How to access a property of a ViewModel from another ViewModel?

If CommentViewModel and ClientViewModel strongly coupled you can use it direct if the CommentView visual tree has anywhere the datacontext from the ClientViewModel you can use RelativeSource Binding to get there you can use MEF for Export the "Client" property and Import it in the CommentViewModel you can use some sort of Messenger or EventAggregator to communicate from your CommentViewModel to your ClientViewmodel and some more I think :).

Pass reference of ClientViewModel or ClientViewModel. Client to CommentViewModel when creating it.

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