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Overview

Hashtables have got to be the greatest thing since sliced bread.  Especially for VB programmers, they also are surpisingly under-utilized.  Every programmer should learn how to effectively use hashtables as they will likely prove useful in a wide variety of situations.

Hashtables are an extremely effecient way of storing key/value pairs.  The key can be an integer (similar to an array), but because of how Hashtables work, the key can actually be anything from a string to an object.

Let's assume for example that you had a class that represented a list of people, and you wanted to be able to access that list of people based on their social security number.  You could store them in an array (or array list) and then iterate over that list each time you wanted to access on of those people.  The two most pressing problems are that this type of access is both cumbersome and inefficient.

A common use of hashtables is to collect other classes.  For example, you might have a class the represents a person, and another class that represents a collection of people.  Hashtables can be used to reference the specific people, to allow you to very quickly access specific people based criteria like name or Social Security Number.

For Example

Let's say that you had  a class that represents a person.  This class has two public member variables (name & ssn (social security number)) as follows:

CSharp public class Person {     public Person(string name, string ssn)     {         this.Name = name;         this.SSN = ssn;     }     public string Name;     public string SSN; } VB Public Class Person     Public Sub New(ByVal name As String, ByVal ssn As String)         Me.Name = name         Me.SSN = ssn     End Sub     Public Name As String     Public SSN As String End Class

We could create a list of "people" by just creating an array of person as follows:

Person[] People;

We could also create a new class called people as follows.  In order to allow multiple people to be managed, we need a public method called Add that would take a Person object as a parameter.   We can then use an array list to store the actual person.

CSharp public class People     // Could also be called PersonCollection {   public People()   {      People = new ArrayList();   }   public ArrayList People;   public void Add(Person person) {     People.Add(person);   } } VB Public Class People ' Could also be called PersonCollection   Private Function People() As Public      People = New ArrayList()   End Function   Public People As ArrayList   Public Sub Add(ByVal person As Person)     People.Add(person)   End Sub End Class

Now, we can create objects of type person, and add them to the collection of people as follows:

CSharp // Create the people collection People people = new People(); // Create a person and add them to the collection Person ej = new Person("EJ Alexandra", "775-11-1234"); people.Add(ej) // Create another person and add them Person peter = new Person("Peter Jackson", "123-45-6789"); people.Add(peter); VB ' Create the people collection Dim people As People = New People() ' Create a person and add them to the collection Dim ej As Person = New Person("EJ Alexandra","775-11-1234") people.Add(ej) ' Create another person and add them Dim peter As Person = New Person("Peter Jackson","123-45-6789") people.Add(peter)

The problem is that without iterating over all of the people in the collection, how do you find 'ej' or the person with the social security number '775-11-1234'. 

Enter the Hashtable

This is where hashtables really start to shine.  Let's say that we not only used an array list to keep track of the people, but also used a Hashtable, who's key was based on name and ssn.   The "People" class would look like this:

CSharp public class People  // Could also be called PersonCollection {     public People()     {         m_byName = new Hashtable();         m_bySSN = new Hashtable();         m_byIndex = new ArrayList();     }       private Hashtable m_byName;     private Hashtable m_bySSN;     private ArrayList m_byIndex;     public Person ByName(string name)     {         return (Person)m_byName[name];     }     public Person BySS(string ssn)     {         return (Person)m_bySSN[ssn];     }     public void Add(Person person)     {         m_byName.Add(person.Name, person);         m_bySSN.Add(person.SSN, person);         m_byIndex.Add(person);     }     public void Add(string name, string ssn)     {         Person person = new Person(name, ssn);         Add(person);     } } VB Public Class People ' Could also be called PersonCollection     Private Function People() As Public         m_byName = New Hashtable()         m_bySSN = New Hashtable()         m_byIndex = New ArrayList()     End Function     Private m_byName As Hashtable     Private m_bySSN As Hashtable     Private m_byIndex As ArrayList     Public Function ByName(ByVal name As String) As Person         Return CType(m_byName(name), Person)     End Function     Public Function BySS(ByVal ssn As String) As Person         Return CType(m_bySSN(ssn), Person)     End Function     Public Sub Add(ByVal person As Person)         m_byName.Add(person.Name, person)         m_bySSN.Add(person.SSN, person)         m_byIndex.Add(person)     End Sub     Public Sub Add(ByVal name As String, ByVal ssn As String)         Dim person As Person = New Person(name,ssn)         Add(person)     End Sub End Class

Notice that in addition to the ArrayList that we had before, we also now have 2 hashtables.  m_byName and m_bySSN. 

