 <p>try to do it,like below code...</p> <p>private void Form1_Load(object sender, EventArgs e) { FillCountry();</p> <pre><code> } private void FillCountry() { string str = "SELECT CountryID, CountryName FROM Country"; SqlCommand cmd = new SqlCommand(str,con); //cmd.Connection = con; //cmd.CommandType = CommandType.Text; // cmd.CommandText = "SELECT CountryID, CountryName FROM Country"; DataSet objDs = new DataSet(); SqlDataAdapter dAdapter = new SqlDataAdapter(); dAdapter.SelectCommand = cmd; con.Open(); dAdapter.Fill(objDs); con.Close(); comboBox1.ValueMember = "CountryID"; comboBox1.DisplayMember = "CountryName"; comboBox1.DataSource = objDs.Tables[0]; } private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) { if (comboBox1.SelectedValue.ToString() != "") { int CountryID = Convert.ToInt32(comboBox1.SelectedValue.ToString()); FillStates(CountryID); comboBox3.SelectedIndex = 0; } } private void FillStates(int countryID) { string str = "SELECT StateID, StateName FROM State WHERE CountryID =@CountryID"; SqlCommand cmd = new SqlCommand(str, con); // SqlConnection con = new SqlConnection(Con); // cmd.Connection = con; // string str="SELECT StateID, StateName FROM State WHERE CountryID =@CountryID"; // cmd.Connection = con; //cmd.CommandType = CommandType.Text; // cmd.CommandText = "SELECT StateID, StateName FROM State WHERE CountryID =@CountryID"; cmd.Parameters.AddWithValue("@CountryID", countryID); DataSet objDs = new DataSet(); SqlDataAdapter dAdapter = new SqlDataAdapter(); dAdapter.SelectCommand = cmd; con.Open(); dAdapter.Fill(objDs); con.Close(); if (objDs.Tables[0].Rows.Count &gt; 0) { comboBox2.ValueMember = "StateID"; comboBox2.DisplayMember = "StateName"; comboBox2.DataSource = objDs.Tables[0]; } } private void comboBox2_SelectedIndexChanged(object sender, EventArgs e) { int StateID = Convert.ToInt32(comboBox2.SelectedValue.ToString()); FillCities(StateID); } private void FillCities(int stateID) { //SqlConnection con = new SqlConnection(str,con); string str = "SELECT CityID, CityName FROM City WHERE StateID =@StateID"; SqlCommand cmd = new SqlCommand(str,con); // cmd.Connection = con; //cmd.CommandType = CommandType.Text; // cmd.CommandText = "SELECT CityID, CityName FROM City WHERE StateID =@StateID"; cmd.Parameters.AddWithValue("@StateID", stateID); DataSet objDs = new DataSet(); SqlDataAdapter dAdapter = new SqlDataAdapter(); dAdapter.SelectCommand = cmd; con.Open(); dAdapter.Fill(objDs); con.Close(); if (objDs.Tables[0].Rows.Count &gt; 0) { comboBox3.DataSource = objDs.Tables[0]; comboBox3.DisplayMember = "CityName"; comboBox3.ValueMember = "CItyID"; } </code></pre>