Guffa answered your question correctly for class instances: passing a class instance to a method will *NOT* copy it; you're passing a pointer to that instance. As Urs mentioned, if you pass a struct instance (say, a System.Int32, a DateTime, or your own custom struct), it *WILL* pass a copy of the struct instance to the method. In summary: class instances are passed by reference (no copying going on), and struct instances are passed by value (copying). As a side note, if you do want to pass values by reference, you can do so using the ref keyword.
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