“EnumerableToArray” (Solution)

Answer

GetString: Foo
EnumerableToArray: Foo
GetString: Bar
EnumerableToArray: Bar
GetString: Foo
GetString: Bar

Explanation

LINQ queries use deferred/lazy execution. It means, a LINQ query without a cast method like ToArray() or ToList() is not executed immediately. The execution will be deferred until we do not explicitly require the results. Thus, the line

var strings = GetStringEnumerable();

will not print anything to the console. Next, in the loop

foreach (var s in strings)
  Console.WriteLine("EnumerableToArray: " + s);

query execution will be performed. Moreover, first will be evaluated the first yield (print GetString: Foo), next the loop body will be evaluated for the first enumerable element (print EnumerableToArray: Foo). Next, the foreach loop will require the second enumerable element, the second yield will be evaluated (print GetString: Bar), the loop body will be evaluated for the second element (print EnumerableToArray: Bar).

Next, the following line will be executed:

return strings.ToArray();

Our LINQ query will be performed again. So, the lines GetString: Foo and GetString: Bar will be printed again.

Problem