C# strange behaviour

Question

3.67/5 (3 votes)

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consider this code

MIDL

int j = 1;
j = j++;

amazingly enough it produses 1, 2nd line has no affect.
MSIL looks like

MSIL

---------------------Loc 0  Stack
IL_0000:  nop
IL_0001:  ldc.i4.1            1           Push 1 onto the stack as int32
IL_0002:  stloc.0     1       -           Pop a value from stack into local variable 0
IL_0003:  ldloc.0     1       1           Load local variable 0 onto stack
IL_0004:  dup         1       1,1         Duplicate the value on the top of the stack
IL_0005:  ldc.i4.1    1       1,1,1       Push 1 onto the stack as int32
IL_0006:  add         1       2,1         Add two values, returning a new value
IL_0007:  stloc.0     2       1           Pop a value from stack into local variable 0
IL_0008:  stloc.0     1       -           Pop a value from stack into local variable 0

quite obvious last line is the culprit. It works the same in Java, JavaScript. C/C++ works fine!

Posted 21-Jan-11 7:41am

asxzasxz

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Nish Nishant 21-Jan-11 14:00pm

Good question, take a 5!

Rob Philpott 21-Jan-11 14:02pm

I think I'm missing something here. Are you suggesting in C++, the answer is 2?

Nish Nishant 21-Jan-11 14:31pm

The VC++ compiler will give 2. It's undefined behavior in the standard, so any other compiler or a future version of the VC++ compiler might return 1 (or some other result).

2 solutions

Solution 2

> C/C++ works fine!

It does not always work - the behavior is not specified, because j = j++ lacks a sequence point[^] between the two assignments.

Posted 21-Jan-11 7:55am

dasblinkenlight

Comments

Sergey Alexandrovich Kryukov 21-Jan-11 14:39pm

Good note - 5

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Nish Nishant · Accepted Answer · 2011-01-21T07:46:00

Solution 1

I would not say it's a bug, C# has different evaluation rules compared to C++.

If you want what you expect, you can do it as:

j = ++j;

And by the way, the behavior for that line of code is undefined in C/C++ and it's up to the individual compiler implementations to decide how to handle this. For more information, google for "C++ sequence points".

BTW the C# behavior is undefined too. A future version of C# may decide to implement this differently (since it's not in the standard). In general it's best to avoid writing code where the same variable is modified multiple times on either side of an assignment operator.

Posted 21-Jan-11 7:46am

Nish Nishant

Updated 21-Jan-11 7:50am

v3

Comments

#realJSOP 21-Jan-11 14:14pm

Damn - beat me to it. Proposed as answer

Nish Nishant 21-Jan-11 14:17pm

Well it was posted half an hour ago :-)

And it's also one of the better and more interesting threads today!

Thank you.

#realJSOP 21-Jan-11 14:29pm

As you might guess, I prefer the questions that are actually worthy of code instead of the ones that scream "I'm too lazy to use google".

Sergey Alexandrovich Kryukov 21-Jan-11 14:38pm

Funny question :) my 5 for answer.