back button causes NullReferecenEx ception

[Steve]

Hello,

In my first (of 2) aspx page I have 2 listboxes. I
populate the first listbox in the PageLoad event

If Not IsPostBack Then
....

When I select an item from the first listbox, lst1, the
2nd listbox gets populated and writes some text to a
hidden textbox called txtSource. Then I redirect to
page2.aspx on the selectedItem event of lst2 and pass the
selections to the next page using Cache("var1") (by the
way, which is better between passing data -- using cache,
session or response.Form?) . But back to the problem,
when I click on the back button of the browser from
page2.aspx I was getting this error:

Exception Details: System.NullRefe renceException: Object
reference not set to an instance of an object.

and modified the code where the error was happening. Then
I could get back to page1.aspx, but when I click on the
first listbox, lst1, I get the same error as above. Here
is the code snipet where the error happens

Sub lst2_Selected.. .
Dim strGetFile As String
If Not txtSource.Text. Equals("") And Not
lst2.SelectedIt em.Value.Equals ("") Then
strGetFile = txtSource.Text & "\" &
lst2.SelectedIt em.Value
Cache("strFile" ) = strGetFile
Response.Redire ct("page2.aspx" )
End If
End Sub

The error happens at

If Not txtSource.Text. Equals("") And Not
lst2.SelectedIt em.Value.Equals ("") Then

What is the correct way to refer to txtSource when it is
empty/null?

How can I eliminate the above error?

TIA,
Steve

[Steve]

OK. I moved the response.Redire ct line in the lst2 event
to a button control. This seems to have fixed the problem
(for now). It appears that all the listbox events were
being called when I clicked on the back button, and also
the page load event of page2.aspx appeared to be getting
called from the lst1 selected... event. Moving ..redirect
to the button resolved that. But I suspect there will be
other not fun things in store with my setup. Any
suggestions welcome if any sees anything whacky here.

-----Original Message-----
Hello,

In my first (of 2) aspx page I have 2 listboxes. I
populate the first listbox in the PageLoad event

If Not IsPostBack Then
....

When I select an item from the first listbox, lst1, the
2nd listbox gets populated and writes some text to a
hidden textbox called txtSource. Then I redirect to
page2.aspx on the selectedItem event of lst2 and pass the
selections to the next page using Cache("var1") (by the
way, which is better between passing data -- using cache,
session or response.Form?) . But back to the problem,
when I click on the back button of the browser from
page2.aspx I was getting this error:

Exception Details: System.NullRefe renceException: Object
reference not set to an instance of an object.

and modified the code where the error was happening. ThenI could get back to page1.aspx, but when I click on the
first listbox, lst1, I get the same error as above. Here
is the code snipet where the error happens

Sub lst2_Selected.. .
Dim strGetFile As String
If Not txtSource.Text. Equals("") And Not
lst2.SelectedI tem.Value.Equal s("") Then
strGetFile = txtSource.Text & "\" &
lst2.SelectedI tem.Value
Cache("strFile" ) = strGetFile
Response.Redire ct("page2.aspx" )
End If
End Sub

The error happens at

If Not txtSource.Text. Equals("") And Not
lst2.SelectedI tem.Value.Equal s("") Then

What is the correct way to refer to txtSource when it is
empty/null?

How can I eliminate the above error?

TIA,
Steve
.