When Expressions

[AdamSpeight2008]

When Expressions on Case Clause Statements

Within the language we are able to add a condition to a catch handler.

Catch ex As Expection When Retries < 3

I think we should also be able extend clause statement with a when expression.
So the body of the case it executed when both clause matches and the when expression evaluates to true.

Syntax

Select Case expr
  Case clause When expression
  Case Else
End Select

Syntax Node Layout

Syntax WhenExpression
  WhenKeyword  As WhenKeywordToken
  Expression   As ExpressionSyntax
End Syntax

The feature would also be useful, combined with #304
Eg

Select TypeOf obj
  Case Byte  Out value When value > 0
    '
  Case Int16 Out value When value > 0
  Case Int32 Out value When value < 0
  Case Else When
  Case Else
End Select

[bandleader]

100%, but I think this is anyway being considered as part of pattern matching.

[KathleenDollard]

Seems reasonable.

[AdamSpeight2008]

@KathleenDollard An example of a parsing a digit of a some specific number base using Case ... When ....

Enum Base
  None = 0
  BIN  = 2
  OCT  = 8
  DEC  = 10
  HEX  = 16
End Enum

Function TryParse_Digit(OfBase As Base, Here As Int32, <Out> Output As Digit) As Boolean
  ' Presumption: The parsing is going to fail, unless proven otherwise.
  ' So set default value for output to nothing.
  Output = Nothing
  Select Case Ch When TryGet(Here, Out Ch)
    Case "0"c To "1"c When OfBase >= Base.BIN : Output = New Digit( Here, Ch, OfBase )
    Case "2"c To "7"c When OfBase >= Base.OCT : Output = New Digit( Here, Ch, OfBase )
    Case "8"c To "9"c When OfBase >= Base.DEC : Output = New Digit( Here, Ch, OfBase )
    Case "A"c To "F"c When OfBase >= Base.Hex : Output = New Digit( Here, Ch, OfBase )
    Case "a"c To "f"c When OfBase >= Base.Hex : Output = New Digit( Here, Ch, OfBase )
  End Select
  Return (Output IsNot Nothing)
End Function