Option Infer - Round Three
Another Sneaky Option Infer Bug
I've been working on an upgrade to one of the tutorials to add information about the new LINQ - Language INtegrated Query - capability in Visual Basic 9 and the .NET Framework 3.5. Only VB.NET 2008 versions has it.
So imagine my dismay when the Microsoft examples and tutorials simply refused to work in my code. To be specific, code similar to this example can be found in more than one Microsoft tutorial:
Dim x = _
From proc In _
System.Diagnostics. _
Process.GetProcesses _
Select proc.ProcessName
For Each y In x
Debug.WriteLine(y.ToString)
Next
A specific error is reported by VB.NET. In my existing project, the variable y is "not declared".
In hunting the bug, I finally discovered that a new project worked just fine. The "not declared" error doesn't show up. But adding the code to my existing project did not! I carefully checked namespace references, Imports clauses, everything, and finally discovered that the bug actually starts with the variable x. In a new project, x is an IEnumerable type. In my existing project, it's just a generic object type.
Why is that?
It's the sneaky Option Infer that doesn't get set correctly when a project is migrated from VB.NET 2005 to VB.NET 2008. These articles describe the basic problem:
In this case, however, it pretty much makes it impossible to code a lot of LINQ statements because what LINQ calls "implicit typing" doesn't create the variable types that you need unless Option Infer is turned on. To solve the problem, simply change the default in your project properties as shown above.
Comments
Hi Dan,
I like your new page layout, and the fact that I can open your examples in new tabs or windows.
I’m not sure that I like the idea of ‘Option Infer’.
It reminds me of an issue that I believe still exists with generic collections, and using a numeric ‘Key’ to identify an Item. It just doesn’t work.
EX:
************
Dim MyClass as New SomeClass ‘ has a single property “Name”
Dim MyClassCollection as New Collection
MyClass.Name = “Tom”
MyClassCollection.Add(MyClass, “28″)
‘ The next line will generate an error
MsgBox(”Name = ” & MyClassCollection(”28″).Name)
***************
In order to use a numeric value as the string value for a ‘Key’, it had to be disguised with a non-numeric character in it. “28″ = “_28″.
It performed as though there were already an ‘Option Infer” action going on behind the scenes.
Thanks for the Sunday reading, and keep up the great work.
Tom
Hi backatcha Tom!
Thanks for the nice comments about the site. We’ve been working hard on it for quite a while.
I wasn’t aware of the problem you reported, so I coded up a quick example to see if I could duplicate it. In brief, I couldn’t. It worked for me.
On the other hand, I had to modify the code in your message a bit. Maybe I haven’t hit exactly on what you were trying to illustrate. Here’s the code I wrote.
Public Class Form1 Private Sub Button1_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button1.Click ' has a single property "Name" Dim MySomeClass As New SomeClass Dim MyClassCollection As New Collection MySomeClass.Name = "Tom" MyClassCollection.Add(MySomeClass, "28") ' The next line will generate an error MsgBox("Name = " & MyClassCollection("28").Name) End Sub Public Class SomeClass Private m_Name As String Property Name() As String Get Return m_Name End Get Set(ByVal value As String) m_Name = value End Set End Property End Class End Class