Question

I have the following code that works fine in VB.net ( VS 2012 ). Note that m.Listen is returning an IObservable(of Maybe(of NominalObject))

    Dim d As IDisposable = (From x In m.Listen(Of Maybe(Of NominalObject))()
            From y In x
            Select New LAPViewCommands(y)).
            BindToControl(Me, Function(x) x.ViewModel)

    d.DisposeWith(Me)

This is part of a custom user control and I have some extension methods for
binding IObservables to models. However the details are irrelevant. When
I change the code to

    (From x In m.Listen(Of Maybe(Of NominalObject))()
            From y In x
            Select New LAPViewCommands(y)).
            BindToControl(Me, Function(x) x.ViewModel).
            DisposeWith(Me)

I have a compile error. Note there is no problem with the fluent syntax.

    m.
        Listen(Of Maybe(Of NominalObject)).
        SelectMany(Function(x) x.Select(Function(y) New LAPViewCommands(y))).
        BindToControl(Me, Function(x) x.ViewModel).
        DisposeWith(Me)

Is this problem a strange edge case of the VB.NET parser or is there something
important here I’m missing? In summary the question is. Why can’t I bracket a
LINQ query expression and then use fluent extension methods that return void

Answer 1

The problem is that your expression doesn’t start with an identifier.

It’s a limitation of VB.Net and not limited to LINQ query expressions.

---

Compare the following C# code (Dump is an extension method)

"123".Dump();

(from a in "1223" select a).Dump();

new object().ToString();

which will work fine. Here’s the VB.Net equivalent (which more or less looks the same):

"123".Dump()

(From a in "123" Select a).Dump()

(New Object()).ToString()

and each of the three statements will throw a syntax error.

You can fix it by using the Call statement:

Call "123".Dump()

Call (From a in "123" Select a).Dump()

Call New Object().ToString()