtml = divBooks.InnerText + "<br /> " + DE.Key + " - " + DE.Value;
        }

        divBooks.InnerHtml = divBooks.InnerText + "<br /> <br /> " + " Book <b>The Final Frontier</b> Added to Hashtable ";
    }
}

Vb.Net

Partial Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        Dim htBooks As New Hashtable

        htBooks.Add("CMP", "Computer Architecture")
        htBooks.Add("MSc", "Advanced Composite Materials")
        htBooks.Add("BUSI", "Challenging Times")

        ' SHOW ORIGINAL LIST BEFORE APPLYING Remove().
        For Each DE As DictionaryEntry In htBooks
            divBooks.InnerHtml = divBooks.InnerText & "<br /> " & DE.Key & " - " & DE.Value
        Next

        htBooks.Remove("BUSI")

        divBooks.InnerHtml = divBooks.InnerText & "<br /> <br /> " & " Book <b>Challenging Times</b> Removed from Hashtable <br /> "
        htBooks.Add("SPA", "The Final Frontier")

        For Each DE As DictionaryEntry In htBooks
            divBooks.InnerHtml = divBooks.InnerText & "<br /> " & DE.Key & " - " & DE.Value
        Next

        divBooks.InnerHtml = divBooks.InnerText & "<br /> <br /> " & " Book <b>The Final Frontier</b> Added to Hashtable "
    End Sub
End Class

Property "Count"

Normally, a Hashtable will have a big list of elements. To get the exact count of elements, we can use the Count property.

for (int i = 0; i <= htBooks.Count; i++)
{
    divBooks.InnerHtml = divBooks.InnerText + "<br /> " + htBooks[i] ;
}

Vb.Net

For i = 0 To htBooks.Count
    divBooks.InnerHtml = divBooks.InnerText & "<br /> " & htBooks(i)
Next

Method clear()

The clear() method will remove all the elements from the Hashtable. However, we can re initialize the Hashtable to make way for a fresh list of elements.

protected void Page_Load(object sender, EventArgs e)
{
    Hashtable htBooks = new Hashtable();

    htBooks.Add(1, "Computer Architecture");
    htBooks.Add(2, "Advanced Composite Materials");
    htBooks.Add(3, "Asp.Net 4 Blue Book");

    divBooks.InnerHtml = "Hashtable elements before clear(): " + htBooks.Count;
    htBooks.Clear(); // CLEAR THE HASHTABLE.
    divBooks.InnerHtml = divBooks.InnerHtml + "<br /> Hashtable elements after clear(): " + htBooks.Count;
}

Vb.Net

Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load

    Dim htBooks As New Hashtable

    htBooks.Add(1, "Computer Architecture")
    htBooks.Add(2, "Advanced Composite Materials")
    htBooks.Add(3, "Asp.Net 4 Blue Book")

    divBooks.InnerHtml = "Hashtable elements before clear(): " & htBooks.Count
    htBooks.Clear()     ' CLEAR THE HASHTABLE.
    divBooks.InnerHtml = divBooks.InnerHtml & "<br /> Hashtable elements after clear(): " & htBooks.Count
End Sub

Method Contains()

The Contains() method returns a boolean (true or false) value if the specified key is found in the Hashtable.

The example below uses the Contains() method to check if the keys CMP and PHY (Computer and Physics) exists in the Hashtable.

Hashtable htBooks = new Hashtable();

htBooks.Add("CMP", "Computer Architecture");
htBooks.Add("MSc", "Advanced Composite Materials");
htBooks.Add("BUS", "Challenging Times");

divBooks.InnerHtml = htBooks.Contains("CMP").ToString();
divBooks.InnerHtml = divBooks.InnerHtml + "<br /> " + htBooks.Contains("PHY").ToString();

Vb.Net

Dim htBooks As New Hashtable

htBooks.Add("CMP", "Computer Architecture")
htBooks.Add("MSc", "Advanced Composite Materials")
htBooks.Add("BUS", "Challenging Times")

divBooks.InnerHtml = htBooks.Contains("CMP")
divBooks.InnerHtml = divBooks.InnerHtml & "<br /> " & htBooks.Contains("PHY")

Output

True
False

Optionally, you can use a similar function in this category, also called the “ContainsKey()”. This method does exactly what the “Contains()” method did, in the above example.

htBooks.ContainsKey("PHY").toString();

Method ContainsValue()

The ContainsValue() method returns a boolean (true or false) value if the specified value is found in the Hashtable.

divBooks.InnerHtml = htBooks.ContainsValue("Challenging Times").ToString();

Vb.Net

divBooks.InnerHtml = htBooks.ContainsValue("Challenging Times")

Property "Keys"

We can display all the listed keys in a Hashtable using the property keys. The keys can either be an integer or string value. Depending upon the keys type, we can use the foreach loop (For Each in Vb.Net) to extract all the keys and display it.

Hashtable htBooks = new Hashtable();

htBooks.Add("CMP", "Computer Architecture");
htBooks.Add("MSc", "Advanced Composite Materials");
htBooks.Add("BUSI", "Challenging Times");

foreach (string key in htBooks.Keys)
{
    divBooks.InnerHtml = divBooks.InnerHtml + "<br /> " + key;
}

Vb.Net

Dim htBooks As New Hashtable

htBooks.Add("CMP", "Computer Architecture")
htBooks.Add("MSc", "Advanced Composite Materials")
htBooks.Add("BUSI", "Challenging Times")

For Each key As String In htBooks.Keys
    divBooks.InnerHtml = divBooks.InnerHtml & "<br /> " & key
Next

Output

CMP
Msc
BUSI

Property "Values"

We have checked the list of keys in the above example; similarly, we can extract all the values from the Hashtable using the Values property.

Hashtable htBooks = new Hashtable();

htBooks.Add("CMP", "Computer Architecture");
htBooks.Add("MSc", "Advanced Composite Materials");
htBooks.Add("BUS", "Challenging Times");

foreach (string val in htBooks.Values)
{
    divBooks.InnerHtml = divBooks.InnerHtml + "<br /> " + val;
}

Vb.Net

Dim htBooks As New Hashtable

htBooks.Add("CMP", "Computer Architecture")
htBooks.Add("MSc", "Advanced Composite Materials")
htBooks.Add("BUSI", "Challenging Times")

For Each val As String In htBooks.Values
    divBooks.InnerHtml = divBooks.InnerHtml & "<br /> " & val
Next

Output

Computer Architecture
Advanced Composite Materials
Challenging Times

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