What does `{0:X2}` mean in this code sample?

This uses the same format as String.Format(). Check out the following reference:

http://msdn.microsoft.com/en-us/library/fht0f5be.aspx

Beware the length specified is not respected if the number is too large to fit the length.

 long a = 123456789;
 Console.Write("{0:X2}", a);
 ->   75BCD15

This is especially important if you want to show negative hex numbers where all the high bits are set to 1's.

 long a = -1;
 Console.Write("{0:X2}", a);
 ->  FFFFFFFFFFFFFFFF

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