Question

Why can’t I use the same variablename e.g. index in a method?

Why can’t the compiler see the different, when I clearly can?

Example:

private void Foo()
{
    for (int index = 0; index < 10; index++) // "first"-index
    {
         // I'm in no doubt, use "first"-index here 
         // (and only within the scope of the for loop)
    }
    int index = 0; // "second"-index
    // I'm in no doubt, use "second"-index here 
    // (and below)
}

Is it because the allocation is made at compile time? But then, why can’t the compiler, under the hood, just call the “first”-index for index_1 and the “second”-index for index_2?

If I have

    private void Foo()
    {
        for (int index = 0; index < 10; index++)
        {
        }
        // the runtime don't know index here
    }

If the runtime don’t know about index below the for-loop, why can’t we have another variable with that name?

Answer 1

The declaration space of those variables overlaps even if the the scope doesn’t. Check Eric Lippert’s blog on that topic:

Simple names are not so simple

The purpose of all of these rules is to prevent the class of bugs in which the reader/maintainer of the code is tricked into believing they are referring to one entity with a simple name, but are in fact accidentally referring to another entity entirely. These rules are in particular designed to prevent nasty surprises when performing what ought to be safe refactorings.

What’s The Difference, Part Two: Scope vs Declaration Space vs Lifetime

A declaration space, by contrast, is a region of program text in which no two entities are allowed to have the same name.

The declaration space of a variable is larger that its scope in order to prevent those misleading situations.