variable scope in statement blocks

问题:

for (int i = 0; i < 10; i++)
{
    Foo();
}
int i = 10; // error, 'i' already exists

----------------------------------------    

for (int i = 0; i < 10; i++)
{
    Foo();
}
i = 10; // error, 'i' doesn't exist

By my understanding of scope, the first example should be fine. The fact neither of them are allowed seems even more odd. Surely 'i' is either in scope or not.

Is there something non-obvious about scope I don't understand which means the compiler genuinely can't resolve this? Or is just a case of nanny-state compilerism?

回答1:

By my understanding of scope, the first example should be fine.

Your understanding of scope is fine. This is not a scoping error. It is an inconsistent use of simple name error.

int i = 10; // error, 'i' already exists

That is not the error that is reported. The error that is reported is "a local variable named i cannot be declared in this scope because it would give a different meaning to i which is already used in a child scope to denote something else"

The error message is telling you what the error is; read the error message again. It nowhere says that there is a conflict between the declarations; it says that the error is because that changes the meaning of the simple name. The error is not the redeclaration; it is perfectly legal to have two things in two different scopes that have the same name, even if those scopes nest. What is not legal is to have one simple name mean two different things in nested local variable declarations spaces.

You would get the error "a local variable named i is already defined in this scope" if instead you did something like

int i = 10;
int i = 10;

Surely 'i' is either in scope or not.

Sure -- but so what? Whether a given i is in scope or not is irrelevant. For example:

class C 
{
    int i;
    void M()
    {
        string i;

Perfectly legal. The outer i is in scope throughout M. There is no problem at all with declaring a local i that shadows the outer scope. What would be a problem is if you said

class C 
{
    int i;
    void M()
    {
        int x = i;
        foreach(char i in ...

Because now you've used i to mean two different things in two nested local variable declaration spaces -- a loop variable and a field. That's confusing and error-prone, so we make it illegal.

Is there something non-obvious about scope I don't understand which means the compiler genuinely can't resolve this?

I don't understand the question. Obviously the compiler is able to completely analyze the program; if the compiler could not resolve the meaning of each usage of i then how could it report the error message? The compiler is completely able to determine that you've used 'i' to mean two different things in the same local variable declaration space, and reports the error accordingly.

回答2:

It is because the declaration space defines i at the method level. The variable i is out of scope at the end of the loop, but you still can't redeclare i, because i was already defined in that method.

Scope vs Declaration Space:

http://csharpfeeds.com/post/11730/Whats_The_Difference_Part_Two_Scope_vs_Declaration_Space_vs_Lifetime.aspx

You'll want to take a look at Eric Lippert's answer (who by default is always right concerning questions like these).

http://blogs.msdn.com/ericlippert/archive/2009/08/03/what-s-the-difference-part-two-scope-vs-declaration-space-vs-lifetime.aspx

Here is a comment from eric on the above mentioned post that I think talks about why they did what they did:

Look at it this way. It should always be legal to move the declaration of a variable UP in the source code so long as you keep it in the same block, right? If we did it the way you suggest, then that would sometimes be legal and sometimes be illegal! But the thing we really want to avoid is what happens in C++ -- in C++, sometimes moving a variable declaration up actually changes the bindings of other simple names!

回答3:

From the C# spec on local variable declarations:

The scope of a local variable declared in a local-variable-declaration is the block in which the declaration occurs.

Now, of course, you can't use i before it is declared, but the i declaration's scope is the entire block that contains it:

{
    // scope starts here
    for (int i = 0; i < 10; i++)
    {
        Foo();
    }
    int i = 10;
}

The for i variable is in a child scope, hence the collision of variable names.

If we rearrange the position of the declaration, the collision becomes clearer:

{
    int i = 10;

    // collision with i
    for (int i = 0; i < 10; i++)
    {
        Foo();
    }
}

回答4:

In the first example, the declaration of i outside of the loop makes i a local variable of the function. As a result, it is an error to have another variable name i declared within any block of that function.

The second, i is in scope only during the loop. Outside of the loop, i can no longer be accessed.

So you have seen the errors, but there is nothing wrong with doing this

for (int i = 0; i < 10; i++)
{
  // do something
}

foreach (Foo foo in foos)
{
   int i = 42;
   // do something 
}

Because the scope of i is limited within each block.

回答5:

Yea, I second the "nanny-state compilerism" comment. What's interesting is that this is ok.

for (int i = 0; i < 10; i++)
{

}

for (int i = 0; i < 10; i++)
{

}

and this is ok

for (int i = 0; i < 10; i++)
{

}

for (int j = 0; j < 10; j++)
{
    var i = 12;                
}

but this is not

for (int i = 0; i < 10; i++)
{
    var x = 2;
}

var x = 5;

even though you can do this

for (int i = 0; i < 10; i++)
{
    var k = 12;
}

for (int i = 0; i < 10; i++)
{
    var k = 13;
}

It's all a little inconsistent.

EDIT

Based on the comment exchange with Eric below, I thought it might be helpful to show how I try to handle loops. I try to compose loops into their own method whenever possible. I do this because it promotes readability.

BEFORE

/*
 * doing two different things with the same name is unclear
 */
for (var index = 0; index < people.Count; index++)
{
    people[index].Email = null;
}
var index = GetIndexForSomethingElse(); 

AFTER

/*
 * Now there is only one meaning for index in this scope
 */
ClearEmailAddressesFor(people); // the method name works like a comment now
var index = GetIndexForSomethingElse();

/*
 * Now index has a single meaning in the scope of this method.
 */
private void ClearEmailAddressesFor(IList<Person> people)
{
    for (var index = 0; index < people.Count; index++)
    {
        people[index].Email = null;
    }
}

回答6:

Or is just a case of nanny-state compilerism?

Exactly that. There is no sense in "reusing" variable names in the same method. It's just a source of errors and nothing more.

回答7:

Me thinks that the compiler means to say that i has been declared at the method level & scoped to within the for loop.

So, in case 1 - you get an error that the variable already exists, which it does

& in case 2 - since the variable is scoped only within the for loop, it cannot be accessed outside that loop

To avoid this, you could:

var i = 0;

for(i = 0, i < 10, i++){
}

i = 10;

but I can't think of a case where you would want to do this.

HTH

回答8:

you need to do

            int i ;
            for ( i = 0; i < 10; i++)
            {

            }
            i = 10;

回答9:

class Test
{
    int i;
    static int si=9; 

    public Test()
    {
        i = 199;
    }

    static void main()
    {
        for (int i = 0; i < 10; i++)
        {
            var x = 2;
        }

        { var x = 3; }

        {    // remove outer "{ }" will generate compile error
            int si = 3; int i = 0;

             Console.WriteLine(si);
             Console.WriteLine(Test.si);
             Console.WriteLine(i);
             Console.WriteLine((new Test()).i);
        }
    }
}