Saturday, September 19, 2009

Impress your colleagues with your knowledge about... the ICustomTypeDescriptor interface.

Sometimes when working with C# you discover some hidden gems. Some of them very usefull, other ones a little bit harder to find a good way to benefit from their functionality. One of those hidden gems that I discovered some days ago is the ICustomTypeDescriptor interface.

This interface lives in an obscure part of the System.ComponentModel namespace where a lot of other interesting components could be found. As I had no clue about the possible usage of this interface, I started to play around with it. It seems that the ICustomTypeDescriptor interface can be used to craft at run time your own pseudo-properties that wrap each of an object's fields. What does this mean?

For example it makes it possible to display and edit fields in the WinForms PropertyGrid control. This feature also works in a databinding context.When binding an object to a Windows Form or ASP.NET control, the Framework uses reflection to get bindable properties of the object. However, when an object implements the ICustomTypeDescriptor interface, the framework gets object properties from ICustomTypeDescriptor.GetProperties() method. So the solution is to include all the properties we need to display.

In this way it allows you to bind to not only properties but also the fields of an object. You fool the databinding system and let it think that your object has some extra properties available. You even can create properties that don't exist at all!

I don't see immediately a situation where I can use this interface but I'm sure I'll find something ;-). In the meanwhile you can use it to impress your colleagues with your knowledge of C#.

If you want to know more about it, I suggest reading ICustomTypeDescriptor, Part 1 and ICustomTypeDescriptor, Part 2 articles in the MSDN magazine.

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