Java – meant by Scope of a variable

What is meant by the "scope" of a variable? think of the scope of a variable as "The scope of a particular variable is the range within a program's source code in which that variable is recognized by the compiler".

That definition matches the definition of scope in the C# specification; it says:

The scope of a name is the region of program text within which it is possible to refer to the entity declared by the name without qualification of the name

Notice that the C# specification calls out that it is the name which has a scope. Some variables do not have names, and therefore have no scope. And some things have names but are not variables; class names, namespace names, function names, and so on, all have a scope.

The scope of a local variable is frequently confused with its lifetime. The lifetime of a local variable is sometimes related to its scope; when control leaves the scope of a local (either through normal means or via an exception) then the local can be destroyed without anyone noticing. However, the runtime is permitted to make the lifetime of a local variable shorter if it can do so without anyone noticing. The runtime is also permitted to make a local variable live longer than its scope. And there are some situations in which a local variable is required to live beyond when control leaves its scope. (For instance, when the local is a closed-over outer variable of a lambda.)

Therefore it is unwise to use "scope" as a synonym for "lifetime". They are often related but can be quite different. Use "scope" to mean "the region of text in which a name is valid", not "the period of time in which a variable is alive". Scope is fundamentally a compile-time concept.

What exactly is the difference between the scope of variables in C# and (C99, C++, Java)

I am not sufficiently familiar with the exact scoping rules of C, C++ or Java to give a short list of the differences. I can however point out some oddities of C#.

The first is that in C#, a local variable is in scope throughout its block.

class C
{
    int x;
    void M()
    {
        int y = x;
        int x = y;
    }
}

In C++, the first "x" means "this.x" because the local variable x is not in scope until its declaration. In C#, the local variable x is in scope throughout the block. Using a local that is in scope before its declaration is an error in C#.

The second is that in C#, a simple name must mean the same thing throughout the scope in which it is first used:

class C
{
    int x;
    void M()
    { // M starts
        int y = x;
        if (y > 100)
        {
            int x = y;
        }
    }
}

This is illegal because inside the block labelled "M starts" the simple name "x" is used to mean two different things. Now the first "x" means "this.x" because the local is not in scope. But it is illegal to use the same name to mean two different things, because that is a source of bugs.

If this subject interests you, I have written a number of articles on it. See

http://blogs.msdn.com/b/ericlippert/archive/tags/scope/

http://blogs.msdn.com/b/ericlippert/archive/tags/declaration+spaces/

UPDATE: The rule that a simple name must have a unique meaning throughout the block which encloses it has been relaxed in C# 6. The language design team decided that the cost of user confusion was too high for the benefit in catching bugs.