Override a property defined in base class

Question

I have case where the class hierarchy is something like this,

   +---------------+
   | UIElement     |
   |---------------|                            +----------------------+
   | ...           |                            | My Windows Application
   | SomePropert{} |                            |----------------------|
   |               |<---+                       |+--------------------+|
   |               |    |                       ||MyUserControl       ||
   +---------------+    |                       ||--------------------||
         +--------------+-----+                 ||                    ||
         |FrameWorkElement    |                 |+--------------------+|
         |--------------------|                 |//Want to use         |
         |    ...             |<-+              |// SomeProperty;      |
         +--------------------+  |              |                      |
                     +-----------+-+            |                      |
                     |Control      |            |                      |
                     |-------------|            +----------------------+
                     |  ...        |<---+
                     +-------------+    |
                            +-----------+---+
                            | UserControl   |
                            |---------------|<---+
                            |  ...          |    |
                            +---------------+    |
                                      +----------+-------+
                                      | MyUserControl    |
                                      |------------------|
                                      | SomeProperty{}   |
                                      | //Want to override
                                      |                  |
                                      +------------------+

Now in my app (and all other apps where I can export this MyUserControl) can I set the SomeProperty that is handled by the MyUserControl class rather than UIElement?

I am right now doing this by creating an object to MyUserControl and assigning that to the control that I added in my xaml page.So right now looks like this,

MyUserControl newControl = new MyUserControl();
web = windowsPhoneControl11;  //windowsPhoneControll1 is the 
                              //one that I added from toolbox.
                              // i.e., mycustomecontrol added from toolbox.

So now since I am using the 'new' it gets overriden. But when I export this control I can't expect the user to create a new object and assign it to the control that one is using in the xaml page.

So is there any other way I could override this one property so that the assignment of that property is handled by MyUserControl class rather than the UIElement class? What I mean about MyUserControl having the control to set this property is that I need to check for some value before assigning it. If it is not atleast an expected value then I need to set it to a default value.

Ps: I am sorry for such a long question but I couldn't express it more precise and I was not able to find anyother question related to this. And it is WindowsPhoneApp... Not the ordinary windowsapplication.

Solution

Forgive me if I've interpreted this incorrectly but would the following work:

public class BaseClass
{
    public int MyProperty
    {
       get; set;
    }
}
public class ChildClass : BaseClass
{
    public new int MyProperty
    {
       get
       {
           return base.MyProperty;
       }
       set
       {
           if(DoYourCheckingStuff(value))
           {
               base.MyProperty = value;
           }
       }
    }
}

Didn't test this.

Although this feels like a really hack-ish way of doing it. What property are you actually trying to 'have control' over? Since there may be easier ways of doing this.

An example: Change a UserControl so that it's width can't be set between 100 and 200 (Although this is probably a pretty bad way to do it), by hiding it's Width property:

public class MyUserControl : UserControl
{
    public new double Width
    {
        get
        {
            return base.Width;
        }
        set
        {
             if(!(value > 100 && value < 200))
                 base.Width = value;
        }
    }
}

Comments

[Tigran]

what is the meaning of MyUserControl newControl = new MyUserControl(); this line in code provided?

[Kevin Gosse]

One vote for the ASCII diagram :D If the base property isn't set as 'virtual', you have no way to override it other than new (with the drawback you've mentioned). But maybe you can reach your goal another way. Which property are you trying to override? Also, you can wrap your control inside of a custom control, and set the properties as you wish.

[Henk Holterman]

Since it's XAML, is this about a Dependency or Attached Property? It's quite a picture but you'll need to provide some more code too.

[Henk Holterman]

UIElement is a library class so please just name SomeProperty. Then someone can look up the specs. The answer: it depends.

[Ajai]

@Tigran: MyUserControl newControl = new MyUserControl(); signifies that I am already having a control in my xaml form and that I am creating a new object for MyCustomeControl and assigning it with the control that I added in xaml.

[Ajai]

@all: I have no control over the base classes that I had mentioned.. So all I can do is to play with my MyUserControl class.

[Ajai]

@HenkHolterman: I am trying to create a new webbrowser control and I am trying to override isHitTestVisible property in UIElement class. I want to override it because I need to check something before assigning it.

[Ajai]

@KooKiz I want to override the isHitTestVisible property for the webbrowser control defined in UIElement class.

[Henk Holterman]

possible duplicate of WPF use override Property in inherit class in XAML

[Kevin Gosse]

@HenkHolterman It isn't a duplicate. You can't override a dependency property's metadata with WP7, so he can't use the same solution.

[Henk Holterman]

@kookiz - you're right, I overlooked the WP7 tag.

[Kevin Gosse]

That is, if the base method is virtual or abstract.

[mircea]

You should use "new" instead of "override". "override" is used for "virtual" properties.

[Ajai]

I have no control over the base classes. I can just do my changes in MyUserControl class..

[K893824]

You don't need any control over the base classes. See my example. You just 'hide' the base classes' property and then create a sort of 'wrapper' for it, adding whatever you want.

[Ajai]

Yes I actually got it now! Thanks for the suggestion.

[K893824]

No problem, glad I could help.

[Henk Holterman]

Just a pity it makes no sense (at all) for a Dependency property.

[Kevin Gosse]

But note that if the user changes the property by using the base class, you won't be notified: ((UIElement)yourControl).IsHitTestVisible = false;

[Ajai]

@KooKiz: That is interesting. But I just now tried it and I was not able to assign the property without without skipping my logic.

[Admin]

Wow, that is not advisable; its an awful hack. Solution should be to not name your property the same as one in the base class.

[Ajai]

@Will: I agree with you. This might not be the best design decision for designing a control. But I deliberately need to override it as I am not the one who designed the base class and I need to check for a few conditions before setting that one particular property.

[IAbstract]

Using the new keyword is not overriding the member, it is hiding the member - big difference. And, as @Will states in a comment above ...*'that is not advisable'*

[IAbstract]

I am completely with @Will: at least you should rename the method/property.

[Chris McCowan]

Common problems are 1) base functions will still access the base property. 2) BaseClass thing = new ChildClass(); thing.Age = 2; // this acts on BaseClass.Age not ChildClass.Age