Autogenerate id based on DropDown value in using C# ASP.Net

Hi soum,

I have modified your code please check.

HTML

 <div>
        <div>
            Table Name:
            <asp:DropDownList ID="DropDownList2" runat="server">
                <asp:ListItem Value="Data" Text="Employees" />
                <asp:ListItem Value="Customers" Text="Customers" />
                <asp:ListItem Value="Contact" Text="Contact" />
            </asp:DropDownList>
            <asp:TextBox ID="TextBox6" runat="server" />
            <br />
            <br />
        </div>
        <asp:Button ID="btnAuto" Text="AutoGenerate" OnClick="Onclick" runat="server" />
        <br />
        <br />
        <asp:TextBox ID="TextBox1" ReadOnly="true" runat="server" />
    </div>

C#

private string constr = ConfigurationManager.ConnectionStrings["constring"].ToString();
public static int totalCount;
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {

    }
}
private void autogenerated()
{
    totalCount++;
    string dp = DropDownList2.SelectedValue;
    dp = dp.ToLower();
    SqlConnection con = new SqlConnection(constr);
    SqlCommand cmd = new SqlCommand("SELECT COUNT(*) FROM " + dp + " ", con);
    cmd.CommandType = CommandType.Text;
    con.Open();
    object count = Convert.ToInt32(cmd.ExecuteScalar()) + totalCount;
    string dt = TextBox6.Text;
    string st = dt.Substring(3, 2);
    string st1 = dt.Substring(8, 2);
    string cu = DropDownList2.Text.Substring(0, 2);
    Session["e"] = "ST" + st + st1 + cu + totalCount.ToString();
    con.Close();
    TextBox1.Text = Session["e"].ToString();
}
protected void Onclick(object sender, EventArgs e)
{
    this.autogenerated();
}

Screenshot

I hope works for you.