- Article
This article introduces how to use the HashTable
collection in Visual C#.
Original product version: Visual C#
Original KB number: 309357
Summary
Because hashing eliminates the need for costly searching of data to retrieve the data, you can use hashing to efficiently retrieve data. Hashing uses the value of the key itself to locate of the data.
The Base Class Libraries offer a HashTable
class that is defined in the System.Collections
namespace so that you are not required to code your own hash tables.
Steps to build the sample
A HashTable
collection stores a (Key
, Value
) pair and uses the Key
to hash and obtain the storage location. The Key
is immutable and cannot have duplicate entries in the HashTable
. This sample uses several instances of a simple Person
class to store in a HashTable
. The last name is used as the Key
.
Open Microsoft Visual Studio, and create a Windows Forms Application project in Visual C#. Form1 is added to the project by default.
In Solution Explorer, right-click the project name, point to Add, and then select Class to add a class module.
Class1
is added to the project by default.Replace any code in the
Class1
module with the following code:public class Person{ public string Fname, Lname;d public Person (string FirstName, string LastName) { Fname = FirstName; Lname = LastName; } public override string ToString () { return Fname + " " + Lname; }}
The
Person
class has one constructor that takes theFirstName
andLastName
parameters and assigns these parameters to the local variables. TheToString
function overridesToString
from theObject
class to returnFname
andLname
concatenated together.Create a form-level
Hashtable
object, and declare three variables of typePerson
. Add the following code to theForm1
class:<?xm-deletion_mark author="v-bobbid" time="20080711T172143-0800"data="private Hashtable MyTable = new Hashtable();//For simplicity, create three Person objects to add to the HashTable collection.Person Person1,Person2,Person3; "?><?xm-insertion_mark_start author="v-bobbid" time="20080711T172143-0800"?>System.Collections.Hashtable MyTable = newSystem.Collections.Hashtable();//For simplicity, create three Person objects to add to the HashTable collection.Person Person1,Person2,Person3;<?xm-insertion_mark_end?>
In the following steps, use the
Add
method of theHashtable
object to add threePerson
objects to theHashtable
in atry-catch
block. Thetry-catch
block catches the exception and displays a message if duplicate keys exist:Place a Button control on Form1, and change the Text property to Add Items.
Double-click the button to open its Code window, and paste the following code in the
Button1_Click
event:Person1 = new Person("David", "Burris");Person2 = new Person("Johnny", "Carrol");Person3 = new Person("Ji", "Jihuang");//The Add method takes Key as the first parameter and Value as the second parameter.try{ MyTable.Add(Person1.Lname, Person1); MyTable.Add(Person2.Lname, Person2); MyTable.Add(Person3.Lname, Person3);}catch (ArgumentException ae){ MessageBox.Show("Duplicate Key"); MessageBox.Show(ae.Message);}
The
Hashtable
object provides an indexer. In the following steps, index with theKey
to access the value that is stored at the hashed location:Add a Button control to Form1, and change the Name property to Get Items.
Double-click the button, and paste the following code in the
Button2_Click
event://Use the indexer of the Hashtable class to retrieve your objects. The indexer takes//Key as a parameter and accesses it with the Hashed location.try{ MessageBox.Show(MyTable[Person1.Lname].ToString()); MessageBox.Show(MyTable[Person2.Lname].ToString()); MessageBox.Show(MyTable[Person3.Lname].ToString());}catch (NullReferenceException ex){ MessageBox.Show("Key not in Hashtable"); MessageBox.Show(ex.Message);}
In the following steps, use the
Remove
method to remove a single item from theHashTable
collection:Add a Button control to Form1, and change the Text property to Remove Item.
Double-click the button, and paste the following code in the
Button3_Click
event:<?xm-deletion_mark author="v-bobbid" time="20080711T173011-0800" data="if (MyTable.Count == 0){ MessageBox.Show("There are no items in HashTable");}else{ MessageBox.Show("The count before removing an Item is" + " " + MyTable.Count); MessageBox.Show("Removing value stored at key value (Burris)"); Remove the object that is stored at the Key value Person1.Lname. MyTable.Remove(Person1.Lname);}"?><?xm-insertion_mark_start author="v-bobbid" time="20080711T173011-0800"?>if (MyTable.Count == 0){ MessageBox.Show("There are no items in HashTable");}else{ MessageBox.Show("The count before removing an Item is" + " " + MyTable.Count); MessageBox.Show("Removing value stored at key value (Burris)"); // Remove the object that is stored at the Key value Person1.Lname. MyTable.Remove(Person1.Lname);}<?xm-insertion_mark_end?>
In the following steps, enumerate the items that are stored in the
HashTable
collection:Add a Button control to Form1, and change the Text property to Enumerate.
Double-click the button, and paste the following code in the
Button4_Click
event:<?xm-deletion_mark author="v-bobbid" time="20080711T174252-0800"data="IDictionaryEnumerator Enumerator;if (MyTable.Count == 0)MessageBox.Show("The hashtable is empty");else{ MessageBox.Show("Enumerating through the Hashtable collection"); Enumerator = MyTable.GetEnumerator(); while (Enumerator.MoveNext()) { MessageBox.Show(Enumerator.Value.ToString()); }}ICollection MyKeys;if (MyTable.Count == 0) MessageBox.Show("The hashtable is empty");else{ MessageBox.Show("Accessing keys property to return keys collection"); MyKeys = MyTable.Keys; foreach (object Key in MyKeys) { MessageBox.Show(Key.ToString()); }}"?><?xm-insertion_mark_start author="v-bobbid" time="20080711T174252-0800"?>System.Collections.IDictionaryEnumerator Enumerator;if (MyTable.Count == 0) MessageBox.Show("The hashtable is empty");else{ MessageBox.Show("Enumerating through the Hashtable collection"); Enumerator = MyTable.GetEnumerator(); while (Enumerator.MoveNext()) { MessageBox.Show(Enumerator.Value.ToString()); }}System.Collections.ICollection MyKeys;if (MyTable.Count == 0) MessageBox.Show("The hashtable is empty");else{ MessageBox.Show("Accessing keys property to return keys collection"); MyKeys = MyTable.Keys; foreach (object Key in MyKeys) { MessageBox.Show(Key.ToString()); }}<?xm-insertion_mark_end?>
This code declares a variable of type
IDictionaryEnumerator
and calls theGetEnumerator
method of theHashTable
collection. With theEnumerator
returned, the code enumerates through the items in the collection and uses theKeys
method of theHashTable
to enumerate through the keys.
In the following steps, use the
Clear
method to clear theHashTable
:Add a Button control to Form1, and change the Text property to Clear.
Double-click the button, and paste the following code in the
Button5_Click
event:MyTable.Clear();MessageBox.Show("HashTable is now empty");
Follow these steps to build and run the application:
- Select Add Items. Three
Person
objects are added to theHashTable
collection. - Select Get Items. The indexer obtains the items in the
HashTable
collection. The three newly added items are displayed. - Select Remove Item. The item at the
Burris
key location is deleted. - Select Enumerate.
IDictionaryEnumerator
enumerates through the items in theHashTable
collection, and theKeys
property of theHashTable
returns a Keys Collection. - Select Clear. All the items are cleared from the
HashTable
collection.
- Select Add Items. Three
Note
The example companies, organizations, products, domain names, e-mail addresses, logos, people, places, and events depicted herein are fictitious. No association with any real company, organization, product, domain name, email address, logo, person, places, or events is intended or should be inferred.