.net - Why doesn't C# allow me to use the same variable name in different scopes?

Question

Like for instance:

if ( this.IsValid )
{
    Matrix matrix = new Matrix();
}

Matrix matrix = new Matrix();

The compiler warns me saying:

"A local variable named 'matrix' cannot be declared in this scope because it would give a different meaning to 'matrix', which is already used in a 'child' scope to denote something else.

Aren't these variables in different scopes, so I wouldn't be able to access the first matrix from outside the if statement anyway?

Answer 1

UPDATE: The answer below from 2011 is correct for earlier versions of C#; in more recent versions, the rule described the answer has been removed from C#. The design team determined that the rule caused more confusion amongst developers leading to questions like this one than the buggy programs prevented would warrant, even after I greatly improved the error messages to more clearly diagnose the problem.

---

The answers given so far are very confusing. The correct analysis of the problem starts by reading the error message. The error message is telling you what is actually wrong:

"A local variable named 'matrix' cannot be declared in this scope because it would give a different meaning to 'matrix', which is already used in a 'child' scope to denote something else.

Read that carefully. It is telling you precisely which rule of C# is being violated, namely that you are not allowed to use the same name to refer to two different things in the same scope. (Actually, the error message is slightly wrong; it should say "local variable declaration space" where it says "scope", but that is pretty wordy.)

This rule is documented in the C# 4.0 specification, section 7.6.2.1: Simple names, Invariant meaning in blocks.

(It is also illegal to have two local variables of the same name in overlapping declaration spaces. The compiler could be reporting that error as well, but it reports the more general error in this case.)

Aren't these variables in different scopes, so I wouldn't be able to access the first matrix from outside the if statement anyway?

Yes. That statement is true but irrelevant. The error here is that the same simple name has been used to refer to two different things in the same local variable declaration space.

Consider this scenario:

class C 
{
    int x;
    void M()
    {
        x = 10; // means "this.x"
        for(whatever)
        {
            int x = whatever;
        }
    }
 }

Same deal. The error here is that the simple name "x" was used in the outer declaration space to refer to this.x, and was used in the inner declaration space to mean "local variable". Using the same simple name to refer to two different things in the same declaration space -- remember, the inner declaration space is a part of the outer one -- is both confusing and dangerous, and is therefore illegal.

It is confusing for obvious reasons; one has a reasonable expectation that a name will mean the same thing everywhere throughout the declaration space in which it is first used. It is dangerous because small code edits are prone to changing the meaning:

class C 
{
    int x;
    void M()
    {
        int x;
        x = 10; // no longer means "this.x"
        for(whatever)
        {
            x = whatever;
        }
    }
 }

If the declaration spaces in which the simple names are first used are not overlapping then it is legal for the simple names to refer to different things:

class C 
{
    int x;
    void M()
    {
        {
            x = 10; // means "this.x"
        }
        for(whatever)
        {
            int x = whatever; // Legal; now the 
        }
    }
 }

For more information, and an amusing story about fried food, see

http://blogs.msdn.com/b/ericlippert/archive/tags/simple+names/