How do I add dynamic controls to htmltable using ASP.NET C#

Question

1.00/5 (1 vote)

See more:

Hey All,
trying to add some dynamic controls to HtmlTable in code behind but getting error on FindControl and the error is as follows

Object reference not set to an instance of an object.

And FindControl is causing this error
when i put control id manually in findcontrol it runs ok but when i use it with loop to find all controls it gives error.
i will deeply appreciate your help

Have a good one.

What I have tried:

C#

HtmlTable table = new HtmlTable();
            HtmlTableRow tr = new HtmlTableRow();
            HtmlTableCell td = new HtmlTableCell();
            foreach (Control c in pnl.Controls)
            {
                TextBox temp_txt = (TextBox)pnl.FindControl(c.ID);  //causing error
                                td.Controls.Add(temp_txt);
                tr.Controls.Add(td);
                table.Controls.Add(tr);
            }            
            pnl.Controls.Add(table);

Posted 14-Oct-20 19:46pm

Aftab Iqbal Clips

Updated 15-Oct-20 0:41am

Add a Solution

Comments

BillWoodruff 15-Oct-20 3:41am

see if this helps: https://stackoverflow.com/a/3731146

2 solutions

Solution 2

Change

C#

foreach (Control c in pnl.Controls)

to

C#

foreach (Control c in pnl.Controls.OfType<TextBox>())

The way asp.net works under the covers is that it converts your html into Literal controls so although your aspx page is like;

HTML

<asp:Panel id="pnl" runat="server">
    <p>Some text</p>
    <asp:Textbox id="txt1" runat="server"/>
</asp:Panel>

asp.net converts it to this when compiling

HTML

<asp:Panel id="pnl" runat="server">
    <asp:Literal id="literal1" runat="server"><p>Some text</p></asp:Literal>
    <asp:Textbox id="txt1" runat="server"/>
</asp:Panel>

It even does this for "non text" like new lines, spacing etc, so there are more controls in your panel than you think and the .OfType makes sure you only get the ones you are interested in.

Posted 15-Oct-20 0:41am

F-ES Sitecore

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sandeep Mewara · Accepted Answer · 2020-10-14T21:51:00

Quote:
Object reference not set to an instance of an object.

Details about the error you see: NullReferenceException Class (System) | Microsoft Docs[^]

Quote:
A NullReferenceException exception is thrown when you try to access a member on a type whose value is null

Possibly because:
1. You've forgotten to instantiate a reference type
2. You've forgotten to dimension an array before initializing it.
3. You get a null return value from a method, and then call a method on the returned type.
4. You're using an expression (for example, you're chaining a list of methods or properties together) to retrieve a value
5. You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws
6. Exception is thrown by a method that is passed null

Quote:
getting error on FindControl

This means that the name of the control is incorrect or not present in that control hierarchy where you are trying to find it.

foreach (Control c in pnl.Controls)
{
    TextBox temp_txt = (TextBox)pnl.FindControl(c.ID);  //causing error
    td.Controls.Add(temp_txt);

With above code, it's not clear on what you want to do actually. Above code is looping through all the controls present in the panel and then add them to a table and then in end add table back to panel. It's like controls are at two places. Then you have tr/td added to table per control in a loop. It's messed up - mostly this leads to mismatch of controls and error. Further, are all controls in panel a textbox that you cast into?