When a new person is added, it adds them to the array list, and to the two hashtables.  The 1st parameter is the key (Name or SSN), and the 2nd parameter is the object that is being stored. 

This let's us also add 2 acces methods ByName and BySSN.  By just passing in a name or SSN, these access methods can very easily return the associated Person, by just passing that key into the appropriate Hashtable.  The code is very simple, easy to undersand and very efficient on the back end of things.

Final Steps

The final step that makes this class even more helpful, is to make the People class inherit from the IEnumerable class.  This will allow us to interate over the collection of people, just like any other collection as follows:

CSharp // We can iterate over each Person in the People collection as follows: foreach (Person person in people) {   // Print person to the screen   Response.Write(person.Name + " - " + person.SSN + "<br>"); } VB ' We can iterate over each Person in the People collection as follows: Dim person As Person For Each person In people   ' Print person to the screen   Response.Write(person.Name + " - " + person.SSN + "<br>") Next

For more information about foreach loops, see this article.  Using for and foreach with arrays, array...

In order for this to work, we need to inherit from the interface IEnumerable and provide a GetEnumerator() method.   people.Count returns how many people have been added.   The full people class would look like this:

CSharp // We can iterate over each Person in the People collection as follows: public class People : IEnumerable // Could also be called PersonCollection {     public People()     {         m_byName = new Hashtable();         m_bySSN = new Hashtable();         m_byIndex = new ArrayList();     }     private Hashtable m_byName;     private Hashtable m_bySSN;     private ArrayList m_byIndex;     public Person ByName(string name)     {         return (Person)m_byName[name];     }     public Person BySS(string ssn)     {         return (Person)m_bySSN[ssn];     }     public Person this[int index]     {         get         {             return (Person)m_byIndex[index];         }     }     public void Add(Person person)     {         m_byName.Add(person.Name, person);         m_bySSN.Add(person.SSN, person);         m_byIndex.Add(person);     }     public void Add(string name, string ssn)     {         Person person = new Person(name, ssn);         Add(person);     }     public int Count()     {         return m_byIndex.Count;     }     public IEnumerator GetEnumerator()     {         return m_byIndex.GetEnumerator();     } } VB ' We can iterate over each Person in the People collection as follows: Public Class People Implements IEnumerable ' Could also be called PersonCollection     Public Sub New()         m_byName = New Hashtable()         m_bySSN = New Hashtable()         m_byIndex = New ArrayList()     End Sub     Private m_byName As Hashtable     Private m_bySSN As Hashtable     Private m_byIndex As ArrayList     Public Function ByName(ByVal name As String) As Person         Return CType(m_byName(name), Person)     End Function     Public Function BySS(ByVal ssn As String) As Person         Return CType(m_bySSN(ssn), Person)     End Function     Default Public ReadOnly Property Item(index As Integer) As Person     Get             Return CType(m_byIndex(index), Person)     End Get     End Property     Public Sub Add(ByVal person As Person)         m_byName.Add(person.Name, person)         m_bySSN.Add(person.SSN, person)         m_byIndex.Add(person)     End Sub     Public Sub Add(ByVal name As String, ByVal ssn As String)         Dim person As Person = New Person(name,ssn)         Add(person)     End Sub     Public Function Count() As Integer         Return m_byIndex.Count     End Function     Public Function GetEnumerator() As IEnumerator         Return m_byIndex.GetEnumerator()     End Function End Class

Implementation example

This web page provides two textboxes (TextBox1 and TextBox2) that add pesons's to a collection of people which is stored in the Session object.  On pre-render, the collection of people is printed in a listbox.

