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.
Download and install Northwind Database
 
 

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 HTML Markup consists of:
DropDownList – For selecting Customer ID.
The DropDownList has been assigned OnSelectedIndexChanged event and also the AutoPostBack property is set to True.
TextBox – For display city.
Label – For display country.
<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.
Here Select Query is executed and it gets the CustomerID and the ContactName fields.
Then 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 (!this.IsPostBack)
    {
        ddlCustomers.Items.Add(new ListItem("--Select Customer--",""));
        ddlCustomers.AppendDataBoundItems = true;
        string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        string sql = "select CustomerID, ContactName from Customers";
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand(sql, 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(sender As Object, e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        ddlCustomers.Items.Add(New ListItem("--Select Customer--", ""))
        ddlCustomers.AppendDataBoundItems =  True
        Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Dim sql As String = "select CustomerID, ContactName from Customers"
        Using con As New SqlConnection(constr)
            Using cmd As New SqlCommand(sql, 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 Using
        End Using
    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.
Then fetched values are displayed in the TextBox and Label controls respectively.
C#
protected void ddlCustomers_SelectedIndexChanged(object sender, EventArgs e)
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    string sql = "select City, Country from Customers where CustomerID = @CustomerID";
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand(sql, con))
        {
            cmd.Parameters.AddWithValue("@CustomerID", ddlCustomers.SelectedItem.Value);
            try
            {
                con.Open();
                using (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(sender As Object, e As EventArgs)
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim sql As String = "select City, Country from Customers where CustomerID = @CustomerID"
    Using con As New SqlConnection(constr)
        Using cmd As New SqlCommand(sql, con)
            cmd.Parameters.AddWithValue("@CustomerID", ddlCustomers.SelectedItem.Value)
            Try
                con.Open()
                Using sdr As SqlDataReader = cmd.ExecuteReader()
                    While sdr.Read()
                        txtCity.Text = sdr(0).ToString()
                        lblCountry.Text = sdr("Country").ToString()
                    End While
                End Using
            Catch ex As Exception
                Throw ex
            Finally
                con.Close()
                con.Dispose()
            End Try
        End Using
    End Using
End Sub
 
 

Screenshot

Populate TextBox based on DropDownList Selection in ASP.Net using C# and VB.Net
 
 

Demo

View Demo
 
 

Downloads

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