更改下拉列表值时，ASP.NET不会更新文本框

Question

I have four text boxes whose value I need to change based on the selection in a dropdown list. but when I change the value, the textboxes don't update

My codes:

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
 DataSourceID="Employees"  DataTextField="FullName" DataValueField="FullName"           
 OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>
    <asp:SqlDataSource ID="Employees" runat="server" 
     ConnectionString="<%$ConnectionStrings:MyConnectionString %>"     
     SelectCommand="SELECT [FullName] FROM [Employees]   ORDER BY      [FirstName]">           
   </asp:SqlDataSource>

And class file:

public partial class WebForm1 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string cn = "Data Source=.;Initial Catalog=DBRE ;Integrated Security=True";
        SqlConnection scon = new SqlConnection(cn);

        SqlCommand scmd = new SqlCommand("Select * from Employees where FullName = '" + DropDownList1.SelectedItem.Value + "'", scon);
        SqlDataReader sdr;

        try
        {
           scon.Open();
           sdr = scmd.ExecuteReader();
           txtName.Text = sdr["FirstName"].ToString();
           txtSurname.Text = sdr["LastName"].ToString();
           txtDepartment.Text=sdr["Dept"].ToString();
           txtCostCentre.Text=sdr["CostCentre"].ToString();
        }
        catch (Exception ex)
        {
        }

        finally
        {
            scon.Close();
        }

What am i doing wrong here?

Answer 1

Remove AutoPostBack="True"

<asp:DropDownList ID="DropDownList1" runat="server"
 DataSourceID="Employees"  DataTextField="FullName" DataValueField="FullName"           
 OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">

And add if (!ispostback) condition

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
       if (!isPostback)
        {
        string cn = "Data Source=.;Initial Catalog=DBRE ;Integrated Security=True";
        SqlConnection scon = new SqlConnection(cn);

        SqlCommand scmd = new SqlCommand("Select * from Employees where FullName = '" + DropDownList1.SelectedItem.Value + "'", scon);

        SqlDataReader sdr;

        try
        {
           scon.Open();

           sdr = scmd.ExecuteReader();
           txtName.Text = sdr["FirstName"].ToString();
           txtSurname.Text = sdr["LastName"].ToString();
           txtDepartment.Text=sdr["Dept"].ToString();
           txtCostCentre.Text=sdr["CostCentre"].ToString();
        }

        catch (Exception ex)
        {
        }   
        finally
        {
            scon.Close();
        }
    }
}

Answer 2

May be i think check the exception, Use Reader like or use ExecuteScalar

using (var reader = cmd.ExecuteReader())
    {
        if (reader.Read())
        {
            learerLabel.Text = reader.GetString(reader.GetOrdinal("somecolumn"))
        }
    }

Check this

dynamically filled DropDownList does not retain value on postback ASP.net c#

Try EnableViewState="true"

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"  EnableViewState="true" DataSourceID="Employees"  DataTextField="FullName" DataValueField="FullName"          
 OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>

Answer 3

Set your dropdownlist with AutoPostBack="true" with the on selected index changed like this:

     <asp:DropDownList ID="ddl" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlPNo_selectedChanged" ></asp:DropDownList>

And in your code behind load data into your dropdownlist with the following code:

     String conn = System.Configuration.ConfigurationManager.ConnectionStrings["CONSTRING"].ConnectionString;
    protected void Page_Load(object sender, EventArgs e)
    {
      if (!IsPostBack)
        {
            SqlConnection con = new SqlConnection(conn);
            SqlDataAdapter da = new SqlDataAdapter("select Code from table1", con);
            DataSet ds = new DataSet();
            con.Open();
            da.Fill(ds);
            con.Close();
            ddl.DataSource = ds.Tables[0];
            ddl.DataValueField = "Code";
            ddl.DataBind();
            ddl.Items.Insert(0, new ListItem("-- select --"));
        }
    }

Now you can bind data on dropdownlist changed with the following code:

     protected void ddl_selectedChanged(object sender, EventArgs e)
    {
        String strConnString = ConfigurationManager.ConnectionStrings["CONSTRING"].ConnectionString;
        String strQuery = "select description from table1 where Code = @Code";
        SqlConnection con = new SqlConnection(strConnString);
        SqlCommand cmd = new SqlCommand();

        cmd.Parameters.AddWithValue("@Code", ddl.SelectedItem.Value);
        cmd.CommandType = CommandType.Text;
        cmd.CommandText = strQuery;
        cmd.Connection = con;

        try
        {
            con.Open();
            SqlDataReader dr = cmd.ExecuteReader();

            while (dr.Read())
            {
                txtbdescription.Text = dr["description"].ToString();
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {
            con.Close();
            con.Dispose();
        }
    }

where txtbdescription is the id of your textbox.

Hope this will help.