[Proposal] TypeOf over multiple types.

[AdamSpeight2008]

(Ported from Roslyn Repo)

TypeOf ... IsNot ..

If TypeOf obj IsNot TypeA AndAlso
   TypeOf obj IsNot TypeB Then

If TypeOf obj IsNot {TypeA, TypeB} Then

TypeOf ... Is ...

If TypeOf obj Is TypeA OrElse
   TypeOf obj Is TypeB Then

If TypeOf obj Is {TypeA, TypeB} Then

Grammar

TypeOf_Obj	 ::= "TypeOf" ws+ identifier
TypeOf_IsExpr    ::= TypeOf_Obj ws+ "Is" ws+ MatchAgaist
TypeOf_IsNotExpr ::= TypeOf_Obj ws+ "IsNot" ws+ MatchAgainst
MatchAgainst	 ::= Single | MultipleTypes 
SingleType       ::= typeIdentifier
MultipleTypes    ::= '{' typeIdentifier (ws* ',' ws* typeIdentifier)+ ws* '}'

Ruled out TypeOf obj Is ( Type0, Type1. Type2 ) as that conflicts with tuple literal syntax.

[rolfbjarne]

I'm not sure I like this, but an improvement might be to allow Or or And instead of commas to separate the types.

This means your examples become:

If TypeOf obj IsNot {TypeA And TypeB} Then
If TypeOf obj Is {TypeA Or TypeB} Then

And it also allows you to test if a type implements multiple interfaces:

If TypeOf obj Is {InterfaceA And InterfaceB} Then

A downside is that it sounds grammatically weird in the negative case (IsNot .. Nor would sound better than IsNot ... And, but that would introduce another keyword for a dubious reason).

[AdamSpeight2008]

The issue with TypeA And TypeB and TypeA Or TypeB is that they are operators, and would affect the order of evaluation. Eg

If TypeOf obj IsNot {TypeA And TypeB} Then
' becoming 
If TypeOf obj IsNot {Boolean} Then

[rolfbjarne]

The And and Or keywords would have to be treated like something other than operators in this context.

[AdamSpeight2008]

Alternative Syntax is to use the syntax of generic type parameters.

TypeOf obj Is (Of T0, T1, T2)
TypeOf obj IsNot (Of T0, T1, T2)

[Bill-McC]

I wonder if we can relax the rules around Select Case such that Is doesn't require a comparison operator when using a Select Case TypeOf(obj)
eg:
`Select Case TypeOf(obj)
Case Is T1, T2, T3

Case Else

End Select`

[aarondglover]

If Mads and the other c# fan boys didnt believe that Pattern Matching is too much for the simple brain of a VB developer then there wouldn't be much need to do the select case approach and VB would have a powerful feature.

[AdamSpeight2008]

The use case I've seen are in the expression part of an if-statement.
Select Case ... End Select isn't an expression.

[AdamSpeight2008]

@AnthonyDGreen
I am looking at implementing this feature, in the form.
TypeOf obj Is (Of T0, T1) TypeOf obj Is { T0,T1 } (in a prototype form)

Initial research suggest that I'd only need to change the binder to bind to generic type parameter list and new type TypeArray. Then in lowering apply the compiler generated transform the to the expanded version.