What if the Query String is empty?

  • Thread starter Thread starter MU
  • Start date Start date
M

MU

Hello

I have some code that sets a dropdownlist control with a parameter
from the querystring. However, when the querystring is empty, I get
an error.

Here is my code:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Load
If Not Page.IsPostBack Then
Dim intProductID As Integer = Trim(Request("productid"))
If IsNumeric(intProductID) Then
drpProducts.SelectedValue = intProductID
End If
End If
End Sub

Here is the error:
Input string was not in a correct format.
Description: An unhandled exception occurred during the execution of
the current web request. Please review the stack trace for more
information about the error and where it originated in the code.

Exception Details: System.FormatException: Input string was not in a
correct format.

Source Error:
Line 18: Protected Sub Page_Load(ByVal sender As Object, ByVal e
As System.EventArgs) Handles Me.Load
Line 19: If Not Page.IsPostBack Then
Line 20: Dim intProductID As Integer =
Trim(Request("productid"))

How do I just have it continue without giving me the error?

Thanks
 
My guess this line
Dim intProductID As Integer = Trim(Request("productid"))
assumes that productid is Integer....and blows when it's not (empty)

As of "how do you continue without getting an error" I would say stop
assuming it's Integer and start assuming it's String
Dim intProductID As String = Trim(Request("productid"))


George.
 
You just need to rearrange your logic a bit...
If Not Page.IsPostBack Then
If IsNumeric(Trim(Request("productid"))) Then
drpProducts.SelectedValue =
Trim(Request.QueryString("productid"))
End If
End If

Or if you also want to make sure the value actually exists, try
this...

If Not Page.IsPostBack Then
Dim productID As String = Request.QueryString("productID")
If
drpProducts.Items.IndexOf(drpProducts.Items.FindByValue(productID)) >
-1 Then
drpProducts.SelectedValue = productID
End If
End If
 
And what if the QueryString is completely empty, as per the title of the
thread...?
Click to expand...

Than IsNumeric will not evaluate to true. I just tested it and it
works...
 
re:
!> Than IsNumeric will not evaluate to true

Querystrings always are Strings, and never Numeric.

If IsNumeric(Trim(Request("productid"))) will *always* evaluate to false.




Juan T. Llibre, asp.net MVP
asp.net faq : http://asp.net.do/faq/
foros de asp.net, en español : http://asp.net.do/foros/
======================================
 
wrong....

from msdn

IsNumeric returns True if the data type of Expression is Short, Integer,
Long, Decimal, Single, or Short. It also returns True if Expression is a
String that can be successfully converted to a Double. It returns False if
Expression is of data type Date."

George
 
re:
!> wrong....

You should be a bit more careful when accusing somebody of being "wrong".

To repeat : a querystring is *always* a string.

A query string is a NameValueCollection containing
the collection of query string variables sent by the client.

The datatype for a *query string* can never be Numeric.
The very name itself is a hint for its data type : query *string*.




Juan T. Llibre, asp.net MVP
asp.net faq : http://asp.net.do/faq/
foros de asp.net, en español : http://asp.net.do/foros/
======================================
 
ok , now you lost me....

I quoted MSDN where it says that IsNumeric(string) will return true if
string can be converted to Double.

You said that
IsNumeric(Trim(Request("productid")))
will always evaluate to false....

I pointed out that you wrong and it will evaluate to TRUE if
Request("productid") returns something like "123"

PS: accusing probably is a strong word.. I did not offer a jail term for
you. May be I should have said "mistaken"? Applogies then... I was in a
hurry...


George.
 
You should be a bit more careful when accusing somebody of being "wrong".
To repeat : a querystring is *always* a string.

A query string is a NameValueCollection containing
the collection of query string variables sent by the client.

The datatype for a *query string* can never be Numeric.
The very name itself is a hint for its data type : query *string*.
Click to expand...


I'm sorry, but you are in fact wrong. The querystring is a string.
We are all in agreement on that. But you said that IsNumeric(string)
will always evaluate to false, which is incorrect. If the string can
be cast to a double such as "1", "345", "6.4", or some other variant
the expression will evaluate to true. George found the appropriate
documentation and correctly pointed this out. Open VS and try it
yourself....
 
re:
!> I quoted MSDN where it says that IsNumeric(string) will return true if
!> string can be converted to Double.
!> I pointed out that you wrong and it will evaluate to TRUE if
!> Request("productid") returns something like "123"

That's a really large *if*...

Sure, but not all product id's are simple numeric strings.
You might have a product id like : w2344.

That won't evaluate the way you think it should.

re:
!> Request("productid")

Request works in a funny way.

When you do a plain request, without identifying the exact type of request you want,
the ASP.NET engine cycles through all request types and will return the first one it finds.

If you ask for Request("productid"), and there's a cookie named "productid",
that's what will be returned instead of the value of the querystring named "productid".

If there's a form with a field named "productid", you may get the form field's value.

You can't trust what a simple "request" returns.

You should always specify Request.QueryString, if you want to go that way
....and you must make sure that the potential value returned can only be numeric.

re:
!> May be I should have said "mistaken"?

Maybe you should have... ;-)

re:
!> Apologies then... I was in a hurry...

No problem...



Juan T. Llibre, asp.net MVP
asp.net faq : http://asp.net.do/faq/
foros de asp.net, en español : http://asp.net.do/foros/
======================================
 
re:
!> I'm sorry, but you are in fact wrong

Explain to me what happens if the product id is w2344...
Click to expand...


You're arguing a completely different point now than what I said you
were wrong about, but ok. If you look at the original post, the
question contained the code...

