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Breaking Changes in Argument List Evaluation in C# 5.0

, 30 Jul 2012 CPOL 18.3K 11
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In C# 4.0 there was an error in the C# compiler in the order of the evaluation of the arguments in argument lists.

The C# Language Specification states on §7.5.1.2 that “(…) the expressions or variable references of an argument list are evaluated in order, from left to right (…)”.free hit counters

So, when this code is compiled with the C# 4.0 compiler:

static void M(
    int x = 10,
    int y = 20,
    int z = 30)
{
    Console.WriteLine(
        "x={0}, y={1}, z={2}", x, y, z);
}

static void Main(string[] args)
{
    int a = 0;

    M(++a, z: ++a);
}

and run, this unexpected output is obtained:

x=2, y=20, z=1

In fact, fixing this compiler flaw was the cause of one of the few breaking changes introduced in C# 5.0.

Using the 5.0 compiler, the expected result is obtained:

x=1, y=20, z=2

To avoid this type of surprises, expression evaluation should be avoided in argument lists.

With this code:

int a = 0;

int i = ++a;
int j = ++a;

M(i, z: j);

the same result is obtained for both C# 4.0 and C# 5.0:

x=1, y=20, z=2

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Paulo Morgado
Software Developer (Senior) Paulo Morgado
Portugal Portugal

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GeneralNot an article Pin
Richard MacCutchan25-Jul-12 23:09
mvpRichard MacCutchan25-Jul-12 23:09 
GeneralRe: Not an article Pin
Paulo Morgado26-Jul-12 0:34
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GeneralRe: Not an article Pin
Richard MacCutchan26-Jul-12 1:17
mvpRichard MacCutchan26-Jul-12 1:17 
GeneralRe: Not an article Pin
Paulo Morgado26-Jul-12 3:46
memberPaulo Morgado26-Jul-12 3:46 
GeneralRe: Not an article Pin
Richard MacCutchan26-Jul-12 5:18
mvpRichard MacCutchan26-Jul-12 5:18 
GeneralRe: Not an article Pin
Paulo Morgado26-Jul-12 5:27
memberPaulo Morgado26-Jul-12 5:27 
GeneralRe: Not an article Pin
Richard MacCutchan26-Jul-12 5:56
mvpRichard MacCutchan26-Jul-12 5:56 
GeneralRe: Not an article Pin
Paulo Morgado26-Jul-12 6:14
memberPaulo Morgado26-Jul-12 6:14 
GeneralRe: Not an article Pin
Matt T Heffron26-Jul-12 8:29
memberMatt T Heffron26-Jul-12 8:29 
GeneralRe: Not an article Pin
Paulo Morgado26-Jul-12 8:50
memberPaulo Morgado26-Jul-12 8:50 

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