C# static variables persisting through vs2012 sessions, static variables NOT initializing correctly on re-run.

public static string s = "NOT initialized"; //my default value
s = "Initialized";

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VS 2012, What I am seeing is;

Start Debugging(F5)
Stop Debugging (Shift+F5)
Start Debugging(F5) //not continue

s still equals "Initialized" when I expect it to equal "NOT initialized".

Why is this and how do I make my static variables re-initialize each time I "run" my code.

ps: This does not happen if I change anything in the code and rebuild it.

Thank You
Sam