Why is a private variable defined in for conflict with the same variable outside?

Question

In my mental model, counter variables defined inside for become private because it cannot be access from outside as follows.

    for (int x = 0; x < 2; x++) ;
    //Console.WriteLine(x); x does not exist in the current context

My confusion occurs when I declared the same variable as follows,

    for (int x = 0; x < 2; x++) ;
    //Console.WriteLine(x); x does not exist in the current context, but why cannot I declare
    //int x = 1;

Why does the compiler disallow me to declare a variable with the same name as another inaccessible variable? It does not make sense to me. :-)

To make more confusing compare it with

    {
        int x = 1;
    }
    {
        int x = 2;
    }

which is allowed by the compiler. But both below are not allowed.

static void Foo()
{
    int x = 1;
    { int x = 2; }
}

static void Bar()
{
    { int x = 2; }
    int x = 3;
}

This is explained in [C# Variable Scoping](http://stackoverflow.com/questions/2049330/c-sharp-variable-scoping). — CodeCaster, Mar 02 '14 at 12:31

score 3 · Accepted Answer · answered Mar 02 '14 at 12:34

This behaviour is covered in section 3.7 of the language spec. It says, “The scope of a local variable declared in a local-variable-declaration (8.5.1) is the block in the which the declaration occurs”.

The scope of x is therefore the entire function, and that means that the use in the for loop is a re=use, and therefore is not allowed.

This behavior is inteded to make incorrect re-use of variable names (such as in a cut and paste) less likely.

score 1 · Answer 2 · edited May 23 '17 at 11:49

1

aleroot already gave the answer to your first part. I want to answer second part of your question.

{
     int x = 1;
}
{
     int x = 2;
}

which is allowed by the compiler.

Yes, because there is no local variable x in the same scope or any nested scope. For example;

static void Main()
{
    int x;
    {
        int y;
        int x; // Error - x already defined in same scope or parrent scope.
    }
    {
        int y; // OK - there is no y in the same scope or any nested scope.
    }
    Console.Write (y); // Error - there is no y in same scope of any parrent scope.
}

edited May 23 '17 at 11:49

Community

1
1

answered Mar 02 '14 at 12:49

Soner Gönül

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If the first `int x;` is moved to the last row, for example after `Console.WriteLine(y);` the compiler is still unhappy. In my mental model, it should not be a problem because it comes last. – kiss my armpit Mar 02 '14 at 12:52
1

@TheLastError It is because there is an already defined `x` in child scope. You can't declare _local-variable-declaration_ in current or parrent scope if there is an already in child scope (doesn't matter comes last or first). – Soner Gönül Mar 02 '14 at 13:02
OK. Thanks. It is counter intuitive at a glance. :-) – kiss my armpit Mar 02 '14 at 13:03

score -1 · Answer 3 · answered Mar 02 '14 at 12:34

-1

Couse for loop almost declared x varible inside it, so u cant redeclared x varible, as can exists only one declaration in scope.

answered Mar 02 '14 at 12:34

andrey.rtv

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3

Why is a private variable defined in for conflict with the same variable outside?

3 Answers3