Web Application : C# Function for Insert,Get Data and Fill the Dropdown List

//To get data from database
public object getdata(string str) {
        string constr;
        constr = "ConnectionString";
        SqlConnection con = new SqlConnection(constr);
        con.Open();
        object a;
        SqlCommand cmd = new SqlCommand(str, con);
        a = cmd.ExecuteScalar();
        con.Close();
        return a;
    }
    //To inserting data into the database
public void insertdata(string str) {
        string constr;
        constr = "ConnectionString";
        SqlConnection con = new SqlConnection(constr);
        con.Open();
        SqlCommand cmd = new SqlCommand(str, con);
        cmd.ExecuteNonQuery();
        con.Close();
    }
    //To fill the dropdownlist
public void dataset(DropDownList ddl, string str) {
    string constr;
    constr = "ConnectionString";
    SqlConnection con = new SqlConnection(constr);
    SqlDataAdapter dap = new SqlDataAdapter(str, con);
    con.Open();
    DataSet ds = new DataSet();
    dap.Fill(ds);
    ddl.Items.Add("Select Option");
    for (int i = 0; i < ds.Tables[0].Rows.Count; i++) {
        if (!string.IsNullOrEmpty(ds.Tables[0].Rows[i].ItemArray[0].ToString())) {
            ddl.Items.Add(ds.Tables[0].Rows[i].ItemArray[0].ToString());
        }
    }
    con.Close();
}