VB Collection with ? gets an error

[paul1956]

Version Used:
VS 2017 15.5.2
Steps to Reproduce:

                   For Each a In node.AccessorList?.Accessors
                       ' Anything
                   Next

1. With the above code I would expect no errors
2. The problem is with the ?. before Accessors

Expected Behavior:
No Error
Actual Behavior:
Error BC32023 Expression is of type 'SyntaxList(Of AccessorDeclarationSyntax)?', which is not a collection type

[jcouv]

@AnthonyDGreen Can you confirm the expected behavior for this?

[paul1956]

FYI: In C# it works as expected.
'''
foreach (var a in node.AccessorList?.Accessors) {
}

[KathleenDollard]

@paul1956

I tried to repo this with the code below. Can you see why this is different and/or let me know which version of VB and .NET you're having this issue with.

Public Class Class1

    Public Sub Foo()

        Dim B As B = Nothing
        Dim x = B.A?.Bar

        For Each y In B.A?.Bar

        Next
    End Sub
End Class

Public Class B
    Public A As A
End Class

Public Class A

    Public Property Bar As IEnumerable(Of String)
End Class

[svick]

The question here is whether foreach works on a nullable struct enumerable (SyntaxList(Of AccessorDeclarationSyntax)? is such type).

In C#:

using System.Collections.Generic;

class C
{
    void M(StructList? list)
    {
        foreach (var x in list) { }
    }
}

struct StructList
{
    public IEnumerator<int> GetEnumerator() => null;
}

In VB:

Imports System.Collections.Generic

Class C
    Sub M(list As StructList?)
        For Each x In list
    	Next
    End Sub
End Class

Structure StructList
    Public Function GetEnumerator() As IEnumerator(Of Integer)
        Return Nothing
    End Function
End Structure

Indeed, the C# code compiles, while the VB code does not.

Interestingly, the C# spec says that this code shouldn't compile, but Roslyn deliberately violates the spec:

roslyn/src/Compilers/CSharp/Portable/Binder/ForEachLoopBinder.cs

Lines 508 to 509 in a550ee1

	// If collectionExprType is a nullable type, then use the underlying type and take the value (i.e. .Value) of collectionExpr.
	// This behavior is not spec'd, but it's what Dev10 does.

I didn't check the VB spec, but I assume the behavior of the VB compiler is correct. So I think there is nothing to do here, unless you'd like to propose that VB changes its behavior (which would belong to the vblang repo) or that the C# spec should be changed to accommodate how the compiler currently behaves (which would belong to the csharplang repo).

[jaredpar]

Closing as the compiler is behaving as designed for VB here.