How to call the events according to the desired situation

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C#

ASP.NET

Hi Everyone,

I have a textbox 'txtQuantity' and a button 'ADD'. I have written the code for generating dynamic textboxes inside the txtQuantity_TextChanged event. i want to generate the textboxes automatically when i write some text or quantity inside the textbox. after filling all the dynamic textboxes i want to add that values in database.
But my problem is on clicking the button 'ADD' the dynamic textboxes are generating as well as the button_Click event is working inserting null value into database. But i want to valid the textchanged event first.How will i do it ? kindly help.

protected void txtQuantity_TextChanged(object sender, EventArgs e)
 {
     con.Open();
     using (SqlCommand comm = new SqlCommand("select [Serial_Number_Required] from Products where Product=@Product ", con))
     {
         //string Role = (string)(Session["Role"]);
         // 2. define parameters used in command object
         SqlParameter para = null;
         para = new SqlParameter();
         para.ParameterName = "@Product";
         para.Value = ddlProductName.SelectedValue;
         comm.Parameters.Add(para);

         //Pass @Pages parameter

         SqlDataReader reade = comm.ExecuteReader();
         while (reade.Read())
         {
             Session["Serial_Number_Required"] = Convert.ToString(reade["Serial_Number_Required"]);
         }
         con.Close();
         //Button Add = (Button)PreviousPage.FindControl("Button2");
         if (Session["Serial_Number_Required"].ToString() == "Y")
         {

             fun1();
         }


     }

         }

<asp:TextBox ID="txtQuantity" runat="server"  Width="150px" 
                    Height="16px" BackColor="White" AutoPostBack="True"  ontextchanged="txtQuantity_TextChanged" 
                    ></asp:TextBox>

<asp:Button ID="Button2" runat="server" BackColor="#F4F4F4" CssClass="style4" Text="Add to GatePass"
                    Font-Size="8.5pt"   onchange="txtQuantity.TextChanged" Height="22px" onclick="Button2_Click"   
                                OnClientClick="javascript:return ValidateField()" Font-Bold="False" 
                    Width="120px" CausesValidation="False" >