Implicit typing; why just local variables?

Question

Does anyone know or care to speculate why implicit typing is limited to local variables?

var thingy = new Foo();

But why not...

var getFoo() {
    return new Foo(); 
}

Answer 1

Eric Lippert did an entire blog post on the subject.

• http://blogs.msdn.com/ericlippert/archive/2009/01/26/why-no-var-on-fields.aspx

In summary, the main problem is that it would have required a major re-architecture of the C# compiler to do so. Declarations are currently processed in a single pass manner. This would require multiple passes because of the ability to form cycles between inferred variables. VB.net has roughly the same problem.

Answer 2

Jared has a fantastic link in his answer, to a fantastic topic.

I think it does not answer the question explicitly.

Why not?

var getFoo() {
    return new Foo(); 
}

The reason for this is:

What if?

class Foo {}

var GetFoo() {
   return GetBar(); 
}

var GetBar() {
  return GetBaz(); 
}

var GetBaz() {
   return new Foo();
}

You could deduce that GetFoo is going to return Foo, but you will have to trace through all the calls that method makes and its children makes just to infer the type. As it stands the C# compiler is not designed to work in this way. It needs method and field types early in the process before the code that infers types can run.

On a purely aesthetic level I find the var definitions on methods confuse things. Its one place where I think being explicit always helps, it protects you from shooting your self in the foot by accidentally returning a type that causes your signature and a ton of other dependent method signatures to change. Worst still, you could potentially change all you signatures of a method chain without even knowing you did so if you return the value of a method that returns object and happened to be lucky.

I think var methods are best left for dynamic languages like Ruby

Answer 3

Because it is much easyer to do. If you were to inference all types, one would need something like Hindley Milner type inference system which will in make your beloved C# into Haskel derivative language.

Answer 4

Essentially, the issue you are running into is that C# (thus far) is a statically typed language. A local variable defined as var is still statically typed, but syntactically hidden. A method returning var, on the other hand, has many implications. It becomes more of an interface for usage, and you don't gain anything by using var.

Answer 5

you can use in vs 2010 Dynamic

Dynamic getFoo() { 
    return new Foo();  
} 

Answer 6

I think it's because the scope of that implied type is much wider and therefore more likely to cause problems than within the scope of a single method.