MVVM - Binding to TreeView.SelectedItem

At first, binding to the TreeView's SelectedItem property sounds like a simple task, but if you give it a go you will soon realise it's not so straight forward. That's because the WPF TreeView's SelectedItem property is read-only so we can't set the binding in XAML like normal.

WPF - Different Data Templates for Different Child Types in a TreeView

The TreeView control in WPF allows us to display hierarchical data and allows us to customise the appearance of this by setting the ItemTemplate property using a HierarchicalDataTemplate, giving us expandable nodes within our tree. However, we can only set the ItemTemplate property once with one data template. Imagine the scenario where you want to display a directory structure. A directory can contain files or other directories which in turn can also contain files or other directories and so on. You might want to use a different data template for files than for directories but as stated earlier you can only set the ItemTemplate property once, so how can we achieve this?