Calling a C# function in asp.net

I have a WebForm before_adm.aspx.cs which has the code as follows:

.
.
.
public partial class _Default : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
    {
          DropDown abs =new DropDown();
          abs.DropDown();
    }
.....

I want to call a function DropDown() which is in file DropDown.cs The code in DropDown.cs is as follows:

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.Odbc;
using System.Data.SqlClient;
using System.Collections;
using System.IO;
using iTextSharp.text.html.simpleparser;
using iTextSharp.text;
using iTextSharp.text.pdf;
using iTextSharp.text.html;

/// <summary>
/// Summary description for DropDown
/// </summary>
public class DropDown
{
    public DropDown()
    {
        try
        {

            OdbcConnection myConn = new OdbcConnection("Driver={MySQL ODBC 3.51 Driver};Server=10.155.160.130;Database=testcase;User=root;Password=;Option=3;");
            OdbcCommand myCmd = new OdbcCommand("select skey,casecode from casetype", myConn);
            myConn.Open();
            OdbcDataReader myReader = myCmd.ExecuteReader();

            //Set up the data binding.
            DropDownList3.DataSource = myReader;
            DropDownList3.DataTextField = "skey";
            DropDownList3.DataValueField = "casecode";
            DropDownList3.DataBind();
            DropDownList3.Items.Insert(0, "<-- Select -->");

        }
        catch (Exception ex)
        {
            Response.Write(ex.StackTrace);
        }

        if (DropDownList2.SelectedItem.Text == "Register")
        {
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("STA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("STM"));
        }
        else if (DropDownList2.SelectedItem.Text == "Stamp")
        {
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("WP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("FA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("LPA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("SA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CAPL"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("MCA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("XFER"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("ARP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("TXA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRIR"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("WPCR"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AO"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AUA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CONP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("ELEP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("COAP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("COA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("ARA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CREF"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("ITA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AAR"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("XFA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("TACR"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("EXAP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("COP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AUE"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AUW"));
开发者_开发问答            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRMA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CAREV"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CONF"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("PILWP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("SMWP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("XOB"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("SMCP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AELEP"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("RUIT"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CS"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("RUCA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRIA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CUSA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("EXA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("SMCRC"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("EA"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRMAB"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRMAM"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("COAPM"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CPM"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("MCAM"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AS"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("PILSM"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("CRREF"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("SMCR"));
            DropDownList3.Items.Remove(DropDownList3.Items.FindByText("AUSFC"));

        }
        else if (DropDownList2.SelectedItem.Text == "<-- Select -->")
        {
            DropDownList3.Items.Clear();
        }
    }

}

Whatever i have done till now,is it correct? I am now getting the error as:

The name 'DropDownList2' does not exist in the current context
The name 'DropDownList3' does not exist in the current context  
The name 'Response' does not exist in the current context

I want to create this function because i will be creating many such WebForms and i want to call the same function on

DropDownList2_SelectedIndexChanged What parameters do i need to pass. i want to know the way to pass the parameters. Please guide me. i want to create many such functions in .cs file which will have many other user controls.

The problem is that the DropDown class does not have a reference to DropDownList2, and it doesn't have a reference to 'Response' because that is a property on the Page object. We can fix this by passing a reference to the DropDownList and the Page, and tweaking things a bit to use those variables.

1. Change DropDown abs =new DropDown(); to DropDown abs =new DropDown(this.DropDownList2, Page page);
2. Change public DropDown() to public DropDown(DropDownList DropDownList2, Page page)
3. Change Response.Write(ex.StackTrace); to page.Response.Write(ex.StackTrace); `

If you're still confused or getting stuck at this point, it might be too early for you to be writing ASP.NET programs - I would start by learning the basics of C# and OO by going through some tutorials.

Try this way

public class DropDown
{
    DropDownList dropDownList3 = null;
    DropDownList dropDownList2 = null;

    public DropDown()
    {
    }

    public void Register(DropDownList dropDown2, DropDownList dropDown3)
    {
        this.dropDownList2 = dropDown2;
        this.dropDownList3 = dropDown3;
        // your code goes here
     }
}

Use HttpContext.Current.Response.Write

Use this class as

DropDown abs = new DropDown();
abs.Register(dropdown1, dropdown2);

Try not to write code in constructor which might throw exception

You shoud pass dropDown as parameter into yours DropDown method:

DropDown abs =new DropDown((DropDownList)sender);
....
public DropDown(DropDownList DropDownList2)..

And instead of:

Response.Write

use:

HttpContext.Current.Response.Write

If you want to call selected index changed event manually you can place following code:

DropDownList2_SelectedIndexChanged(this,null);

I think you are trying to achieve a cascaded dropdown. You have a utility class DropDown.CS. Thementhod "Dropdown" is a constrcutor and not a method so its gets called whenever you initialize the class Dropdown. Selected index changed will be called when you cahnge the first dropdown.

Is this helpful ?

Don't know if it helps, but you can also use extension methods to extend functionality of stock classes.