Set custom MarkupExtension from code

How do you set a custom MarkupExtension from code?

You can easily set if from Xaml. The same goes for Binding and DynamicResource.

<TextBox FontSize="{Binding MyFontSize}"
         Style="{DynamicResource MyStyle}"
         Text="{markup:CustomMarkup}"/>

Setting the same values through code behind requires a little different approach

Binding: Use textBox.SetBinding or BindingOperations.SetBinding

Binding binding = new Binding("MyFontSize");
BindingOperations.SetBinding(textBox, TextBox.FontSizeProperty, binding);

DynamicResource: Use SetResourceReference

textBox.SetResourceReference(TextBox.StyleProperty, "MyStyle");

CustomMarkup: How do I set a custom MarkupExtension from code? Should I call ProvideValue and it that case, how do I get a hold of a IServiceProvider?*
```
CustomMarkupExtension customExtension = new CustomMarkupExtension();
textBox.Text = customExtension.ProvideValue(??);
```

I found surprisingly little on the subject so, can it be done?

H.B. has answered the question. Just adding some details here to why I wanted to do this. I tried to create a workaround for the following problem.

The problem is that you can't derive from Binding and override ProvideValue since it is sealed. You'll have to do something like this instead: A base class for custom WPF binding markup extensions. But then the problem is that when you return a Binding to a Setter you get an exception, but outside of the Style it works fine.

I've read in several places that you should return the MarkupExtension itself if the TargetObject is a Setter to allow it to reeavaluate once it is being applied to an actual FrameworkElement and this makes sense.

Markup Extension in Data Trigger
Huge limitation of a MarkupExtension
A base class for custom WPF binding markup extensions (in the comments)

However, that only works when the TargetProperty is of type object, otherwise the exception is back. If you look at the source code for BindingBase you can see that it does exactly this but it appears the framework has some secret ingredient that makes it work.

Solution

I think there is no code-equivalent, the services are only available via XAML. From MSDN:

MarkupExtension has only one virtual method, ProvideValue. The input serviceProvider parameter is how the services are communicated to implementations when the markup extension is called by a XAML processor.