WPF: Support custom RelativeSource on bindings

_This issue has been moved from [a ticket on Developer Community](https://developercommunity.visualstudio.com/t/WPF:-Support-custom-RelativeSource-on-bi/1636305)._

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I'm not sure if this is the proper place for a feature request for WPF, please redirect me if not.

I would like to be able to implement custom RelativeSource for bindings. This is not currently possible as the RelativeSource class only holds the configuration, while the actual resolution to a source object happens in RelativeObjectRef, which is sealed and internal.

My concrete use case is that I would like a RelativeSource that searches for the type of the DataContext, not the element itself.

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### Original Comments

#### Feedback Bot on 1/14/2022, 00:32 PM:

(private comment, text removed)