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Breaking Changes In Argument List Evaluation In C# 5.0

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

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

static void M(
    int x = 10,
    int y = 20,
    int z = 30)
        "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


  • I would modify "expression evaluation should be avoided in argument lists" to "expression evaluation that changes state should be avoided in argument lists." It works perfectly fine if the evaluation causes no side effects.

  • Chris,

    I thought about that.

    Would you consider DateTime.Now something that changes state? Or is it something where state changes by itself?

    And you may not have state changing statements, but than add one, and later another...

    The safest is never doing it. But like most superlatives, it might be overkill.

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