Dependency Property Uses in WPF

Question

I am having a hard time figuring out good reasons for the dependency property. Why the System.Controls.TextBox "Text" property a dependency property and not a normal property? What benefits does it serve being a dependency property?

One of the things I am trying to accomplish is to add a ValidationRules property to my UserControl which will contain other validation rules. Like here:

<customControls:RequiredTextBox.ValidationRules>
                        <validators:NotNullOrEmptyValidationRule ErrorMessage="FirstName cannot be null or empty"/>
                    </customControls:RequiredTextBox.ValidationRules>

The problem is that I am not sure if ValidationRules property should be DependencyProperty or just a normal property.

The above code gives the following error:

{"Cannot add element to 'ValidationRules'; the property value is null.  Error at object 'LearningWPF.ValidationRules.NotNullOrEmptyValidationRule' in markup file 'LearningWPF;component/addcustomerwindow.xaml' Line 35 Position 66."}

Here is the ValidationRules property:

 public static readonly DependencyProperty ValidationRulesProperty =
            DependencyProperty.Register("ValidationRules",
                                        typeof (Collection<ValidationRule>), typeof (RequiredTextBox),
                                        new FrameworkPropertyMetadata(null)); 

        public Collection<ValidationRule> ValidationRules
        {
            get { return (Collection<ValidationRule>)GetValue(ValidationRulesProperty); }
            set { SetValue(ValidationRulesProperty, value); }
        }

Answer 1

The benefits are primarily two fold:

Firstly a dependency property is only created when it is used, this means the TextBox class can be very efficient, with a low memory footprint since it has a minimal number of real properties taking up space on the heap. This is especially important in WPF where all controls are just collections of more and more specific types. If each of those inner types declared tens of properties to define behaviour and look then a high level control like a button would end up having the size of a class with something in the ballpark of a hundred properties.

Secondly dependency properties can be tied to an object other than the type that they are created for. This allows the case where a control can set a Grid.Column property, which the Grid control can read and use for layout. This means that we don't hundreds of decorator classes supplying tiny pieces of functionality required by other controls. This means that xmal is far more intuitive and readable.

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Edited to address the example in your revised question:

While your validation property won't gain much benefit from being a dependency property (basically out of the reasons in all the answers so far I can only really see my comment of memory footprint coming in to play), and it certainly isn't advantageous as it is in the case of the Text property of a text box where you may want to bind it, or change it based on some other input, I would still implement it as a dependency property. My reasoning for this is simple; you don't gain much, but it also doesn't cost you anything - I have never wished I had used a basic property in a custom control whereas when I first started writing them I was constantly upgrading my basic properties to dependencies because I wanted some extra functionality.

Simply put, while the dependency property is more complex to define that a normal property I would still use it as the de facto standard for WPF controls unless there was some good reason to do otherwise. In much the same way as a property is the standard for classes, even though a field is easier to implement.

Answer 2

The primary benefits I would say are:

1. First-class data binding support.
2. Clean Attached Property semantics
3. Property values which "depend".

That last point is key

Before Dependency Properties, having a value have a local value, an animatable value, an overridable value, a styleable value, a templatable value would require the declaration of multiple Properties/Fields/Dictionary entries, as well as complex state+precedence management.

Dependency Properties give you all these features out of the box, while declaring just ONE property.

That being said, in your case, you may not want to declare your ValidationRules as a DependencyProperty if you won't need to take advantage of these features.

If you do, you'll want to have different handling for your collections (non-empty collections for example). In this particular example, I would use Reflector and see how the .NET TextBox implements their validation collections, and see if you can reuse or copy the code.

There's no point re-inventing the wheel unless you're certain your wheel will be better. My personal experience is that my reinvented wheels tend to be missing things ;).

As Martin Harris already pointed out, DependencyProperties can limit the memory footprint by throwing the property values into a dictionary, however this can (and I believe was?) done by MSFT before the advent of DependencyProperties.

Martin also mentions Attached Properties, but those were also available (at least in designer) before the advent of DependencyProperties. The Attached Property implementation using DependencyProperties is much cleaner.

Answer 3

Dependency Properties are required if you want to use binding to populate the value of a property. If it was just a normal property, you wouldn't be able to bind the Text property to a property of your View Model object, for example.