Accessing server control from within app_code class

Question

My question is about accessing a server control (listbox) that is located in default.aspx.

I wish to access this control in Functions.cs (this class is located in the App_Code folder).

My page structure is:

1 masterpage with 1 content holder
Default.aspx (all the controls are within the content place holder)
Functions.cs (located in App_Code)

Now when I try to fill up the listbox elements I get the error "object reference not set to an instance of an object."

What I have tried to gain access to this control: (this code is located in Functions.cs in App_Code). This is basically showing some items in the listbox that are located in an xml file

private static string file = HttpContext.Current.Server.MapPath("~/App_Data/Questions.xml");

public static void ListItems()
{
    XmlDocument XMLDoc = new XmlDocument();
    XMLDoc.Load(file); 
    XPathNavigator nav = XMLDoc.CreateNavigator();

    XPathExpression expr;
    expr = nav.Compile("/root/file/naam");
    XPathNodeIterator iterator = nav.Select(expr);

    //ATTEMPT to get access to ServerControl(listbox)
    Page page = (Page)HttpContext.Current.Handler;
    ListBox test = (ListBox)page.FindControl("lbTest"); //control is called lbTest in Default.aspx

    test.Items.Clear();

    while (iterator.MoveNext())
    {
        test.Items.Add(iterator.Current.Value);
    }
}

Code from the default.apx file

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterFile.master" AutoEventWireup="true" CodeFile="default.aspx.cs" Inherits="default" Debug="true" %>

<%@ MasterType TypeName="Master" %>

<asp:Content ID="Content1" ContentPlaceHolderID="cphContent" Runat="Server" >

    <asp:MultiView ID="mvTest" runat="server" >

        <asp:View ID="vCollection" runat="server">
            <asp:ListBox ID="lbTest" runat="server" CssClass="listbox" ></asp:ListBox>
        </asp:View>


    </asp:MultiView>

</asp:Content>

The masterfile itself just has 1 placeholder.

Then I call upon the function ListItems in the Default.aspx.cs file

protected void Page_Load(object sender, EventArgs e)
{    
    Functions.ListItems();
}

SLaks · Accepted Answer · 2010-12-26 22:59:42Z

2

FindControl will not search nested controls.

You need to search recursively:

private Control FindControlRecursive(Control rootControl, string controlID) {
    if (rootControl.ID == controlID) return rootControl;

    foreach (Control child in rootControl.Controls) {
        var match = FindControlRecursive(child, controlID);
        if (match != null) return match;
    }
    return null;
}

However, your design is much too coupled.
You should consider a design which does not involve this.

For example, you can pass the ListBox as a parameter, move the method to the page class, or create a base Page class with a virtual property to get the ListBox.

answered Dec 26, 2010 at 22:59

SLaks

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Comments

jball · Accepted Answer · 2010-12-26 22:59:56Z

1

You should pass the control you wish to populate with items into your function.

Function.cs:

public static void ListItems(ListBox test)
{
     //same as what you had before, minus the
     //ATTEMPT to get access to ServerControl(listbox)
     //lines
}

Codebehind:

protected void Page_Load(object sender, EventArgs e)
{
    Functions.ListItems(lbTest); //pass your control in here...
}

answered Dec 26, 2010 at 22:59

jball

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