Window Application : C# Function for Insert,get data and fill the dropdown list

//To get data from database
string constr;
constr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\database.mdb";
public object getdata(string str)
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
object a;
OleDbCommand cmd = new OleDbCommand(str, con);
a = cmd.ExecuteScalar();
con.Close();
return a;
}
//To inserting data into the database
public void insertdata(string str)
{
OleDbConnection con = new OleDbConnection(constr);
con.Open();
OleDbCommand cmd = new OleDbCommand(str, con);
cmd.ExecuteNonQuery();
con.Close();
}
//To fill the dropdownlist
public void dataset(ComboBox ddl, string str)
{
OleDbConnection con = new OleDbConnection(constr);
OleDbDataAdapter dap = new OleDbDataAdapter(str, con);
con.Open();

DataSet ds = new DataSet();
dap.Fill(ds);

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();
}