C# Variable Scoping: 'X' Cannot Be Declared in This Scope Because It Would Give a Different Meaning to 'X'

C# variable scoping: 'x' cannot be declared in this scope because it would give a different meaning to 'x'

The issue here is largely one of good practice and preventing against inadvertent mistakes. Admittedly, the C# compiler could theoretically be designed such that there is no conflict between scopes here. This would however be much effort for little gain, as I see it.

Consider that if the declaration of var in the parent scope were before the if statement, there would be an unresolvable naming conflict. The compiler simply does not differentiate between the following two cases. Analysis is done purely based on scope, and not order of declaration/use, as you seem to be expecting.

The theoretically acceptable (but still invalid as far as C# is concerned):

if(true)
{
    string var = "VAR";
}

string var = "New VAR!";

and the unacceptable (since it would be hiding the parent variable):

string var = "New VAR!";

if(true)
{
    string var = "VAR";
}

are both treated precisely the same in terms of variables and scopes.

Now, is there any actual reason in this secenario why you can't just give one of the variables a different name? I assume (hope) your actual variables aren't called var, so I don't really see this being a problem. If you're still intent on reusing the same variable name, just put them in sibling scopes:

if(true)
{
    string var = "VAR";
}

{
    string var = "New VAR!";
}

This however, while valid to the compiler, can lead to some amount of confusion when reading the code, so I recommend against it in almost any case.

Local variable named 'i' cannot be declared in this scope because it would give a different meaning to 'i'

You need to have different names for your iterators with nested for loops.

Example:

for(int i = 0; i < 10; i++)
{
    for(int x = 0; x < 10; x++)
    {
        //some code
    }
}

Notice i and x. Both cannot be i in the same scope.

This, on the other hand, is valid as each exists in a different scope.

for(int i = 0; i < 10; i++)
{
    //some code
}

for(int i = 0; i < 10; i++)
{
    //some code
}

Fix:

Here are the loops in question. In the declarations, change all instances of i to x (or whatever) as I have shown above.

for (int i = 0; i < rectSpaceInvader.Length; i++)
{
    if (AlienAlive[i].Equals("Yes"))
        CountAliensAlive = CountAliensAlive + 1;
}

for (int i = 0; i < rectSpaceInvader.Length; i++)
{
    {
        if (AlienAlive[i].Equals("Yes"))
            if (rectSpaceInvader[i].Y + rectSpaceInvader[i].Height > rectSpaceShip.Y)
                this.Exit();*
    }
}

They are both contained inside of this one (showing if for reference).

if (ShipBulletVisible.Equals("Yes"))
{
    for (int i = 0; i < rectSpaceInvader.Length; i++)
    {
        //code
    }
}

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

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.

A local variable named 'port' cannot be declared

You have a variable named port in your code already. So it doesn't allow you to declare it again in your foreach loop. Change it's name to something else:

foreach (string port2 in ports)
{
    comboBox1.Items.Add(port2);
}

C# scope question

This is a frequently asked question; see also:

Lambda variable scope

C# going nuts when I declare variables with the same name as the ones in a lambda

C# variable scoping: 'x' cannot be declared in this scope because it would give a different meaning to 'x'

Variable scope confusion in C#

The answer is: read the error message more carefully. The error message states exactly what the problem is: you are not allowed to use the same simple name to mean two different things in the same block.

For example:

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

Perfectly legal. Note that the scope of the outer x overlaps the scope of the inner x. It is not illegal to have overlapping scopes with the same name in scope in both.

This is not legal:

class C
{
    int x;
    void M()
    {
        Console.WriteLine(x); // this.x
        {
            int x;
        }
    }
}

Again, the two scopes both with an x overlapping is perfectly legal. What is illegal is that the simple name x is used two mean two different variables in the same block - that is, within the outer block of M(), which contains the inner block of M.

Programs where the same simple name is used to mean two different things in the same block are confusing and bug prone and therefore are illegal in C#.

For more details read my articles on the subject:

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

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