Null Reference exception could occur...

jrmrcol · March 26th, 2009, 01:15 PM

[IMG]file:///C:/DOCUME%7E1/raldana/Local%20Settings/Temp/moz-screenshot.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/raldana/Local%20Settings/Temp/moz-screenshot-1.jpg[/IMG]Hi, I am learning visual studio .net with the following book:

Beginning Visual BasicÂ® 2005
Thearon Willis and Bryan Newsome

On chapter 5 : Deleting from an ArrayList

Try it Out, Step 3

I write the following code accordingly to the instruction and I am getting a warning:

Property 'SelectedCustomer' does not return a value on all code paths. A null reference exception could occur at run time when the result is used

The question is why and how to fix this null issue?

Public ReadOnly Property SelectedCustomer() As Customer

Get
If lstCustomers.SelectedIndex <> -1 Then
âReturn the selected customer
Return lstCustomers.Items(lstCustomers.SelectedIndex)

End If

End Get

End Property

Thank you,

Ricardo

gbianchi · March 26th, 2009, 01:30 PM

Hello.

This is happening because if the selected index is -1, then you will be returning a null pointer (There is no else). It's said that not all code path returns a value because of that.

To solve it, just add return null after the end if. Be aware that if there is not a selected index, this in fact will return null, so you will have to check for a null pointer when you call this function.

jrmrcol · March 26th, 2009, 01:43 PM

Hi Gonzalo,

I am very new with VB.net, I try

Code:

Get
            If lstCustomers.SelectedIndex <> -1 Then
                'Return the selected customer
                Return lstCustomers.Items(lstCustomers.SelectedIndex)

            End If

            Return DBNull

        End Get

but, it says that cannot be use with as an expresion.(i try it as well with Return Null, null is no longer supported), so I write "Return Nothing" will that be ok?

gbianchi · March 26th, 2009, 01:49 PM

That's perfect. null, nothing, everything depends of the language...