CSharp/C#, Extension methods for value types

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CSharp/C# >> Extension methods for value types

by Jesper Lund Stocholm » Thu, 21 Aug 2008 02:43:21 GMT

I am currently working on converting an old .Net 1.1-application to .Net
3.5. Due to the lack of nullable datatypes in those days, a "decimal
wrapper" was created that had various methods to parse objects (of
varying types) as well as methods to determine if the parsed decimal was
"valid", i.e. not null.

With nullable types we have some of it covered when creating the
functionality in .Net 3.5, but I am unsure how to create the
"sophisticated parsing". I thought about implementing an extension method
to facilitate code like

decimal? d = default (decimal);
d.ParseMe(1.2d);
if (d.HasValue)
{
// do stuff
}

A simple version of .ParseMe() would be

public static decimal? ParseMe(object o)
{
decimal? d = default(decimal);
try
{
d = Decimal.Parse(o.ToString());
}
catch {}
return d;
}

So my question is: Can I make an extension method on a value type - and
how would I do this in my case?

Thanks, in advance

:o)

--
Jesper Lund Stocholm
http://idippedut.dk
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CSharp/C# >> Extension methods for value types

by Peter Duniho » Thu, 21 Aug 2008 03:07:33 GMT


On Wed, 20 Aug 2008 11:43:21 -0700, Jesper Lund Stocholm



You can extend classes and interfaces, not value types.

But I'm not clear on why you want to do this. Why not just call
Decimal.Parse() or Decimal.TryParse() directly in your own code?

Pete
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CSharp/C# >> Extension methods for value types

by Peter Duniho » Thu, 21 Aug 2008 03:13:50 GMT

On Wed, 20 Aug 2008 12:07:33 -0700, Peter Duniho




Sorry, spoke too soon. You can extend value types too.

But I still don't know why you want to. :)
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CSharp/C# >> Extension methods for value types

by Jon Skeet [C# MVP] » Thu, 21 Aug 2008 03:36:25 GMT


<snip>


I've found the following methods very handy for unit testing, when I
want a quick way of getting dates and times. They're not what you'd
often use in production (where you're typically specifying the month
via a variable rather than effectively hard-coding it) but "just for
unit tests" is still useful :)

public static DateTime January(this int day, int year)
{
return new DateTime(year, 1, day);
}

public static DateTime February(this int day, int year)
{
return new DateTime(year, 2, day);
}

etc

Then:

public static TimeSpan Milliseconds(this int milliseconds)
{
return TimeSpan.FromMilliseconds(milliseconds);
}

public static TimeSpan Seconds(this int seconds)
{
return TimeSpan.FromSeconds(seconds);
}

public static TimeSpan Minutes(this int minutes)
{
return TimeSpan.FromMinutes(minutes);
}

etc.


So then we have in the unit tests:

DateTime birthday = 19.June(1976) + 8.Hours();

Very readable, IMO.

--
Jon Skeet - < XXXX@XXXXX.COM >
Web site: http://www.pobox.com/ ~skeet
Blog: http://www.msmvps.com/jon.skeet
C# in Depth: http://csharpindepth.com
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CSharp/C# >> Extension methods for value types

by Jesper Lund Stocholm » Thu, 21 Aug 2008 03:37:45 GMT


Ok - thanks for clearing this out. Earlier today I stumpled over this
article [0] - so he is wrong? As I read the code (that I cannot get to
work or even compile, btw) it seems he is doing exactly that.

Well, I am a little unsure of this too. The issue is that the existing
fucntionality has the following ctor:

class MyDecimalWrapper
{
private decimal m_decimal = 0;

public MyDecimalWrapper(object o)
{
// ...
}

public override string ToString()
{
if (IsValid)
{
return "";
}
else
{
return m_decimal.ToString();
}
}
}

Where o can be literally everything. The method parses this object and
sets a property ( .IsValid ) if conversion goes through. This enables
me to assigning e.g. the ToString()-value to controls etc without worrying
about the internal value being valid (or null) and thus throwing a
NullReferenceException.

With the nullable decimal I have the .NasValue-property that I can use
to replace the above - but the method Decimal.TryParse() does not
accept a nullable decimal ... just the "regular" one.

I am very much open for suggestions to how to accomplish this.

:o)

Thank you for your time so far.

[0] http://www.codemeit.com/code-collection/c-extension-method-getvalueornull.html

--
Jesper Lund Stocholm
http://idippedut.dk
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CSharp/C# >> Extension methods for value types

by Arne Vajh » Thu, 21 Aug 2008 07:25:40 GMT


For inspiration:

using System;

namespace E
{
public static class MyExtensions
{
public static decimal? SpecialParse(this decimal o, string s)
{
decimal res;
if(decimal.TryParse(s, out res))
{
return res;
}
else
{
return null;
}
}
public static decimal? ToDecimal(this string s)
{
decimal res;
if(decimal.TryParse(s, out res))
{
return res;
}
else
{
return null;
}
}
}
public class Program
{
public static void Main(string[] args)
{
Console.WriteLine(default(decimal).SpecialParse("123,45"));
Console.WriteLine(default(decimal).SpecialParse("ABC") ==
null);
Console.WriteLine("123,45".ToDecimal());
Console.WriteLine("ABC".ToDecimal() == null);
Console.ReadKey();
}
}
}

Arne
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CSharp/C# >> Extension methods for value types

by Peter Duniho » Thu, 21 Aug 2008 08:25:42 GMT

On Wed, 20 Aug 2008 12:36:25 -0700, Jon Skeet [C# MVP] < XXXX@XXXXX.COM >




Sorry...my most recent statement is easily misunderstood out of context.

It's not that I don't see a use for extension methods on value types
generally. It's just that the OP's example doesn't seem to call for an
extension method. Maybe I'm missing something here, but it seems like he
ought to just call Parse() directly.

If he were actually returning null from the extension method on failure,
it might make more sense to me. But he's not, so it doesn't. :(

Pete
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CSharp/C# >> Extension methods for value types

by Peter Duniho » Thu, 21 Aug 2008 08:29:42 GMT

On Wed, 20 Aug 2008 12:37:45 -0700, Jesper Lund Stocholm



As I mentioned in my immediate follow-up, my statement about not being
able to extend value types was incorrect.

As for further suggestions, I'm still having trouble understanding your
own examples, but it seems to me that the examples that Arne posted might
be useful. They do make more sense to me, anyway :), in that they return
null on failure. Especially the ToDecimal() method (I'm not particularly
fond of the use of a dummy default value for the SpecialParse() method).

Pete
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CSharp/C# >> Extension methods for value types

by Jesper Lund Stocholm » Sun, 07 Sep 2008 05:31:48 GMT

Arne Vajh鴍 < XXXX@XXXXX.COM > wrote in





[snip code]

Thanks, Arne - that was exactly what I was looking for.

:o)

--
Jesper Lund Stocholm
http://idippedut.dk
Top
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