>>Thanks a lot in advance.
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>prefsRetailViewModel.AvailableTrackingColumns = this.trackingLookupOrderColumns.Keys.Except(prefsRetailViewModel.Keys);

This is close, but also not working. The AvailableTrackingColumns is also a Dictionary, so I need to get them with the values. If I can get a list of keys with the above method, can I then somehow grab that portion of the Dictionary (using these keys) in one simple call?

I can, of course, just iterate through keys, but I wanted to use LINQ capabilities and I'm sure there is some smart way of grabbing the relevant dictionary entries.