generate automatic empcode in the textbox by company wise when the page is load

1. You will have to set AutoPostBack = true for the DropDowbList and also add a SelectedIndexChanged event and the you can code as follows

protected void ddlCustomers_SelectedIndexChanged(object sender, EventArgs e)
{
    String strConnString = ConfigurationManager
                          .ConnectionStrings["conString"].ConnectionString;

    String strQuery = "select * 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())
        {
           lblName.Text = sdr["ContactName"].ToString();
           lblCountry.Text = sdr["Country"].ToString(); 
        }
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        con.Close();
        con.Dispose();
    }
}

hi mudassar thanks for ur reply,i need to display the employee id by company type..i tried like this it show an error,input string wat not in correct format

this is my code,its show an error input string wat not in correct format

 Dim connection As New SqlConnection
    Dim cmd As New SqlCommand

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        connection.ConnectionString = ("Data Source=RIFAYEE-PC;Initial Catalog=trial;Integrated Security=True;Pooling=False")
        connection.Open()
        cmd = New SqlCommand("insert into employee values('" + TextBox1.Text.Trim() + "','" + TextBox2.Text.Trim() + "'," + TextBox3.Text.Trim() + ")", connection)
        cmd.ExecuteNonQuery()
        connection.Close()
        label1.Text = "Records successfully Inserted"
    End Sub

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        'Dim con As New SqlConnection(strConnString)
        connection.ConnectionString = ("Data Source=RIFAYEE-PC;Initial Catalog=trial;Integrated Security=True;Pooling=False")
        Dim count As Integer
        cmd = New SqlCommand("select count(*)from employee", connection)
        connection.Open()
        count = Convert.ToInt16(cmd.ExecuteScalar()) + 1
        TextBox1.Text = "OAD"+count
        connection.Close()
    End Sub