[antlr-interest] Value types - can't be set to null in C#

[Harald Mueller]

datetime returns [DateTime value = DateTime.Now]

-------- Original-Nachricht --------
> Datum: Mon, 7 Jan 2008 08:14:21 -0800 (PST)
> Von: Shawn Poulson <spoulson3 at yahoo.com>
> An: antlr-interest at antlr.org
> Betreff: [antlr-interest] Value types - can\'t be set to null in C#

> Hello,
> 
> Now that I've gotten setup on using ANTLR, I found a small snag that
> doesn't seem to be mentioned in the Wiki or Google for that matter.
> 
> I'm building a grammar that parses a datetime string and returns a
> DateTime object in C#.  So far the grammar looks like:
> 
> ----- 8< ----- 8< -----
> prog: dt=datetime {Debug.WriteLine("datetime=" +
> $dt.value.ToString());};
> 
> datetime returns [DateTime value]:
>    (h=hour24 {Debug.WriteLine("hour24=" + $h.value.ToString());}
>    ':' m=minute {Debug.WriteLine("minute=" + $m.value.ToString());}
>    ':' s=second {Debug.WriteLine("second=" + $s.value.ToString());}
>    '.' ms=millisecond {Debug.WriteLine("millisecond=" +
> $ms.value.ToString());})
>    {
>       $value = new DateTime(2008, 1, 4, $h.value, $m.value, $s.value,
> $ms.value);
>    };
>       
> 
> hour24 returns [int value]:
>    INT {$value = int.Parse($INT.text);};
> 
> minute returns [int value]:
>    INT {$value = int.Parse($INT.text);};
> 
> second returns [int value]:
>    INT {$value = int.Parse($INT.text);};
> 
> millisecond returns [int value]:
>    INT {$value = int.Parse($INT.text);};
> 
> INT: '0'..'9'+;
> WS: (' '|'\t'|'\r\n'|'\r')+ { $channel = HIDDEN; };
> ----- 8< ----- 8< -----
> 
> It generates ok with ANTLR, but when I compile in Visual Studio 2005 I
> get two identical errors for "prog" and "datetime" rules:
> 
> "Cannot convert null to 'System.DateTime' because it is a value type"
> 
> One of the offending lines is:
> ---- 8< ----- 8< -----
>     public void prog() // throws RecognitionException [1]
>     {   
>         DateTime dt = null;
> ...
> ---- 8< ----- 8< -----
> 
> Is there a way I can set the initialization to be something arbitrary,
> like DateTime.Now?
> 
> Thanks.
> 
> ---
> Shawn Poulson
> spoulson at explodingcoder.com

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