"Dim intProductID As Integer = Trim(Request("productid")) "

Based on the original poster's own code, it is going to be an int. If
it is not going to be an int they should use the other piece of code
that I originally provided.


If Not Page.IsPostBack Then
Dim productID As String = Request.QueryString("productID")
If
drpProducts.Items.IndexOf(drpProducts.Items.FindByValue(productID)) >
-1 Then
drpProducts.SelectedValue = productID
End If
End If
 
re:
!> were wrong about, but ok. If you look at the original post, the
!> question contained the code...

!> "Dim intProductID As Integer = Trim(Request("productid")) "

See my explanantion of what a simple Request may return.
It may not necessarily return the value of the querystring.

What you both have proposed amounts to sloppy coding.
No offense intended.




Juan T. Llibre, asp.net MVP
asp.net faq : http://asp.net.do/faq/
foros de asp.net, en español : http://asp.net.do/foros/
======================================
re:
!> I'm sorry, but you are in fact wrong

Explain to me what happens if the product id is w2344...
Click to expand...


You're arguing a completely different point now than what I said you
were wrong about, but ok. If you look at the original post, the
question contained the code...

"Dim intProductID As Integer = Trim(Request("productid")) "

Based on the original poster's own code, it is going to be an int. If
it is not going to be an int they should use the other piece of code
that I originally provided.


If Not Page.IsPostBack Then
Dim productID As String = Request.QueryString("productID")
If
drpProducts.Items.IndexOf(drpProducts.Items.FindByValue(productID)) >
-1 Then
drpProducts.SelectedValue = productID
End If
End If
 
See my explanantion of what a simple Request may return.
It may not necessarily return the value of the querystring.
Click to expand...

I never said otherwise. I provided two solutions, one merely adjusted
the logic the OP had provided, which did have the
Request["productid']. This was not my code, and I was merely
providing some guidance to prevent the problem they posted about.

I than provided my own solution which did use
Request.QueryString("productid").
 
Protected Sub Page_Load(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Load
If Not Page.IsPostBack Then

Dim o as Object = Request("productid")

If Not (o is Nothing) Then

Dim intProductID As Integer = Trim(o.ToString())

If IsNumeric(intProductID) Then
drpProducts.SelectedValue = intProductID
End If

Else
'What do you do when this happens
End If



End If
End Sub

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Are we back to this????

What if the sun will not come up tomorrow.....
Any programming is based on assumptions.... And it's good to have those
assumptions.

Let say ProductId is id in MsSql database which is marked as integer. Then
the page will return an error trying to convert productid to integer and
database will be protected from virus that hits your site with urls like
this

/product.aspx?id=31147'%20and%20char(124)%2Buser%2Bchar(124)=0%20and%20''='


Again, let me repeat myself that any programming is based on assumptions.
Assumptions can be as simple as for example in DB first name is only limited
to 250 characters. It's an assumption. Very realistic but still it is.
George.
 
I do not think you need this IF
If IsNumeric(intProductID) Then

Since you already did
Dim intProductID As Integer = Trim(o.ToString())

Not sure if VB will eats this line like this though... I know C# compiler
will throw an error.

I would do something like this
Dim intProductID As Integer = Int32.Parse(Trim(o.ToString()))


George.
 
"exceptions are handled" and "exceptions are not happening" 2 different
things...

I did not say that user needs to get ASP.NET error screen with bunch of
information if something is not integer.
But if user decided to play with QueryString by hand and put into ProductId
his name instead of number he deserves to be redirected to the default.aspx
how I do it in case of exception that is caught in Application_OnError

So it's perfectly OK to assume that ProductId is a number (if it's in
database) and do a conversion without tipping-toeing around.


George.
 
George,

I think the point is that we can't assume a general rule for product id's.

Even if in this case it were to be numeric,
programmers simply can't assume that to be the general case.

Why base your programming logic on an assumption which might not be true ?
Why not write code which will work regardless of whether the product id is a number or a string ?




Juan T. Llibre, asp.net MVP
asp.net faq : http://asp.net.do/faq/
foros de asp.net, en español : http://asp.net.do/foros/
======================================
George said:
Are we back to this????

What if the sun will not come up tomorrow.....
Any programming is based on assumptions.... And it's good to have those assumptions.

Let say ProductId is id in MsSql database which is marked as integer. Then the page will return an error trying to
convert productid to integer and database will be protected from virus that hits your site with urls like this

/product.aspx?id=31147'%20and%20char(124)%2Buser%2Bchar(124)=0%20and%20''='


Again, let me repeat myself that any programming is based on assumptions.
Assumptions can be as simple as for example in DB first name is only limited to 250 characters. It's an assumption.
Very realistic but still it is.
George.
Click to expand...
 
re:
!> So it's perfectly OK to assume that ProductId is a number (if it's in database)

I'd hate to be in your shoes when the time comes you have to eat those words. <g>




Juan T. Llibre, asp.net MVP
asp.net faq : http://asp.net.do/faq/
foros de asp.net, en español : http://asp.net.do/foros/
======================================
George said:
"exceptions are handled" and "exceptions are not happening" 2 different things...

I did not say that user needs to get ASP.NET error screen with bunch of information if something is not integer.
But if user decided to play with QueryString by hand and put into ProductId his name instead of number he deserves to
be redirected to the default.aspx how I do it in case of exception that is caught in Application_OnError

So it's perfectly OK to assume that ProductId is a number (if it's in database) and do a conversion without
tipping-toeing around.


George.
Click to expand...
 
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