C# local variable scope

Question

Possible Duplicate:
confused with the scope in c#

It appears that in C# a variable defined with the local scope to if/else/loop block is conflicting with variable defined outside following that block - see code snipped. An equivalent code compiles fine under C/C++ and Java. Is this expected behavior in C#?

public void f(){
  if (true) {
    /* local if scope */
    int a = 1;
    System.Console.WriteLine(a);
  } else {
    /* does not conflict with local from the same if/else */
    int a = 2;
    System.Console.WriteLine(a);
  }

  if (true) {
    /* does not conflict with local from the different if */
    int a = 3;
    System.Console.WriteLine(a);
  }

  /* doing this:
   * int a = 5;
   * results in: Error 1 A local variable named 'a' cannot be declared in this scope
   *  because it would give a different meaning to 'a', which is already used in a 
   *  'child' scope to denote something else
   * Which suggests (IMHO incorrectly) that variable 'a' is visible in this scope
   */

  /* doing this: 
   * System.Console.WriteLine(a);
   * results in: Error 1 The name 'a' does not exist in the current context..
   * Which correctly indicates that variable 'a' is not visible in this scope
   */
}

http://blogs.msdn.com/b/ericlippert/archive/2009/11/02/simple-names-are-not-so-simple.aspx — Servy, Aug 21 '12 at 18:25
possible duplicate of [confused with the scope in c#](http://stackoverflow.com/questions/1196941/confused-with-the-scope-in-c-sharp) and [C# Variable Scoping](http://stackoverflow.com/questions/2049330/c-sharp-variable-scoping). — Raymond Chen, Aug 21 '12 at 19:03
Thank you Servy. Per blog: 3) Local variables are in scope throughout the entire block in which the declaration occurs. **This is in contrast with C++, in which local variables are in scope in their block only at points after the declaration** — luchon, Aug 21 '12 at 20:30

score 5 · Answer 1 · answered Aug 21 '12 at 18:20

5

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.

answered Aug 21 '12 at 18:20

Oded

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1

It appears it boils down to "in-scope" definition. In C# unlike in (C++), if local variable is defined inside the block - it is in-scope within entire block regardless of the place of declaration. Whereas, in C++, local variable defined inside the block is in-scope **only at point after the declaration** – luchon Aug 21 '12 at 20:38

Jay · Answer 2 · 2012-08-21T18:28:34.077

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
}

score 3 · Answer 3 · answered Aug 21 '12 at 18:23

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

answered Aug 21 '12 at 18:23

Jensen

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score 3 · Answer 4 · edited Jun 20 '20 at 09:12

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.

score 0 · Answer 5 · answered Aug 21 '12 at 18:21

0

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.

answered Aug 21 '12 at 18:21

BlackHatSamurai

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C# local variable scope

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