Question

I can add a child using two methods, one is

Canvas.AddVisualChild(Visual);
Canvas.AddLogicalChild(Visual);

Where i draw in the visual’s DrawingContext giving corrdinates about where to place each item for example a line.

And second is:

Canvas.SetTop(Visual, location);
Canvas.SetLeft(Visual, (location);
Canvas.SetZIndex(Visual, someZIndex);

this.Children.Add(Visual);

Here I am setting the location directly, and then adding it to the children of the Canvas.

My questions are:

1. What is the difference between these methods.
2. I tried adding another canvas to the canvas, but the first method didn’t work, the second one did, why is it so since Canvas also has System.Windows.Media.Visual in its inheritance hierarchy.
3. In the first method, there is no mention of the position of the Visual, drawing details are in the DrawingContext of the visual, is there a way to retrieve them through code?
4. Which one is ideal in what cases?

Answer 1

The first set of code is not a proper usage of those methods. While those methods manipulate the state of the element passed into the method (namely changing the visual and logical parent respectively) the element whose method you are calling does not do any automatic storage of that information. Basically when one uses AddVisualChild then one has to override VisualChildrenCount and GetVisualChild and return that element. When one uses AddLogicalChild then one must override LogicalChildren and return that object as part of the enumerator it returns. The framework also expects these methods to be called in a specific manner and not doing so can lead to problems – definitely performance but could be others as well. These are advanced methods that are meant to be used by custom element developers who need to have control over what elements are considered logical and/or visual children of a given element. A panel for example will add the elements added to its Children as visual children – it might also add them as logical children if the items weren’t provided by the items host. I would recommend just adding elements to the Children collection of the panel. A decorator (like a Border) will add the Child as its visual/logical child. Each of these classes overrides the members I specified (as well as others like MeasureOverride and ArrangeOverride so it can measure/arrange these child elements).