Closures in VB Part 6: Limitations

For previous articles in this series please see ...

• Part 1: Introduction
• Part 2: Method Calls
• Part 3: Scope
• Part 4: Variable Lifetime
• Part 5: Looping

As powerful as closures are in the language they do have a few limitations.  We worked hard in Orcas to put as few limitations in Orcas as possible.  Below are the current limitations and some insight into why they exist this way. 

1. Cannot use "ByRef" parameters in a closure

Example:

     Sub LiftAByRef(ByRef x As Integer)
        Dim f = Function() x
    End Sub

Message: error BC36639: 'ByRef' parameter 'x' cannot be used in a lambda expression.

The problem here is the expectation surrounding x.  Any change in the value of "x" inside the method "LiftAByRef" should be reflected in the calling function.   Normally for any lifted parameter we add a new field inside the closure and all read/writes are redirected into that value.  For "ByRef" parameters we would additionally have to ensure that all writes are make to the parameter.  Even in the presence of an exception.  Not a trivial task. 

2. Cannot use "Me" in a closure created inside a structure. 

Example:

 Structure S1
    Public F1 As Integer

    Public Sub M1()
        Dim f = Function() F1
    End Sub
End Structure

Message: error BC36638: Instance members and 'Me' cannot be used within a lambda expression in structures

Closures capture values by reference.  It's not possible to capture the "Me" of a structure by reference in VB.  The only other option is to capture them by value.  If we did that then all changes to members of a structure inside a lambda would not affect the structure in which they were created; merely the value copy.  This is very different behavior from every other place that closures are used.  To avoid confusing behavior this is not a legal operation. 

3. Cannot use a Restricted Type in a closure

Example:

     Sub LiftRestrictedType()
        Dim x As ArgIterator = Nothing
        Dim f = Function() x.GetNextArgType().GetModuleHandle()
    End Sub

Message: error BC36640: Instance of restricted type 'System.ArgIterator' cannot be used in a lambda expression.

This hopefully will not affect many users.  There are several types in the CLR that are considered restricted because they have special semantics.  Typically they are special cased by the CLR and as such we can't use them in a closure.  Several of these cannot be used in VB at all.  They are ...

• System.TypedReference
• System.ArgIterator
• System.RuntimeArgumentHandle

4. Cannot Goto into scope that contains a closure

Example:

     Sub BadGoto()
        Dim x = 0

        GoTo Label1
        If x > 5 Then
Label1:
            Dim y = 5
            Dim f = Function() y
        End If
    End Sub

Message: error BC36597: 'GoTo Label1' is not valid because 'Label1' is inside a scope that defines a variable that is used in a lambda or query expression.

If you look back at Part 3 of this series you will see that a lot of work goes into initializing closures inside of a scope.  Unfortunately allowing a user to jump into a block that contains a closure makes respecting these rules very difficult.  In a even trivial example in makes the resulting code mostly unreadable.  We decided to disable this in Orcas and reconsider it in a future release.

It is perfectly legal however to jump into any scope that is currently visible regardless of whether or not in contains a closure.  Because jumping into a visible scope does not affect the creation of a variable lifetime (just the ending), it does not add any complications to the code. 

5. Cannot mix "On Error Goto" and Closures

Message: error BC36595: Method cannot contain both an 'On Error GoTo' statement and a lambda or query expression.

Because of restriction #4 we must disable this scenario as well since it's very easy to hit this scenario with "On Error Goto".