retrieve dropdownlist selection after postback

Question

I add values in a dropdownlist in a method as given below

pageload()
{
method(type);
}
public void method(type)
{
 dropdownlist1.items.clear();
 if(type == "Student")
 {
  dropdownlist1.items.add("abc");
  dropdownlist1.items.add("xyz");
 }
}

when i select dropdownlist value, it will lost after postback and i do this for retrieveing

public void method(type)
{
 string selection = dropdownlist1.selectedItem.text;
 Viewstate["selectionValue"] = selection;
 dropdownlist1.items.clear();
 if(type == "Student")
 {
  dropdownlist1.items.add("abc");
  dropdownlist1.items.add("xyz");
 }
}

but an exception occur at this line:

string selection = dropdownlist1.selectedItem.text;

The exception message:

{object reference is not set to an instance of an object}

And i know why it comes. because when first time dropdownlist1 load it could not find the object of dropdownlist , so that's exception occur. my quection is where i retrieve dropdownlist1 selection value that would not lost even after postback .

Answer 1

This question is really about the asp.net page lifecycle. Here's a reference: http://msdn.microsoft.com/en-us/library/ms178472.ASPX

I'm guessing that you're creating your dropdown on page load, but when postback occurs you're trying to read from your dropdown before the page controls have been initialized.

You should check for IsPostBack in your onload function, and if it's true, then try and rebuild your dropdown there.

Answer 2

I think it could be happening because the dropdown might not be having any items when the line

 string selection = dropdownlist1.selectedItem.text;

is executed.

you would have to make sure data is bound to the dropdown before that line gets executed or add a condition like this.

string selection="";
if(dropdownlist1.items.count>0) 
selection = dropdownlist1.selectedItem.text;

Answer 3

Most probably you dont have even a single item in drop-down and you are trying to invoke selecteditem.text. so showing the error message. Put a break point to find out why you dont have items in dropdownlist1.

Answer 4

You should clarify first which is the null . Is it the dropdownlist1.selectedItem or the dropdownlist1 ? I would suggest using the Viewstate["selectionValue"] = selection; statement on the selectedIndeChanged event of the dropdownlist. Then I would use the viewstate content in the onLoad event handler to populate the dropdownlist selection.

Anyway, I suppose Michael J. Anderson is right. You should invest some time on understanding the asp.net life cycle. Misusing the event mechanism of ASP.NET is very often based on poor understanding of ASP.NET lifecycle.