Your "DestPath" needs to be a property if you want to use it in Binding.
The source of the binding can be any public property, including properties of other controls, common language runtime (CLR) objects, XAML elements, ADO.NET DataSets, XML Fragments, and so forth.
More details at MSDN
As per comments below: In order to update your UI when property gets updates, your VM class needs to implement INotifyPropertyChanged, and in "set" of DestPath you need to raise property changed event. this MSDN link has details and sample code.