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();
}