C# Sql Bağlantısı

string query = @"Select Alan1,Alan2 From TABLO where Alan2='" + comboBox1.Text + "'"; 
              DataTable dt = null; 
              comboBox2.BeginUpdate(); 
              using (SqlConnection conn = new SqlConnection(query)) 
              { 
                  conn.Open(); 
                  using (SqlCommand cmd = new SqlCommand(query, conn)) 
                  { 
                      using (SqlDataReader rdr = cmd.ExecuteReader()) 
                      { 
                          dt = new DataTable(); 
                          dt.Load(rdr); 
                      } 
                  } 
                  conn.Close(); 
              } 
              for (int i = 0; i < dt.Rows.Count; i++) 
              { 
                  comboBox2.Items.Add(dt.Rows[i].ToString()); 
              } 

using System; 
  using System.Collections.Generic; 
  using System.ComponentModel; 
  using System.Data; 
  using System.Drawing; 
  using System.Linq; 
  using System.Text; 
  using System.Windows.Forms; 
  using System.Data.SqlClient; 
   
  namespace WindowsFormsApplication1 
  { 
      public partial class Form1 : Form 
      { 
          public Form1() 
          { 
              InitializeComponent(); 
          } 
   
          private void Form1_Load(object sender, EventArgs e) 
          { 
              // TODO: This line of code loads data into the 'testDataSet.tst' table. You can move, or remove it, as needed. 
              this.tstTableAdapter.Fill(this.testDataSet.tst); 
              SqlConnection con; 
              con = new SqlConnection("Server=SERVERNAME;Database=Test;User ID=sa;Password=1234qwer;Trusted_Connection=False;"); 
              SqlCommand kmt = new SqlCommand("Select name from tst group by name", con); 
              con.Open(); 
              SqlDataReader oku; 
              oku = kmt.ExecuteReader(); 
              while (oku.Read()) 
              { 
                  comboBox1.Items.Add(oku[0]); 
              } 
              con.Close(); 
   
          } 
   
          private void comboBox1_SelectedValueChanged(object sender, EventArgs e) 
          { 
              SqlConnection con; 
              comboBox2.Items.Clear(); 
              comboBox2.Refresh(); 
              con = new SqlConnection("Server=SERVERNAME;Database=Test;User ID=sa;Password=1234qwer;Trusted_Connection=False;"); 
              SqlCommand kmt = new SqlCommand("Select surname from tst where name='" + comboBox1.Text + "'", con); 
              comboBox2.BeginUpdate(); 
              con.Open(); 
              SqlDataAdapter sqDa = new SqlDataAdapter(); 
              SqlDataReader oku; 
              oku = kmt.ExecuteReader(); 
              while (oku.Read()) 
              { 
                  comboBox2.Items.Add(oku[0]); 
              } 
              con.Close(); 
          } 
      } 
  }