C# local variable scope [duplicate]

Answer 1

There are already some good answers but I gave a look to the C# 4 language specs to clarify this.

We can read in §1.24 about scopes:

Scopes can be nested, and an inner scope may redeclare the meaning of a name from an outer scope (this does not, however, remove the restriction imposed by §1.20 that within a nested block it is not possible to declare a local variable with the same name as a local variable in an enclosing block).

And this is the cited part in §1.20:

A declaration defines a name in the declaration space to which the declaration belongs. Except for overloaded members (§1.23), it is a compile-time error to have two or more declarations that introduce members with the same name in a declaration space. It is never possible for a declaration space to contain different kinds of members with the same name

[...]

Note that blocks that occur as or within the body of a function member or anonymous function are nested within the local variable declaration space declared by those functions for their parameters.

Answer 2

Yes, this is how C# works.

When declaring a scope, any local variable from an outer scope is also known - there is no way to qualify that a local variable within the scope should override the local variable from outside.

Answer 3

That's normal behavior.

Sam Ng wrote a nice blog-post about this a while ago: http://blogs.msdn.com/b/samng/archive/2007/11/09/local-variable-scoping-in-c.aspx

Answer 4

Yes. This is expected because you are defining the variable within the local statement. If you were to define your variable at the class level, you would have different results.

Answer 5

It looks like you are concerned with the order of declaration (redeclaring a after the if blocks).

Consider the case that it is declared before the if blocks. Then you would expect it to be available within the scope of those blocks.

int a = 1;

if(true)
{
  var b = a + 1; // accessing a from outer scope
  int a = 2; // conflicts
}

There is not really a concept of "not in scope yet" at compile time.

You can actually create an inner scope with just bare curly braces:

{
   int a = 1;
}

if(true)
{
   int a = 2; // works because the a above is not accessible in this scope
}