[Solved] DropDownList resets SelectedIndex after PostBack using C# and VB.Net in ASP.Net

Hi akhter,

Check this sample.

HTML

Select Employee ID :
<asp:DropDownList ID="ddlEmployeeID" runat="server" AutoPostBack="true" OnSelectedIndexChanged="SelectedCustomer">
</asp:DropDownList><br /><br />
<asp:GridView ID="gvEmployees" runat="server">
</asp:GridView>

Namespaces

C#

using System.Configuration;
using System.Data;
using System.Data.SqlClient;

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindEmployees();
        this.BindDropdownID();
    }
}

protected void SelectedCustomer(object sender, EventArgs e)
{
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constr"].ConnectionString))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT EmployeeID,(FirstName+' ' +LastName) AS Name,City,Country FROM Employees WHERE EmployeeID=@EmployeeID", con))
        {
            cmd.CommandType = CommandType.Text;
            cmd.Parameters.AddWithValue("@EmployeeID", ddlEmployeeID.SelectedValue);
            con.Open();
            this.gvEmployees.DataSource = cmd.ExecuteReader();
            this.gvEmployees.DataBind();
            con.Close();
        }
    }
}

private void BindDropdownID()
{
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constr"].ConnectionString))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT EmployeeID,(FirstName+' ' +LastName) AS Name FROM Employees", con))
        {
            cmd.CommandType = CommandType.Text;
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                sda.SelectCommand = cmd;
                DataTable dt = new DataTable();
                sda.Fill(dt);
                this.ddlEmployeeID.DataTextField = "Name";
                this.ddlEmployeeID.DataValueField = "EmployeeID";
                this.ddlEmployeeID.DataSource = dt;
                this.ddlEmployeeID.DataBind();
                this.ddlEmployeeID.Items.Insert(0, new ListItem { Text = "--Select Customer--", Value = "" });
            }
        }
    }
}

private void BindEmployees()
{
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constr"].ConnectionString))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT EmployeeID,(FirstName+' ' +LastName) AS Name,City,Country FROM Employees", con))
        {
            cmd.CommandType = CommandType.Text;
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                sda.SelectCommand = cmd;
                DataTable dt = new DataTable();
                sda.Fill(dt);
                this.gvEmployees.DataSource = dt;
                this.gvEmployees.DataBind();
            }
        }
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.BindEmployees()
        Me.BindDropdownID()
    End If
End Sub

Protected Sub SelectedCustomer(ByVal sender As Object, ByVal e As EventArgs)
    Using con As SqlConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("constr").ConnectionString)
        Using cmd As SqlCommand = New SqlCommand("SELECT EmployeeID,(FirstName+' ' +LastName) AS Name,City,Country FROM Employees WHERE EmployeeID=@EmployeeID", con)
            cmd.CommandType = CommandType.Text
            cmd.Parameters.AddWithValue("@EmployeeID", ddlEmployeeID.SelectedValue)
            con.Open()
            Me.gvEmployees.DataSource = cmd.ExecuteReader()
            Me.gvEmployees.DataBind()
            con.Close()
        End Using
    End Using
End Sub

Private Sub BindDropdownID()
    Using con As SqlConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("constr").ConnectionString)
        Using cmd As SqlCommand = New SqlCommand("SELECT EmployeeID,(FirstName+' ' +LastName) AS Name FROM Employees", con)
            cmd.CommandType = CommandType.Text
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                sda.SelectCommand = cmd
                Dim dt As DataTable = New DataTable()
                sda.Fill(dt)
                Me.ddlEmployeeID.DataTextField = "Name"
                Me.ddlEmployeeID.DataValueField = "EmployeeID"
                Me.ddlEmployeeID.DataSource = dt
                Me.ddlEmployeeID.DataBind()
                Me.ddlEmployeeID.Items.Insert(0, New ListItem With { .Text = "--Select Customer--", .Value = ""})
            End Using
        End Using
    End Using
End Sub

Private Sub BindEmployees()
    Using con As SqlConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("constr").ConnectionString)
        Using cmd As SqlCommand = New SqlCommand("SELECT EmployeeID,(FirstName+' ' +LastName) AS Name,City,Country FROM Employees", con)
            cmd.CommandType = CommandType.Text
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                sda.SelectCommand = cmd
                Dim dt As DataTable = New DataTable()
                sda.Fill(dt)
                Me.gvEmployees.DataSource = dt
                Me.gvEmployees.DataBind()
            End Using
        End Using
    End Using
End Sub

Screenshot