How do You Create a Read-Only Dependency Property?

Question

How do you create a read-only dependancy property? What are the best-practices for doing so?

Specifically, what's stumping me the most is the fact that there's no implementation of

DependencyObject.GetValue()  

that takes a System.Windows.DependencyPropertyKey as a parameter.

System.Windows.DependencyProperty.RegisterReadOnly returns a DependencyPropertyKey object rather than a DependencyProperty. So how are you supposed to access your read-only dependency property if you can't make any calls to GetValue? Or are you supposed to somehow convert the DependencyPropertyKey into a plain old DependencyProperty object?

Advice and/or code would be GREATLY appreciated!

Solution

It's easy, actually (via RegisterReadOnly):

public class OwnerClass : DependencyObject // or DependencyObject inheritor
{
    private static readonly DependencyPropertyKey ReadOnlyPropPropertyKey
        = DependencyProperty.RegisterReadOnly(
            nameof(ReadOnlyProp),
            typeof(int), typeof(OwnerClass),
            new FrameworkPropertyMetadata(default(int),
                FrameworkPropertyMetadataOptions.None));

    public static readonly DependencyProperty ReadOnlyPropProperty
        = ReadOnlyPropPropertyKey.DependencyProperty;

    public int ReadOnlyProp
    {
        get { return (int)GetValue(ReadOnlyPropProperty); }
        protected set { SetValue(ReadOnlyPropPropertyKey, value); }
    }

    //your other code here ...
}

You use the key only when you set the value in private/protected/internal code. Due to the protected ReadOnlyProp setter, this is transparent to you.

Comments

[Rahul Sonone]

Can you please one real time example or requirement where we use, this ReadOnly dependency property?

[Josh Noe]

@RahulSonone anytime you don't want the property to be set from outside the control class. If you don't make it read only, you can set it in XAML etc.

[Flynn1179]

ActualWidth/ActualHeight of any control are one good example.

[StayOnTarget]

You might want to update "ReadOnlyProp" to nameof(ReadOnlyProp) because a lot of people probably copy/paste this and they might as well use the current & improved syntax.

[Kenan E. K.]

No problem, thanks for the suggestion. To be honest, this is an old answer and I am not "maintaining" updated versions of answers myself. Also, these days I'm much more on javascript than C#.