Implementing DependencyProperty with a single attribute (and CciSharp)

In WPF, one needs to implement the DependencyProperty pattern
to make properties bindable. It usually involves a lot of boiler plate code:

  • adding a static field that names the depency property,

  • adding the instance property to the type (and make sure you follow the naming convention),

  • add validation methods and wire them in the dependency constructor

These are a number steps that need to be done again and again if you are building
new WPF controls.

This is where a little transformation using CciSharp really
helps with such boiler plate code. Using the DependencyAutoProperty mutator,
you can define dependency properties with a single attribute. The rest is taken and
validated by the compiler.

The simple scenario: Add [DependencyAutoProperty]

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The rewritten code will contain a dependency property for the Value property, and
will rewrite the value property getter and setters to use GetValue, SetValue instead.

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Supporting default values

Default values can be specified in the DependencyProperty register method, so we want
to support this as well through the DependencyAutoProperty constructor.

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Supporting validation

Callbacks can be passed in the constructor of DependencyProperty to validate the values
of the property. To support this, we use a simple naming convention: if you specify
the Validate = true constructor argument, the rewritter will look for a static “Validate”
+ property name method whose signature is Func<T, bool>, where T is the property
type. The rewritter will make sure this method is used – or raise a compilation error
if it is missing.

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An interresting point here is that the callbacks passed in the DependencyProperty.Register
method are untyped (Func<object, bool>) and the user needs to make the appropriate
casts himself. This is taken care of by the rewritter:

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Have fun!