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Asked: 2026-05-25T06:05:12+00:00

I am attempting to fill in an ASP.NET page textbox with some predefined text

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I am attempting to fill in an ASP.NET page textbox with some predefined text so that when it is displayed the value is predefined. I have tried

protected void Page_PreRender ()
{
    mytextbox.Text = somestring;
}

which works fine in the development environment but on the server produces…

System.NullReferenceException: Object reference not set to an instance of an object

The same applies when I try this in Page_Load. As I read the answers to this question, what I am trying should work (in at least one of these places).

Can anyone see what I am doing wrong?

EDIT more code, as suggested. The C# looks like this:-

    protected void Page_PreRender (Object sender, EventArgs e)
    {
        try
        {
            string [] file_list;
            int i = 0;

            file_list = Directory.GetFiles(MyProg.Common.GetDirectory(),
                                           MyProg.Common.GetFileNameRoot() + "*.*");

            foreach (string filename in file_list)
            {
                string filenameonly = Path.GetFileName (filename);

                if (filenameonly == MyProg.Common.GetFileNameRoot() + "runlog.log")
                {
                    nametextbox.Text = filenameonly;
                }

            }

        }
        catch (Exception ex)
        {
            string mystring = ex.ToString();
            errorMessage.Text = "Page Load Error : " + mystring;
        }

    }

and the ASP.NET page like this…

<%@ Page Language="C#"
         AutoEventWireup="true"
         CodeBehind="MyDialogue.aspx.cs"
         Inherits="MyDialogue" %>
<%@ Register assembly="ComponentArt.Web.UI"
             namespace="ComponentArt.Web.UI"
             tagprefix="ComponentArt" %>
<%@ Register assembly="ComponentArt.Web.Visualization.Charting"
             namespace="ComponentArt.Web.Visualization.Charting"
             tagprefix="cc1" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
</head>
<body>
<form id="myForm" runat="server">
<div style="visibility:hidden">
<asp:TextBox ID="nametextbox"
             TextMode="MultiLine"
             runat="server"
             Visible="true" />
</div>
</form>
</body>
</html>
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1 Answer

  1. Editorial Team

    There could be several areas that are causing this problem. How are you sure that you’ve narrowed it down to the textbox itself? Was this code completely bug-free before adding the textbox message? I’ll post your code below with where I think potential null references may be occurring (in comments):

    string [] file_list;
int i = 0;

file_list = Directory.GetFiles(MyProg.Common.GetDirectory(),
                               MyProg.Common.GetFileNameRoot() + "*.*");

// it is possible that file_list is null
// potentially due to an invalid path (missing / perhaps?)
foreach (string filename in file_list)
{
    string filenameonly = Path.GetFileName (filename);

    // It's possible that the MixedZone.Kernel.Common library
    // is experiencing the null reference exception because it
    // may not understand what file to get the name root of or 
    // maybe it is not capable of getting the root for some
    // other reason (permissions perhaps?)
    if (filenameonly == MixedZone.Kernel.Common.GetFileNameRoot() + "runlog.log")
    {
        nametextbox.Text = filenameonly;
    }

    Some possible solutions or safer code:

    string [] file_list;
int i = 0;

file_list = Directory.GetFiles(MyProg.Common.GetDirectory(),
                               MyProg.Common.GetFileNameRoot() + "*.*");

if (file_list == null) throw new Exception("File List is null. Something is wrong.");

foreach (string filename in file_list)
{
    string filenameonly = Path.GetFileName (filename);

    string fileroot = MixedZone.Kernel.Common.GetFileNameRoot();
    if(string.IsNullOrEmpty(fileroot) throw new Exception("MixedZone Library failed.");

    if (filenameonly.Equals(fileroot + "runlog.log", StringComparison.OrdinalIgnoreCase)) // Choose your own string comparison here
    {
        nametextbox.Text = filenameonly;
    }
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