C# Language Specification

Declaration statements

A declaration-statement declares a local variable or constant. Declaration statements are permitted in blocks, but are not permitted as embedded statements.

declaration-statement:
local-variable-declaration ;
local-constant-declaration ;

1. Local variable declarations

A local-variable-declaration declares one or more local variables.

local-variable-declaration:
local-variable-type local-variable-declarators

local-variable-type:
type
var

local-variable-declarators:
local-variable-declarator
local-variable-declarators , local-variable-declarator

local-variable-declarator:
identifier
identifier = local-variable-initializer

local-variable-initializer:

expression
array-initializer

The local-variable-type of a local-variable-declaration either directly specifies the type of the variables introduced by the declaration, or indicates with the identifier var that the type should be inferred based on an initializer. The type is followed by a list of local-variable-declarators, each of which introduces a new variable. A local-variable-declarator consists of an identifier that names the variable, optionally followed by an “=” token and a local-variable-initializer that gives the initial value of the variable.

In the context of a local variable declaration, the identifier var acts as a contextual keyword (§2.4.3).When the local-variable-type is specified as var and no type named var is in scope, the declaration is an implicitly typed local variable declaration, whose type is inferred from the type of the associated initializer expression. Implicitly typed local variable declarations are subject to the following restrictions:

• 
  The local-variable-declaration cannot include multiple local-variable-declarators.
  
• 
  The local-variable-declarator must include a local-variable-initializer.
  
• 
  The local-variable-initializer must be an expression.
  
• 
  The initializer expression must have a compile-time type.
  
• 
  The initializer expression cannot refer to the declared variable itself
  

The following are examples of incorrect implicitly typed local variable declarations:

var x; // Error, no initializer to infer type from

var y = {1, 2, 3}; // Error, array initializer not permitted
var z = null; // Error, null does not have a type
var u = x => x + 1; // Error, anonymous functions do not have a type
var v = v++; // Error, initializer cannot refer to variable itself

The value of a local variable is obtained in an expression using a simple-name (§7.6.2), and the value of a local variable is modified using an assignment (§7.17). A local variable must be definitely assigned (§5.3) at each location where its value is obtained.

The scope of a local variable declared in a local-variable-declaration is the block in which the declaration occurs. It is an error to refer to a local variable in a textual position that precedes the local-variable-declarator of the local variable. Within the scope of a local variable, it is a compile-time error to declare another local variable or constant with the same name.

A local variable declaration that declares multiple variables is equivalent to multiple declarations of single variables with the same type. Furthermore, a variable initializer in a local variable declaration corresponds exactly to an assignment statement that is inserted immediately after the declaration.

The example

void F() {

int x = 1, y, z = x * 2;
}

corresponds exactly to

void F() {
int x; x = 1;
int y;
int z; z = x * 2;
}

In an implicitly typed local variable declaration, the type of the local variable being declared is taken to be the same as the type of the expression used to initialize the variable. For example:

var i = 5;
var s = "Hello";
var d = 1.0;
var numbers = new int[] {1, 2, 3};
var orders = new Dictionary();

The implicitly typed local variable declarations above are precisely equivalent to the following explicitly typed declarations:

int i = 5;
string s = "Hello";
double d = 1.0;
int[] numbers = new int[] {1, 2, 3};
Dictionary orders = new Dictionary();

2.