Mebyon Kernow

New .NET Attributes

The very first talk I gave on .NET was on custom attributes – some time way back in late 2000 or early 2001. At the time I was espousing the benefits of using custom attributes to add on details of unit tests and bug fixes.

There have been many additions to .NET since then and I spied a couple of *really* cool new attributes in .NET 4 the other day so had to share them.

Enter (drum roll please) :-

• [Optional]
• [DefaultParameterValue]

Yay!

These are (I believe) a real productivity boost, and will make your code much easier to read. I bet you have some code like the following hanging around, don’t you…

public class Thingy
{
    public Thingy()
        : this("Default", -1)
    {
    }

    public Thingy(string name)
        : this(name, -1)
    {
    }

    public Thingy(string name, int whatever)
    {
        _name = name;
        _whatever = whatever;
    }

    // Other code...

    private string _name;
    private int _whatever;
}

Now there’s nothing desperately wrong with this, although I feel that 3 constructors is a bit ugly, I’d rather just have one. And what if I want to change the default value for the ‘whatever’ value – there are two places it’s used in the code above and I’d need to update both which is a bit on the fragile side.

With these swanky new attributes I can have just the one constructor…

public class Thingy
{
    public Thingy([Optional, DefaultParameterValue("Default")] string name,
                  [Optional, DefaultParameterValue(-1)]int whatever)
    {
    }
}

Now that’s a whole lot better if you ask me! My defaults are in one place, I have only one constructor rather than three (obviously this works on methods too), and Intellisense and the C# environment understand the [Optional] attribute and present the appropriate stuff to me.

Now as if that wasn’t enough we’ve updated C# to support this without even having to write attributes (thanks to Wes Haggard for pointing this out to me)…

public class Thingy
{
    public Thingy(string name = "Default",
                  int whatever = -1)
    {
    }
}

Ooh, how cool is that? All in all I’m very happy with this little addition to .NET and C#.