C# Implicit Variables (var)

In C#, variables must be declared with a data type. These are called explicitly typed variables.

int i =　100;//　Explicit variables

C#3The '.0' introduces the 'var' keyword to declare method-level variables without explicitly specifying the data type. 　 　 　 　 　 　 　 　 　 　

var j　=　100;　//　Implicitly typed local variables

The compiler will infer the type of the variable from the expression on the right side of the = operator. In this case, 'var' will be compiled as int.

The following types are inferred from expressions.

int i =　10;
var j = i　+　1;　//　Compiled as int

The 'var' keyword can be used to declare variables of any built-in data type, user-defined type, or anonymous type. The following example shows how the C# compiler infers the type based on the value: 　 　 　 　

static void Main(string[] args)
{
　　　　var i　=　10;
　　　　Console.WriteLine("Type of i is {0}", i.GetType());
　　　　var str = "Hello World!!";
　　　　Console.WriteLine("Type of str is {0}", str.GetType());
　　　　var dbl =　100.50d;
　　　　Console.WriteLine("Type of dbl is {0}", dbl.GetType());
　　　　var isValid = true;
　　　　Console.WriteLine("Type of isValid is {0}", isValid.GetType());
　　　　var ano = new { name = "Steve" };
　　　　Console.WriteLine("Type of ano is {0}", ano.GetType());
　　　　var arr = new[] {　1,　10,　20,　30　};
　　　　Console.WriteLine("Type of arr is {0}", arr.GetType());
　　　　var file = new FileInfo("MyFile");
　　　　Console.WriteLine("Type of file is {0}", file.GetType());
}

Implicitly typed variables must be initialized at the time of declaration; otherwise, the C# compiler will give an error: Implicitly typed variables must be initialized.

var i;　//　Compile-time error: Implicitly typed variables must be initialized
i　=　100;

var does not allow multiple variable declarations in a single statement.

var i　=　100,　j　=　200,　k　=　300;　//　Error: var variables cannot be declared in a single statement//The following is also valid var　i　=　100;　
var j　=　200;　
var k　=　300;

var cannot be used for function parameters.

void Display(var　param)　//Compile-time error
{
　　　　Console.Write(param);
}

var can be used in for and foreach loops.

for(var　i　=　0;　i　<　10;　i++)
{      Console.WriteLine(i);
}

var can also be used with LINQ queries.

//　String Collection
IList<string> stringList = new List<string>() {　
　　　　"C# Tutorials",
　　　　"VB.NET Tutorials",
　　　　"Learn C++",
　　　　"MVC Tutorials",
　　　　"Java"　
};
//　LINQ Query Syntax
var result = from s in stringList
　　　　　　　　　　　　where s.Contains("Tutorials")　
　　　　　　　　　　　　select s;