Question

Why can I do the following :

Dim qNodes As IQueryable(Of XmlNode) = xDoc.ChildNodes.AsQueryable()
Dim test = qNodes.Where(Function(node) True)

although the following gives the error I stated in the title :

Dim qNodes As IQueryable(Of XmlNode) = xDoc.ChildNodes.AsQueryable()
Dim test = qNodes.Where(Function(node)
                               Return True
                           End Function)

?

I really don’t get it.

Answer 1

This is stated in section 11.1 of the VB.NET 10 Language Specification:

The exact translation between lambda
methods and expression trees may not
be fixed between versions of the
compiler and is beyond the scope of
this specification. For Microsoft
Visual Basic 10.0, all lambda
expressions may be converted to
expression trees subject to the
following restrictions:

1. Only single-line lambda expressions without ByRef parameters
   may be converted to expression trees.
   Of the single-line Sub lambdas, only
   invocation statements may be converted
   to expression trees.
2. Anonymous type expressions cannot be converted to expression trees if an
   earlier field initializer is used to
   initialize a subsequent field
   initializer, e.g. New With {.a=1,
   .b=.a}
3. Object initializer expressions cannot be converted to expression
   trees if a member of the current
   object being initialized is used in
   one of the field initializers, e.g.
   New C1 With {.a=1, .b=.Method1()}
4. Multi-dimensional array creation expressions can only be converted to
   expression trees if they declare their
   element type explicitly.
5. Late-binding expressions cannot be converted to expression trees.
6. When a variable or field is passed ByRef to an invocation
   expression but does not have exactly
   the same type as the ByRef parameter,
   or when a property is passed ByRef,
   normal VB semantics are that a copy of
   the argument is passed ByRef and its
   final value is then copied back into
   the variable or field or property. In
   expression trees, the copy-back does
   not happen.

All these restrictions apply to nested
lambda expressions as well.

It is the restriction in the first item that you are running into. The introduction leaves plenty room to assume that this will be worked on in future releases, there’s much to be gained of course. Connect.microsoft.com is a good place to go to encourage them. I couldn’t check if a feedback item was already opened for this, the site is on the fritz right now.