Implicitly Typed Local Variables (C# Programming Guide)

Artykuł
02/04/2013

Local variables can be given an inferred "type" of var instead of an explicit type. The var keyword instructs the compiler to infer the type of the variable from the expression on the right side of the initialization statement. The inferred type may be a built-in type, an anonymous type, a user-defined type, or a type defined in the .NET Framework class library. For more information about how to initialize arrays with var, see Implicitly Typed Arrays (C# Programming Guide).

The following examples show various ways in which local variables can be declared with var:

// i is compiled as an int
var i = 5;

// s is compiled as a string
var s = "Hello";

// a is compiled as int[]
var a = new[] { 0, 1, 2 };

// expr is compiled as IEnumerable<Customer>
// or perhaps IQueryable<Customer>
var expr =
    from c in customers
    where c.City == "London"
    select c;

// anon is compiled as an anonymous type
var anon = new { Name = "Terry", Age = 34 };

// list is compiled as List<int>                             
var list = new List<int>();

It is important to understand that the var keyword does not mean "variant" and does not indicate that the variable is loosely typed, or late-bound. It just means that the compiler determines and assigns the most appropriate type.

The var keyword may be used in the following contexts:

On local variables (variables declared at method scope) as shown in the previous example.
In a for initialization statement.
```
for(var x = 1; x < 10; x++)
```
In a foreach initialization statement.
```
foreach(var item in list){...}
```

In a using statement.

using (var file = new StreamReader("C:\\myfile.txt")) {...}

For more information, see How to: Use Implicitly Typed Local Variables and Arrays in a Query Expression (C# Programming Guide).

var and Anonymous Types

In many cases the use of var is optional and is just a syntactic convenience. However, when a variable is initialized with an anonymous type you must declare the variable as var if you need to access the properties of the object at a later point. This is a common scenario in LINQ query expressions. For more information, see Anonymous Types (C# Programming Guide).

From the perspective of your source code, an anonymous type has no name. Therefore, if a query variable has been initialized with var, then the only way to access the properties in the returned sequence of objects is to use var as the type of the iteration variable in the foreach statement.

class ImplicitlyTypedLocals2
{
    static void Main()
    {
        string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };

        // If a query produces a sequence of anonymous types, 
        // then use var in the foreach statement to access the properties.
        var upperLowerWords =
             from w in words
             select new { Upper = w.ToUpper(), Lower = w.ToLower() };

        // Execute the query
        foreach (var ul in upperLowerWords)
        {
            Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
        }
    }
}
/* Outputs:
    Uppercase: APPLE, Lowercase: apple
    Uppercase: BLUEBERRY, Lowercase: blueberry
    Uppercase: CHERRY, Lowercase: cherry        
 */

Remarks

The following restrictions apply to implicitly-typed variable declarations:

var can only be used when a local variable is declared and initialized in the same statement; the variable cannot be initialized to null, or to a method group or an anonymous function.
var cannot be used on fields at class scope.
Variables declared by using var cannot be used in the initialization expression. In other words, this expression is legal: int i = (i = 20); but this expression produces a compile-time error: var i = (i = 20);
Multiple implicitly-typed variables cannot be initialized in the same statement.
If a type named var is in scope, then the var keyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration.

You may find that var can also be useful with query expressions in which the exact constructed type of the query variable is difficult to determine. This can occur with grouping and ordering operations.

The var keyword can also be useful when the specific type of the variable is tedious to type on the keyboard, or is obvious, or does not add to the readability of the code. One example where var is helpful in this manner is with nested generic types such as those used with group operations. In the following query, the type of the query variable is IEnumerable<IGrouping<string, Student>>. As long as you and others who must maintain your code understand this, there is no problem with using implicit typing for convenience and brevity.

// Same as previous example except we use the entire last name as a key.
// Query variable is an IEnumerable<IGrouping<string, Student>>
 var studentQuery3 =
     from student in students
     group student by student.Last;

However, the use of var does have at least the potential to make your code more difficult to understand for other developers. For that reason, the C# documentation generally uses var only when it is required.

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Implicitly Typed Local Variables (C# Programming Guide)

var and Anonymous Types

Remarks

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