Why are many properties in WPF an 'Object' instead of an interface?

Question

I may be missing something about the fundamentals of WPF design, but I was wondering why many properties on WPF controls are exposed as the type 'Object'?

For example, MenuItem.Icon is an Object, and so is MenuItem.ToolTip. As a near first time user, this was very confusing to me (it felt like I was using a dynamic programming language, having no idea whether setting ToolTip to a String type would even work or not). Moreover, I tried to set the Icon to a 'System.Drawing.Icon' and I get an ArgumentException of "Argument 'picture' must be a picture that can be used as a Icon." Shouldn't the property be typed so it can at least describe what in the world you're supposed to give it?

Honestly, my guess as to the reason is because you cannot implement an interface on a type you did not create (without creating a wrapper), but that's just a guess.

Thanks very much for your answers and insights!

Answer 1

The main reason in my opinion is that since an Object is the "ultimate base class of all classes in the .Net Framework". This gives you flexibility, in WPF you are not limited to a predefined type. Wpf is different and has a learning curve, but it does give you a lot more options to create a product that looks good.

i.e.

You can assign a TextBox to a ToolTip:

TextBox tb = new TextBox();
tb.Text = "Hello World";
this.ToolTip = tb;

a Bitmap

BitmapImage myBitmapImage = new BitmapImage(new Uri((@"C:\Temp\20100706.jpg")));
Image image = new Image();
image.Source = myBitmapImage;
this.ToolTip = image;

and assigning a Image to a MenuItem

BitmapImage myBitmapImage = new BitmapImage(new Uri((@"C:\Temp\20100706.jpg")));
Image image = new Image();
image.Source = myBitmapImage;
menuItem1.Icon = image;

Answer 2

Consider the ToolTip for example. A ToolTip is a ContentControl, which can contain any type of CLR (Common Language Runtime) object (such as a string or a DateTime object) or a UIElement object (such as a Rectangle or a Panel). This enables you to add rich content to controls such as Button and CheckBox.

For this reason, elements such as ToolTip are exposed as Object, that is the root of the type hierarchy (with resulting ease of use, flexibility and clarity of the code).