Darach (3) [Avatar] Offline
Using the *.pdf version of C# in Depth, 3rd Edition I believe I've found a technical, coding mistake.

Page 534, in Table A.7 GroupBy examples:

The second line of code has a "Result" in column 2 of the table of:

Key: 4; Sequence: "zero", "four"
Key: 3; Sequence: "one", "two"
Key: 5; Sequence: "three"

I believe the way the line of associated code indicates the values should be all UPPERCASE:

"ZERO", "FOUR", etc.