select combobox data then load another field to textbox from SQL database in c#

private void Form1_Load(object sender, EventArgs e)
       {
           //Fill your combo box
       }

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
       {
         String str="select YourFieldName from YourTableName where FieldName='" + Combobox1.Text + "'";//your select query
        //after getting record assign value to your textbox
         //Textbox1.Text=datareader[0];
       }

private void GetColumnOnComboBox()
       {
           try
           {
              SqlConnection ObjConnection = new SqlConnection();
               ObjConnection.ConnectionString = ConfigurationSettings.AppSettings["constring"];
               ObjConnection.Open();
               string str = "select ColumnName from TableName";
               SqlDataAdapter da = new SqlDataAdapter(str, ObjConnection);
               DataSet ds = new DataSet();
               da.Fill(ds, "YourTableName");
               this.comboBox1.DataSource = ds.Tables["ProductMaster"];
               this.comboBox1.DisplayMember = "ColumnName";//column that you want to show on combo Box
               this.comboBox1.ValueMember = "ColumnName";//same ColumnName Here

           }
           catch (Exception ex)
           {
               MessageBox.Show(ex.Message);
           }
           finally { ObjConnection.Close(); }
       }

private void form1_Load(object sender, EventArgs e)
{
GetColumnOnComboBox();
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
       {
         SqlConnection ObjConnection = new SqlConnection();
            ObjConnection.ConnectionString = System.Configuration.ConfigurationSettings.AppSettings["constring"];
            ObjConnection.Open();
            string str = "select * from YourTableName where columnName='"+ComboBox.text+"'";
            SqlCommand cmd = new SqlCommand(str, ObjConnection);
            SqlDataReader dr=cmd.ExecuteReader();
            if(dr.Read())
            {
            textbox1.Text=dr["ColumnName"].ToString());//column name should be that you want to show on textbox
           
            }
       }