Actually this instruction is not C# originated. I know it was present in at least Python before that. The easiest way is to use a static local variable holding the last returned result and continue from there during the next call.

Qt Code: 
  1. int getNextNatural(bool restart=false){
  2.   static int last = -1;
  3.   if(restart)
  4.     last = -1;
  5.   return ++last;
  6. }
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