How to get valid "System.Windows.Media.CharacterMetricsDictionary"?

Question

I tried this:

Code Snippet

System.Windows.Media.FontFamily ff = new System.Windows.Media.FontFamily("Arial");

System.Windows.Media.FamilyTypeface tf = ff.FamilyTypefaces;

System.Windows.Media.CharacterMetricsDictionary md = tf[0].DeviceFontCharacterMetrics;

but (md.Count == 0) is true;

So there are no data in.
Is any other method that can get valid data?