What is the order of evaluation in C#?

Raymond Chen

The C and C++ languages leave the order of evaluation generally unspecified aside from specific locations called sequence points. Side effects of operations performed prior to the sequence point are guaranteed visible to operations performed after it.¹ For example, the C comma operator introduces a sequence point. When you write f(), g(), the language guarantees that any changes to program state made by the function f can be seen by the function g; f executes before g. On the other hand, the multiplication operator does not introduce a sequence point. If you write f() * g() there is no guarantee which side will be evaluated first.

(Note that order of evaluation is not the same as associativity and operator precedence. Given the expression f() + g() * h(), operator precedence says that it should be evaluated as if it were written f() + (g() * h()), but that doesn’t say what order the three functions will be evaluated. It merely describes how the results of the three functions will be combined.)

In the C# language, the order of evaluation is spelled out more explicitly. The order of evaluation for operators is left to right. if you write f() + g() in C#, the language guarantees that f() will be evaluated first. The example in the linked-to page is even clearer. The expression F(i) + G(i++) * H(i) is evaluated as if it were written like this:

temp1 = F(i);
temp2 = i++;
temp3 = G(temp2);
temp4 = H(i);
return temp1 + temp3 * temp4;

The side effects of each part of the expression take effect in left-to-right order. Even the order of evaluation of function arguments is strictly left-to-right.

Note that the compiler has permission to evaluate the operands in a different order if it can prove that the alternate order of evaluation has the same effect as the original one (in the absence of asynchronous exceptions).

Why does C# take a much more restrictive view of the order of evaluation? I don’t know, but I can guess.²

My guess is that the language designers wanted to reduce the frequency of a category of subtle bugs (in this case, order-of-evaluation dependency). There are many other examples of this in the language design. Consider:

class A {
 void f()
  int i = 1;
  if (true) {
   int i = 2; // error - redeclaration
 int x;
 void g()
  x = 3; // error - using variable before declared
  int x = 2;

The language designers specified that the scope of a local variable in C# extends to the entire block in which it is declared. As a first consequence of this, the second declaration of i in the function f() is illegal since its scope overlaps with the scope of the first declaration. This removes a class of bugs that can be traced to one local variable masking another with the same name.

In the function g() the assignment x = 3; is illegal because the x refers not to the member variable but to the local variable declared below it. Notice that the scope of the local variable begins with the entire block, and not with the point of declaration as it would have been in C++.

Nitpicker’s Corner

¹This is a simplified definition of sequence point. For more precise definitions, consult the relevant standards documents.

²I have not historically included the sentence “I don’t know but I can guess” because this is a blog, not formal documentation. Everything is my opinion, recollection, or interpretation. But it seems that people take what I say to establish the official Microsoft position on things, so now I have to go back and add explicit disclaimers.


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