>Actually my idea was overkill. Since your key value is a string then the default comparer should work anyway. What was the problem with your original code? Simple working example:

static void Main(string[] args)
>        {
>            Dictionary<string, string> AssignedTrackingColumns = new Dictionary<string,string>();
>            Dictionary<string, string> TrackingLookupOrderColumns = new Dictionary<string, string>();
>
>            TrackingLookupOrderColumns.Add("A", "One");
>            TrackingLookupOrderColumns.Add("B", "Two");
>            TrackingLookupOrderColumns.Add("C", "Three");
>            TrackingLookupOrderColumns.Add("D", "Four");
>
>            AssignedTrackingColumns.Add("A", "One");
>            AssignedTrackingColumns.Add("C", "Three");
>
>            var result = TrackingLookupOrderColumns.Except(AssignedTrackingColumns);
>
>            foreach (var x in result)
>            {
>                Console.WriteLine(string.Format("Key:{0} Value:{1}",x.Key,x.Value));
>            }
>            Console.ReadLine();
>        }

It gave me an error in compile time that it needs an explicit cast.

My code was:

private Dictionary<string, string> trackingLookupOrderColumns = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

This in the API controller.

In the ViewModel I have

public Dictionary<string, string> AvailableTrackingColumns = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
        public Dictionary<string, string> AssignedTrackingColumns = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

And in the GetPrefs method of the API controller I was trying

prefsRetailViewModel.AvailableTrackingColumns = this.trackingLookupOrderColumns.Except(prefsRetailViewModel.AssignedTrackingColumns);

which gives me

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS0266	Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, string>>' 
to 'System.Collections.Generic.Dictionary<string, string>'. An explicit conversion exists (are you missing a cast?)	

Do you see what is different in my code?

UPDATE. Got this compiled this way

prefsRetailViewModel.AvailableTrackingColumns = this.trackingLookupOrderColumns.Except(prefsRetailViewModel.AssignedTrackingColumns).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);