How to retrive the DropDownListValue to TextBox using asp.n

Question

 Hi Every One ,

 i have Small doubt Asap  solvethem,

 When User Click The DropDown List Display  TextBox ... actually Iam Writing Logic Code Let me know in my code where small Mistake...

protected void dpCity_SelectedIndexChanged(object sender, EventArgs e)

{

AutoNumber();

}

public void AutoNumber()

{

SqlConnection con = new SqlConnection("Data Source=SERVER;Initial Catalog=MuraliIIT;User ID=sa;Password=rits@1234");

con.Open();

string s = "select count(*) from NewStudentRegistrationForm";

SqlCommand com = new SqlCommand(s, con);

int count = Convert.ToInt16(com.ExecuteScalar()) + 2;

// dpCity.SelectedItem.Text = dpCity.SelectedItem.Text + count;

TxtUserid.Text = dpCity.SelectedItem.Text + count;

con.Close();

}