Mind the runtime optimization

You might know this already, but in C# whenever you write something like this:

   1: if (MethodA() || MethodB())

   2: {

   3:     // do something

   4: }

it’s not guaranteed that both methods will be executed, so DO NOT DO THIS if you’re relying on both methods to be expected to cause some desirable side-effects.

The reason for this is simple, at runtime, as soon as one of the methods returns true the whole expression will evaluate to true regardless of the output of the second method. So as far as the runtime is concerned, it can safely skip the second part of the if condition as a form of runtime optimization.

Here’s a quick demo that shows this behaviour in action:

   1: public class MyClass

   2: {

   3:     public bool FlagA { get; set; }    

   4:     public bool FlagB { get; set; }

   5: }

   6:  

   7: public bool MethodA(MyClass myObj)

   8: {

   9:     myObj.FlagA = true;    

  10:     Console.WriteLine("Set FlagA to true");

  11:     

  12:     return true;

  13: }

  14:  

  15: public bool MethodB(MyClass myObj)

  16: {

  17:     myObj.FlagB = true;    

  18:     Console.WriteLine("Set FlagB to true");

  19:     

  20:     return true;

  21: }

  22:  

  23: ...

  24:  

  25: var myObj = new MyClass();

  26:  

  27: // this evaluates to true, but only MethodA is invoked

  28: var isMyObjChanged = MethodA(myObj) || MethodB(myObj);

  29:  

  30: // prints FlagA [True], FlagB[False]

  31: Console.WriteLine("FlagA [{0}], FlagB[{1}]", myObj.FlagA, myObj.FlagB);

This behaviour also applies outside to if statements like the one at the top of the post. I wasted some valuable minutes trying to solve a WTF bug resulting from this, hopefully it won’t catch you out too!

  • Andy

    You can get what you were after by using the bitwise operator instead of the logical one.

    ie, var isMyObjChanged = MethodA(myObj) | MethodB(myObj);

  • theburningmonk

    Andy – ah, that’s a good idea, never came across my mind.

    The only thing I worry about doing this is if someone else comes along, thought it was a typo and changes it to ||! Nothing a little bit of comment won’t solve though ;-)

    Thanks for the suggestion!