Content page throws object reference not set to an instance of an object when adding dynamic control to table in page_preinit

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I have a web page,after selecting particular student/studentID ,I am showing subject offered to him,user can enter marks obtained.I am creating dynamic label and textbox after selecting item/student in dropdownlist.And on save button ,inserting data to database.But on save button click,post back happens,dynamic control disappaer.In order make it appear ,again created same dynamic control in Page_PreInit event and inserting into table(tblStudent) which is already created/exist in aspx page.This works fine when I am using web form,but fail when use Content page.Content page throws object reference not set to an instance of an object when adding dynamic control to table in Page_PreInit event.Line : tblStudent.Rows.Add(NewRow);.I shows tblStudent as null.How to solve this.I can't change master page.

What I have tried:

TableRow NewRow = new TableRow();

                    TableCell NewCell = new TableCell();
                    TextBox txtBox = new TextBox();

                    txtBox.ID = "TextBox_" + i;

                    NewCell.Controls.Add(txtBox);

                    NewRow.Cells.Add(NewCell);

                    tblStudent.Rows.Add(NewRow);

Posted 1-Sep-17 20:32pm

Member 11589429

Updated 1-Sep-17 20:37pm

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OriginalGriff · Answer 1 · 2017-09-01T20:35:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!