I'm trying to figure out what exactly Dependency Properties are, but when I look anywhere for a definition, I only find "how to use" but not "what it is". Imagine you are asked on a job interview - what is a dependency property. What would be your answer?
A DependencyProperty is a property whose value depends (or can depend) on some other source (such as animation, data binding, styles, or visual tree inheritance). A regular property's value is stored in the object it belongs to, while you can think of a dependency property as being stored in a database somewhere. This database is essentially composed of a dictionary that maps (object, property) pairs to their values, along with a mapping of which properties depend on other properties (e.g. so when you change the DataContext of a Panel, it can notify all the children inside the panel).
So why do they store property values in some magic database somewhere? There are a few reasons:
It reduces storage space. Adding a property (even if its value is null) to a class adds 4 bytes (8 for a 64-bit process) of space to every instance of the class. A DependencyProperty only takes up space when an instance has a value. For example, a FrameworkElement has dozens of dependency properties, most of which are never assigned values. If all those properties were stored in the class, each instance would be hundreds of bytes. Instead each instance is only about 40 bytes.
It enables attached properties. Properties like
Canvas.LeftandGrid.Rowhave to be stored on objects that have never heard of aCanvasorGrid, so where do you put them? You put them in a database somewhere.It enables automatic property changes. Imagine how you would implement something like styles or property inheritance (the ability to set something like a font or data context on a parent element and have its value propagate to all child elements). Having all of this stored in a database makes it so the code is all in one place instead of being implemented separately for each object and property that needs it.
"gives you a bunch of infrastructure to do all the things that you often want to do with a normal property - validate it, coerce it into a proper range, give out change notifications, and a number of other aspects."
A dependency property is a property that is backed by the WPF property system instead of by a field in the declaring class.
The significance of this is that, because WPF owns the property, WPF can factor in various considerations when calculating the property value -- such as animations, styles and data bindings. Another consequence is that because properties are managed by WPF they don't have to be declared on the classes that conceptually have the state: hence, atttached properties, which allow e.g. a Grid to associate Grid-specific state with non-Grid objects.
(By the way, I've mentioned WPF above because this is the main framework that uses DPs, but Windows Workflow Foundation also has the notion of dependency properties. So to be strictly correct a DP is a property that is backed by an external property system, specifically one which allows factors other than "the last set value" to come into play when getting the property value.)
A dependency property is a property where the current value is dependent (hence the name) on other aspects such as default values, validation, coercion, value inheritance or animation.
Also a depedency property has inbuilt support for change notifications, data binding and styling.
What are they?
A bunch of classes and defined ways to use them that provide the above functionality to be used on classes that inherit from DependencyObject.
They are not a language feature. They are a feature of the .NET Framework.
A dependency property depends on multiple providers for determining its value at any point in time. These providers could be an animation continuously changing its value, a parent element whose property value propagates down to its children, and so on.
Arguably the biggest feature of a dependency property is its built-in ability to provide change notification.
Whenever the value of a dependency property changes, WPF can automatically trigger a number of actions, depending on the property’s metadata. These actions can be re-render- ing the appropriate elements, updating the current layout, refreshing data bindings, and much more. One of the most interesting features enabled by this built-in change notifica- tion is property triggers, which enable you to perform your own custom actions when a property value changes, without writing any procedural code
来源：https://stackoverflow.com/questions/2505234/need-a-short-and-clear-definition-for-dependency-properties