>prefsRetailViewModel.AvailableTrackingColumns = this.trackingLookupOrderColumns.Except(prefsRetailViewModel.AssignedTrackingColumns);
>>Dictionary<string, string>>
public class StringDictionaryComparer : IEqualityComparer<KeyValuePair<string,string>> { public bool Equals (KeyValuePair<string, string> a , KeyValuePair<string, string> b) { return a.Key == b.Key; } public int GetHashCode(KeyValuePair<string, string> item) { return item.GetHashCode(); } }Your code should then work (assuming you only need to check the key)