Implicit styles in Application.Resources vs Window.Resources?

Question

I was looking at this question and noticed that placing an implicit `TextBlock` style in `Application.Resources` applies that style to all TextBlocks, even those inside other controls such as `Buttons`, `ComboBoxes`, etc ``` ``` Placing the implicit style in `Window.Resources` does not cross control template boundaries, so things like `Buttons` and `ComboBoxes` maintain their default black text. ``` ``` Furthermore, adding the default style in the `Application.Resources` makes it so you can't overwrite that style with another implicit style. ```

Accepted Answer

This is really the only special handling added to WPF and it was done by design. The code that implements it can be found in `FrameworkElement`, in the method `FindImplicitStyleResource`, which effectively does:

```
internal static object FindImplicitStyleResource(FrameworkElement fe, object resourceKey, out object source)
{
        // ...
        DependencyObject boundaryElement = null;
        if (!(fe is Control))
            boundaryElement = fe.TemplatedParent;
        // ...
}
```

So the rule of thumb is that implicit Styles are always applied to controls (i.e. derives from `Control`). Assuming the implicit Style can be found.

For elements used in a `ControlTemplate` that do not derive from `Control`, such as `TextBlock`, the implicit Style look up will not cross it's templated parent. In your case above, this would be the `ComboBox`.

I believe this was done so that non-application implicit Styles for TextBlock were not inadvertently applied to `TextBlock` elements used in control templates, which the developer may or may not have known were there. The implicit Styles would only be applied to TextBlocks actually created by the developer in their own XAML.

The application implicit Styles would still allow global styling, such as increasing font size. But has probably caused more confusion than it's worth.

There is no good answer to say when to use one versus the other, as they each have their function. Obviously if you don't want to affect every `TextBlock` in your application, you shouldn't put the style in the application resources.

But keep in mind that this affects any non-`Control` element, such as `Shape` elements.

Implicit styles in Application.Resources vs Window.Resources?

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