The Moth

A new feature in C# version 3 that comes with Orcas allows you to declare variables within the body of a method like this:

var i = 3;
var s = "hi";
var o = new SomeType();

Note that the compiler generates IL that is identical to what it would generate if you typed:

int i = 3;
string s = "hi";
SomeType o = new SomeType();

In other words the 2nd line below compiles fine but the 3rd does not compile:

var b = true;
b = false; //fine
b = "pull the other one";  //Cannot implicitly convert type 'string' to 'bool'

So, the important point is that this local variable type inference, results in early-bound strongly-typed code. There is no late-binding taking place, besides the similarity with the var keyword in script languages where everything is an object/variant - that is not the case here.

Another point to stress is that variable inference does not work for class level fields or method arguments or anywhere else other than for local variables in a method.

Another example is the following:

IEnumerable<SomeType> col = new SomeCollectionType();
foreach (var a in col) //a is inferred by the compiler to be SomeType
{
    a.MethodOnSomeType();  //intellisense here pops-up as expected
}

It is neat that the compiler can infer what type you expect a variable to be by looking at the right hand-side of the assignment, but I personally will not be using this often as I prefer explicitness. I guess it is a time saver for very long types, for example:

// I'd rather type this
var myCol = new Dictionary<int, SomeType>();
//...than type this
Dictionary<int, SomeType> myCol = new Dictionary<int, SomeType>();

Local variable type inference is a feature that seems not very useful in its own right, but becomes important when used in conjunction with other language enhancements.