default.aspx

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head runat="server">     <title>Untitled Page</title> </head> <body>     <form id="form1" runat="server">     <div>         Name:<br />         <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><br />         SSN:         <br />         <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>         <br />         <asp:Button ID="Button1" runat="server" Text="Add Person" OnClick="Button1_Click" /><br />         <br />         <br />         People:<br />         <asp:ListBox ID="ListBox1" runat="server" Height="221px"             Width="169px"></asp:ListBox></div>     </form> </body> </html>

default.aspx.cs/vb

CSharp using System; using System.Data; using System.Configuration; using System.Web; using System.Web.Security; using System.Web.UI; using System.Web.UI.WebControls; using System.Web.UI.WebControls.WebParts; using System.Web.UI.HtmlControls; public partial class _Default : System.Web.UI.Page {     private People people;     protected void Page_Load(object sender, EventArgs e)     {         if (Session["people"] == null) Session["people"] = new People();         people = (People)Session["people"];     }     protected override void OnPreRender(EventArgs e)     {         base.OnPreRender(e);         ListBox1.Items.Clear();         foreach (Person person in people)         {             ListBox1.Items.Add(String.Format("{0} - {1}",                 person.Name, person.SSN));         }     }     protected void Button1_Click(object sender, EventArgs e)     {         people.Add(TextBox1.Text, TextBox2.Text);     } } VB Imports System Imports System.Data Imports System.Configuration Imports System.Web Imports System.Web.Security Imports System.Web.UI Imports System.Web.UI.WebControls Imports System.Web.UI.WebControls.WebParts Imports System.Web.UI.HtmlControls Public partial Class _Default Inherits System.Web.UI.Page     Private people As People     Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)         If Session("people") Is Nothing Then         Session("people") = New People()         End If         people = CType(Session("people"), People)     End Sub     Protected Overrides Sub OnPreRender(ByVal e As EventArgs)         MyBase.OnPreRender(e)         ListBox1.Items.Clear()         Dim person As Person         For Each person In people             ListBox1.Items.Add(String.Format("{0} - {1}",                 person.Name, person.SSN))         Next     End Sub     Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)         people.Add(TextBox1.Text, TextBox2.Text)     End Sub End Class

Exceptions & Error Checking with Hashtables

This code does not do any error checking.  Some specific things that would need to be checked have to do with how Hashtables handle duplicate keys.  Specifically, they are not accepted.  What this means is that before adding a key/value pair to  a hashtable, you should really check that they key does not already exist.  If you do no, you will likely get an error like the following.

Server Error in '/WebSite1' Application.

---

Item has already been added. Key in dictionary: 'xxx'  Key being added: 'xxx'

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: Item has already been added. Key in dictionary: 'xxx'  Key being added: 'xx'

The way to avoid this error is to check that they key does not exist before adding it as follows:

CSharp  public void Add(Person person)  {    // Check that the name is not already used as a key, add if so    if (!m_byName.ContainsKey(person.Name)) m_byName.Add(person.Name, person);    // Check that the key is not already used as a key, add if so    if (!m_bySSN.ContainsKey(person.SSN)) m_bySSN.Add(person.SSN, person);    // Always add the person to the array list...    m_byIndex.Add(person);  } VB  Public Sub Add(ByVal person As Person)    ' Check that the name is not already used as a key, add if so    If Not m_byName.ContainsKey(person.Name) Then    m_byName.Add(person.Name, person)    End If    ' Check that the key is not already used as a key, add if so    If Not m_bySSN.ContainsKey(person.SSN) Then    m_bySSN.Add(person.SSN, person)    End If    ' Always add the person to the array list...    m_byIndex.Add(person)  End Sub

Also, the reason that I always include an array list in addition to any Hashtables that I use is that order is not preserved with a hashtable.  In other words, I could add keys 1, 2 and 3 and if I access the hashtable by index, it might return 3, 1 and 2.  The array list on the other hand will always return the elements in the order in which they were added.  This can be important if trying to preserve a specific order.

Conclusion

In closing, I think you can see that hashtables are extraordinarly powerful.  They should be used where appropriate and will make a powerful tool in your toolbox under the right circumstances.  Good luck.