Child Scope & CS0136

问题:

The following code fails to compile stating "A local variable named 'st' cannot be declared in this scope because it would give a different meaning to 'st', which is already used in a 'child' scope to denote something else":

        var l = new List<string>();
        l.Find(st => st.EndsWith("12"));
        string st = "why this fails?";

I understand why this won't work:

        string preParent = "";
        {
            string preParent = "Should fail cause we change the meaning";
        }

When we do the following we get "CS0103: The name 'postParent' does not exist in the current context":

        {
            string postParent=string.Empty;
        }
        postParent = "Should this work?";

What I don't get is why is the compiler smart enough to see that postParent is not within scope, but won't let me define a new variable that has the same name as a variable used within a child scope (which is obviously out of scope at this point).

Is the compiler simple enforcing scope by refusing to let me use the variable? If so this makes sense.

===========

Edited:

I guess what I also find interesting is how you can have the same variable within two child scopes in a single method, so this is valid:

        {
            string thisWorks= string.Empty;
        }
        {
            string thisWorks= "Should this work?";
        }

I'm just a little curious that you can have two variables with the same name as long as they are at the same level (if you look at scope as a tree). This makes sense because you can have local variables in two methods of the same class with the same name.

I'm just surprised that the compiler is able to differentiate and allow this, while it wouldn't allow the postParent variable. And is this a technical limitation or was this a design decision? That's what I'm really trying to get at;-)

回答1:

Yes, the compiler is enforcing scope. Note that the scope of a variable is the lexical block it's part of - not just from the point of declaration onwards, but the whole scope.

The compiler is complaining because the assignment to postParent is outside its scope (which is only the nested braces). If you tried to declare a new variable at the point where you're currently assigning to postParent the problem would be with the nested block, because the scope of postParent would include that nested block, even though it was before the declaration.

Scopes are described in section 3.7 of the C# 3.0 specification.

EDIT: To respond to your question edit.

It's just two simple rules:

• you can't declare a local variable when another local variable with the same name is in scope
• the scope of a local variable is the block in which the declaration occurs

I'm sure the language could have been designed such that the scope only began at the point of declaration, but I think it's simpler (in terms of language complexity) to consider scopes as just blocks - so all local variables declared in the same block have the same scope, for example. That makes life a lot simpler when considering captured variables, too - as what gets captured depends on the scope, and nested scopes make life interesting...

EDIT: The language spec has this to say about the original lambda expression example - it's section 7.14.1:

The optional anonymous-function-signature of an anonymous function defines the names and optionally the types of the formal parameters for the anonymous function. The scope of the parameters of the anonymous function is the anonymous-function-body. Together with the parameter list (if given), the anonymous-method-body constitutes a declaration space. For this reason, it is a compile-time error for the name of a parameter of the anonymous function to match the name of a local variable, local constant, or parameter whose scope includes the anonymous-method-expression or lambda-expression.

Does that help?

回答2:

You're declaring a variable in a limited scope and trying to use it outside of that scope. The compiler assumes you don’t want access to it so you can declare a variable with the same name somewhere else in the file. Your trying to do the old C trick of assuming the variable will live immediately outside of the scope. For example this used to work in older versions of C/C++ but no longer does.

for (int i=0; i<10; i++)
{
    cout <<”In the loop i is “<< i << endl;
}
cout << “outside of the loop i is “ << i << endl; //this only compiles with old C/C++ compilers.