• It seems the C# default (starting) solution is missing a
  using System.Collections.Generic;
  (for the List<String> parameter).
• According to the standard C# naming convention, method names should start with a capital (i.e. EncodeStr and Decode).

The Credits section in the description is empty :|.

It works now, thanks!

Is there a reason for the "n" input parameter? numbers.Length holds the same value.

• What is the purpose of the rnd member in the test class?
  Random rnd = new Random();
• Perhaps add the following tests to the Kata test fixture:
  • Single-operand disjunction: result should equal the value of the sole operand.
  • Empty disjunction (non-exclusive and exclusive): formal logic dictates the result should be (vacuously) false.
  • Make sure these non-obvious tests have informative failure-texts.

• The language is sloppy. Specifically, many determinators ("the", "a") are missing: "A race condition, loosely speaking, is a situation when the order of some actions isn't specified, but the output of a program depends on it." This applies to the entire text.
• The last hint ("Enumerators have a number of...") is unintelligible. I couldn't understand the sentence, but I didn't need to in order to solve the problem.