In this article I will explain with an example, how to populate TextBox based on DropDownList selection in ASP.Net using C# and VB.Net.
The selected value of the DropDownList will be displayed in TextBox and Label controls.
 
 
Database
Here I am making use of Microsoft’s Northwind Database. The download and install instructions are provided in the following article.
 
Namespaces
You will need to import the following namespaces.

C#
using System.Data;
using System.Data.SqlClient;
using System.Configuration; 
 
VB.Net
Imports System.Data
Imports System.Data.SqlClient
Imports System.Configuration
 
HTML Markup
The following HTML Markup consists of an ASP.Net DropDownList, a Label and a TextBox control. 
The DropDownList has been assigned OnSelectedIndexChanged event and also the AutoPostBackproperty is set to True.
<form id="form1" runat="server">
<div>
    Select Customer ID
    <asp:DropDownList ID="ddlCustomers" runat="server" AutoPostBack = "true"
          OnSelectedIndexChanged="ddlCustomers_SelectedIndexChanged">
    </asp:DropDownList>
    <br />
    <br />
    <br />
    <p>---Customer Details---</p>
    <hr />
    City :
    <asp:TextBox ID="txtCity" runat="server"></asp:TextBox>
    <br />
    Country :
    <asp:Label ID="lblCountry" runat="server" Text=""></asp:Label>
</div>
</form>
 
Populating DropDownList from Database
Inside the Page Load event of the page, the DropDownList is populated with records from the Customers table of the Northwind database.    
A Select Query is executed and it gets the CustomerID and the ContactName fields. The CustomerID field is assigned to the DataValueField property of the DropDownList and the ContactName field is assigned to the DataTextField property.

C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ddlCustomers.Items.Add(new ListItem("--Select Customer--", ""));
        ddlCustomers.AppendDataBoundItems = true;
        String strConnString = ConfigurationManager
             .ConnectionStrings["conString"].ConnectionString;
        String strQuery = "select CustomerID, ContactName from Customers";
        SqlConnection con = new SqlConnection(strConnString);
        SqlCommand cmd = new SqlCommand();
        cmd.CommandType = CommandType.Text;
        cmd.CommandText = strQuery;
        cmd.Connection = con;
        try
        {
            con.Open();
            ddlCustomers.DataSource = cmd.ExecuteReader();
            ddlCustomers.DataTextField = "ContactName";
            ddlCustomers.DataValueField = "CustomerID";
            ddlCustomers.DataBind();
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            con.Close();
            con.Dispose();
        }
    }
}        
 
VB.Net
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
 If Not IsPostBack Then
   ddlCustomers.Items.Add(New ListItem("--Select Customer--", ""))
   ddlCustomers.AppendDataBoundItems = True
   Dim strConnString As String = ConfigurationManager _
                     .ConnectionStrings("conString").ConnectionString
   Dim strQuery As String = "select CustomerID, ContactName from Customers"
   Dim con As New SqlConnection(strConnString)
   Dim cmd As New SqlCommand()
   cmd.CommandType = CommandType.Text
   cmd.CommandText = strQuery
   cmd.Connection = con
   Try
     con.Open()
     ddlCustomers.DataSource = cmd.ExecuteReader()
     ddlCustomers.DataTextField = "ContactName"
     ddlCustomers.DataValueField = "CustomerID"
     ddlCustomers.DataBind()
   Catch ex As Exception
     Throw ex
   Finally
     con.Close()
     con.Dispose()
   End Try
  End If
End Sub
 
Populating TextBox and Label controls on DropDownList selection
Inside the OnSelectedIndexChanged event handler, the City and Country fields are fetched based on the selected CustomerID value.
The fetched values are then displayed in the TextBox and Label controls respectively.

C#
protected void ddlCustomers_SelectedIndexChanged(object sender, EventArgs e)
{
    String strConnString = ConfigurationManager
                           .ConnectionStrings["conString"].ConnectionString;
    String strQuery = "select City, Country from Customers where" +
                      " CustomerID = @CustomerID";
    SqlConnection con = new SqlConnection(strConnString);
    SqlCommand cmd = new SqlCommand();
    cmd.Parameters.AddWithValue("@CustomerID", ddlCustomers.SelectedItem.Value);     
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = strQuery;
    cmd.Connection = con;
    try
    {
        con.Open();
        SqlDataReader sdr= cmd.ExecuteReader();
        while (sdr.Read())
        {
            txtCity.Text = sdr[0].ToString();
            lblCountry.Text = sdr["Country"].ToString(); 
        }
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        con.Close();
        con.Dispose();
    }
}         
 
VB.Net
Protected Sub ddlCustomers_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
  Dim strConnString As String = ConfigurationManager _
             .ConnectionStrings("conString").ConnectionString
  Dim strQuery As String = "select City, Country from Customers where" _
                           & " CustomerID = @CustomerID"
  Dim con As New SqlConnection(strConnString)
  Dim cmd As New SqlCommand()
  cmd.Parameters.AddWithValue("@CustomerID", ddlCustomers.SelectedItem.Value)
  cmd.CommandType = CommandType.Text
  cmd.CommandText = strQuery
  cmd.Connection = con
  Try
      con.Open()
      Dim sdr As SqlDataReader = cmd.ExecuteReader()
      While sdr.Read()
        txtCity.Text = sdr(0).ToString()
        lblCountry.Text = sdr("Country").ToString()
      End While
  Catch ex As Exception
      Throw ex
  Finally
      con.Close()
      con.Dispose()
  End Try
End Sub
 
Screenshot
Populate TextBox based on DropDownList Selection in ASP.Net using C# and VB.Net
 
Demo
 
 
Downloads