You can Reverse list after creating using Enumerable.Range :
cboYearList.ItemsSource = Enumerable.Range(DateTime.Today.Year, 1950).Reverse().ToList();
Or, if you want to keep your OrderByDescending , you need to pass the key selector ( i => i at the end):
cboYearList.ItemsSource = Enumerable.Range( DateTime.Today.Year,1950).OrderByDescending(i => i).ToList();
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