Why can't I use the Name attribute on UserControl in the same assembly?

Question

When I created a WPF UserControl and tried to use it, I got the following compiler error:

Because 'UserControl1' is implemented in the same assembly, you must set the x:Name attribute rather than the Name attribute.

The message is clear on what to do to fix it, but what is its cause? Why can't I use Name here?

Answer 1

x:Name is simply a more explicit way of saying "The name attribute in this specific XML namespace". The fact that WPF can't compile it without being given this hint because it's in the same assembly is just a limitation of how they wrote the parser.

If you are asking why it is this way, I do not know for sure because I didn't write it. It probably has something to do with it needing to be able to resolve the Name attribute (read: Dependency Property) to something concrete BEFORE building your UserControl1, in other words, a catch-22.

Answer 2

In the beginning the XAML compiler was written to enable to creation of “trees” of .net objects, there were 101 projects within Microsoft that used XAML. The XAML compiler does not know anything about WPF.

The “Name” property is defined by WPF and is not known about by the XAML compiler. WPF maps the name property to be the same as the “Name TAG” that is supported by the XAML compiler. The “x” in “x:name” is saying use “name” as defined by the XAML xml schema, “Name” says find a property called “name” on the given object. See In WPF, what are the differences between the x:Name and Name attributes? for more details.

The XAML compiler is very limited in what it can do with a user control without having to load the assembly that defines the user control. As the XAML needs to be compiled before the assembly can be loaded, the xaml compiler clearly can’t load the assembly for a control that is implemented in the same assembly. Therefore the XAML compiler does not even know the item is a user control.

Properties that are defined on a user control (or its parent class) can therefore not be accessed. “Name” is a property that is defined in the parent (or super-parent) of a custom control.

The XAML compiler could just say “Name is not defined as a property”; if it did, think about how many people will fail to get a simple custom control working! Therefore the XAML compiler has a special case that gives a more useful error message, by “guessing” what the code means. Its guess are mostly correct.

Anything apart from the most simple user control needs to be in its own assembly, however user simple control are so common that a special case was considered worthwhile for them.