Relational DropDownList Using VB.NET Code Example. In this code, we illustrate three steps:
Step 1. Copy and paste the code below on to the newly created aspx file
Step 2. Add <appSettings> tag to the existing web.config
Step 3. Make sure you have Northwind database install in your system******************************

 <!-- ASPX Page --><!-- Step 1. Copy and paste the code below on to the newly created aspx file --><!-- Step 2. Add <appSettings> tag to the existing web.config --><!-- Step 3. Make sure you have Northwind database install in your system --><%@ Import Namespace="System.Data" %><%@ Import Namespace="System.Data.SqlClient" %>
<SCRIPT language=VB runat="server">

Public Sub Page_Load(Source As Object, E As EventArgs)

If Not IsPostBack Then

Dim _Conn As SqlConnection
Dim _Command As SqlCommand
Dim _Reader As SqlDataReader
Dim SQL As String

SQL = "select * from Region"

_Conn = GetConnection()

_Command = new SqlCommand(SQL, _Conn)

_Reader = _Command.ExecuteReader()

Regions.Items.Add(new ListItem("Select Region...", "0")) 

While _Reader.Read()
Regions.Items.Add(new ListItem(_Reader("RegionDescription").ToString(), _Reader("RegionID").ToString()))
End While

Territory.Items.Add(new ListItem("Select Territory...", "0"))

End If

End Sub

Function GetConnection() As SqlConnection
Dim _Connection As SqlConnection
Dim ConnStr As String

ConnStr = ConfigurationSettings.AppSettings("ConnectionString")

_Connection = new SqlConnection(ConnStr)
_Connection.Open()

Return _Connection
End Function 


Sub OnSelectedRegionIndexChanged(Source As Object, E As EventArgs)
Dim _Region As Integer

_Region = Regions.SelectedItem.Value

If _Region > 0 Then 

Dim _Conn As SqlConnection
Dim _Command As SqlCommand
Dim _Reader As SqlDataReader
Dim SQL As String

SQL = "select * from Territories where RegionID ='" & _Region & "'"

_Conn = GetConnection()

_Command = new SqlCommand(SQL, _Conn)

_Reader = _Command.ExecuteReader()

Territory.Items.Clear()
Territory.Items.Add(new ListItem("Select Territory...", "0"))


While _Reader.Read()
Territory.Items.Add(new ListItem(_Reader("TerritoryDescription").ToString(), _Reader("TerritoryID").ToString()))
End While

txtRegion.Text = Regions.SelectedItem.Text 
txtTerritory.Text = "" 
Else

Territory.Items.Clear()
Territory.Items.Add(new ListItem("Select Territory...", "0"))
txtRegion.Text = ""
txtTerritory.Text = ""

End If 

End Sub

Sub OnSelectedTerritoryIndexChanged(Source As Object, E As EventArgs)

Dim _Territory As Integer

_Territory = Territory.SelectedItem.Value

If _Territory > 0 Then

txtTerritory.Text = Territory.SelectedItem.Text
Else
txtTerritory.Text = ""
End If

End Sub

</SCRIPT>

<FORM runat="server"><asp:DropDownList id=Regions runat="server" 
OnSelectedIndexChanged="OnSelectedRegionIndexChanged" 
AutoPostBack="True"></asp:DropDownList>
<asp:DropDownList id=Territory runat="server" 
OnSelectedIndexChanged="OnSelectedTerritoryIndexChanged" 
AutoPostBack="True"></asp:DropDownList>
<asp:Label id=txtRegion runat="server"></asp:Label>
<asp:Label 
id=txtTerritory 
runat="server"></asp:Label></FORM><!-- Web.Config --><CONFIGURATION><APPSETTINGS><ADD 
value="Server=(local)NetSDK;uid=sa;pwd=;database=Northwind" 
key="ConnectionString" /></APPSETTINGS></CONFIGURATION>

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