Var

This C# keyword references a type in an implicit way. It aliases any type. The aliased type is determined by the C# compiler, and has no performance penalty.

The var keyword is an example of syntactic sugar. It makes programs shorter and easier to read. It can be used in method bodies and loops.

Example

Let us look at some C# code examples of the var keyword. To begin, here is a code example that shows the var keyword referencing a List of ints.

using System;
using System.Collections.Generic;

// Hover over the var keyword.
// ... Visual Studio will tell us the referenced type.
var codes = new List<int> { 1, 2, 7, 9 };
Console.WriteLine("LIST COUNT: " + codes.Count);
LIST COUNT: 4

Query expression example

Next we see a query expression. It returns an IEnumerable of ints. The query is evaluated to this type—and var implicitly refers to that type.

using System;
using System.Linq;

int[] numbers = { 1, 2, 3, 4, 5 };

// Use a query expression to get all the odd numbers from the array.
// ... The var stores the result IEnumerable.
var items = from item in numbers
            where (item % 2 == 1)
            select item;

// Each item is an int.
foreach (var item in items)
{
    Console.WriteLine("VAR LOOP: {0}", item);
}
// Use the same loop but without var.
foreach (int item in items)
{
    Console.WriteLine("LOOP 2: {0}", item);
}
VAR LOOP: 1
VAR LOOP: 3
VAR LOOP: 5
LOOP 2: 1
LOOP 2: 3
LOOP 2: 5

Error, null var

We cannot assign a var to null. This will result in a compiler warning. We also can not use var as a parameter type or a return value of a method.

class Program
{
    static void Main()
    {
        var test = null;
    }
}
Error CS0815
Cannot assign <null> to an implicitly-typed variable

Dictionary

Var simplifies the syntax of generic types. It is particularly useful for generic collections such as Dictionary. It allows you to omit the long type parameter lists.

using System;
using System.Collections.Generic;

// Use implicit type keyword var on Dictionary instance.
// ... Then use the collection itself.
var data = new Dictionary<string, int>();
data.Add("cat", 2);
data.Add("dog", 1);
Console.WriteLine("cat - dog = {0}", data["cat"] - data["dog"]);
cat - dog = 1

Case var

We can use the var keyword to capture variables when matching in a switch statement. Usually this is used when dealing with tuples and other complex types.

using System;

var bird = (20, "blue");
// Use var to capture variables in case statements.
switch (bird)
{
    case var (size, color) when size >= 10:
        Console.WriteLine($"SIZE >= 10, {color}");
        break;
    case var (_, color):
        Console.WriteLine($"SIZE < 10, {color}");
        break;
}
SIZE >= 10, blue

Summary

Var is for implicit typing. It can make code easier to read. A variable that uses the "var" type has the same type as one that has its type specified directly.