C# Extension Properties?

In C# 3 you can create an extension method like this:

public static class Extensions {
    public static string ToCamelCase(this string s) {
        // perform action
    }
}

Well what about properties? They could act as an indexer would, by using brackets around parameters:

public static class Extensions {
    public static object Cell[this DataTable t, int row, int column] {
        get { return t.Rows[row][column]; }
        set { t.Rows[row][column] = value; }
    }
}
// ...
// usage
dataTable.Cell[0, 2] = "Hello World";

That example is simple, but I think it could be a powerful concept. Take, for example, Extended Properties in WPF, which can't be added to the class. Extension Properties would allow a simpler syntax. There are also areas in the .NET Framework where a Property would be better suited but a "GetXXX(type param)" method was used because Parameterized Properties don't exist.

I dunno, it could end up cluttering the language. What do you think?

Yeah, I was hoping for extension properties too:

public static TimeSpan Minutes[this int i]
{
   get { return new TimeSpan(0, i, 0); }
}

public static DateTime Ago[this TimeSpan ts]
{
   get { return DateTime.Now.Subtract(ts); }
}

So then I could do:

DateTime d = 20.Minutes.Ago;

No ugly parentheses!

Exactly. It would be great to have these to get/set WPF attached properties. For Example, instead of writing this:

DockPanel.SetDock(btn1, Dock.Left);
DockPanel.SetDock(btn2, Dock.Left);
DockPanel.SetDock(btn3, Dock.Left);

You would write this instead:

btn1.Dock = btn2.Dock = btn3.Dock = Dock.Left;

Much cooler

mabster wrote:

Yeah, I was hoping for extension properties too:

public static TimeSpan Minutes[this int i]
{
   get { return new TimeSpan(0, i, 0); }
}

public static DateTime Ago[this TimeSpan ts]
{
   get { return DateTime.Now.Subtract(ts); }
}

So then I could do:

DateTime d = 20.Minutes.Ago;

No ugly parentheses!

Sweet. That looks Ruby'ish (not to be mistaken by Rubish). Not that I like Ruby.

How is 20.Minutes.Ago more confusing than DateTime.Now.Subtract(new TimeSpan(0, 20, 0))?

TommyCarlier wrote:

How is 20.Minutes.Ago more confusing than DateTime.Now.Subtract(new TimeSpan(0, 20, 0))?

lol. thanks for that.

TommyCarlier said:

How is 20.Minutes.Ago more confusing than DateTime.Now.Subtract(new TimeSpan(0, 20, 0))?

It's not.  However, there are side effects.  Having the ability to write DateTime.Now.Ago is definitely confusing.  The Ago property would show up on all DateTimes (provided the using statement is there).  The idea is good, though.  Having an int extension property named MinutesAgo seems better.

I too would have liked to see extension properties.  We have custom properties that we read from the DB and are generating them onto classes.  We are forced to use the ugly GetPropertyname() syntax.

GaryC said:

TommyCarlier said:

*snip*

It's not.  However, there are side effects.  Having the ability to write DateTime.Now.Ago is definitely confusing.  The Ago property would show up on all DateTimes (provided the using statement is there).  The idea is good, though.  Having an int extension property named MinutesAgo seems better.

I too would have liked to see extension properties.  We have custom properties that we read from the DB and are generating them onto classes.  We are forced to use the ugly GetPropertyname() syntax.

Actually, as he wrote it the "Ago" property would show up on TimeSpan, not on DateTime, which makes a lot more sense. So you wouldn't actually be able to do DateTime.Now.Ago.

Ion Todirel said:

TommyCarlier said:

*snip*

I meant the extension property declaration will be confusing and very close in definition to a indexer

The way I read it, the reason extension properties didn't make it in was because they couldn't reach consensus about a good syntax for declaring them.

mabster said:

Yeah, I was hoping for extension properties too:

public static TimeSpan Minutes[this int i]
{
   get { return new TimeSpan(0, i, 0); }
}

public static DateTime Ago[this TimeSpan ts]
{
   get { return DateTime.Now.Subtract(ts); }
}

So then I could do:

DateTime d = 20.Minutes.Ago;

No ugly parentheses!

Hah.  Reminds me of a query I saw recently.  At my new job, we communicate alot with an ancient DB2 system; someone posted a query like:

Select * from abc.defg
where dt = ? - 180 days

I was like, what? Awesome.

Then someone showed me that TOP 10 is now FETCH FIRST 10 ROWS ONLY.  And the awe was gone.

Don't get me started.

I was wondering about this feature as well. Could the syntax look something like this:

public static class Extensions {
    public static object Cell{
        get(this DataTable t, int row, int column) 
            { return t.Rows[row][column]; }
        set(this DataTable t, int row, int column) 
            { t.Rows[row][column] = value; }
    }
}

// usage
dataTable.Cell[0, 2] = "Hello World";

The usage would be the same, but the compiler would just have to translate this new syntax and it differs from indexing syntax.

public static TimeSpan Minutes
{
   get(this int i) 
    { return new TimeSpan(0, i, 0); }
}

public static DateTime Ago
{
   get(this TimeSpan ts) 
    { return DateTime.Now.Subtract(ts); }
}

They still look like properties.

Andres

I'm probably in the minority here, but I think the whole extension methods concept is confusing.  If you want to add a method to a class, then subclass it.  That's why inheritance exists, and it makes a lot more sense to me when I see MyTimeSpan.Ago rather than TimeSpan.Ago, because I might go searching the MSDN for documentation on TimeSpan.Ago only to find it doesn't exist.

dahat said:

spivonious said:

*snip*

>  If you want to add a method to a class, then subclass it.

What if the class you want to add a method to is sealed or worse, is a struct, preventing you from doing just that? Not unlike DateTime, TimeSpan or String?

Well, without the extension method language feature, you'd have to use the decorator pattern. If you only want to add a single method, that's potentially an awful lot of work, which is why I like extensions.

For me the real strength of extension methods is in adding type-inferenced extension methods to interface, so that classes inheriting that interface automatically get interface-related functionality for free (we discussed this a while ago over on this thread).


Herbie