Why can't the C# constructor infer type?

Question

Why is type inference not supported for constructors the way it is for generic methods?

public class MyType<T>
{
   private readonly T field;
   public MyType(T value) { field = value; }
}

var obj = new MyType(42); // why can't type inference work out that I want a MyType<int>?

Though you could get around this with a factory class,

public class MyTypeFactory
{
   public static MyType<T> Create<T>(T value)
   {
      return new MyType<T>(value);
   }
}
var myObj = MyTypeFactory.Create(42);

Is there a practical or philosophical reason why the constructor can't support type inference?

Answer 1

Is there a philosophical reason why the constructor can't support type inference?

No. When you have

new Foo(bar)

then we could identify all types called Foo in scope regardless of generic arity, and then do overload resolution on each using a modified method type inference algorithm. We'd then have to create a 'betterness' algorithm that determines which of two applicable constructors in two types that have the same name but different generic arity is the better constructor. In order to maintain backwards compatibility a ctor on a non-generic type must always win.

Is there a practical reason why the constructor can't support type inference?

Yes. Even if the benefit of the feature outweighs its costs -- which are considerable -- that's not sufficient to have a feature implemented. Not only does the feature have to be a net win, it has to be a large net win compared to all the other possible features we could be investing in. It also has to be better than spending that time and effort on bug fixing, performance work, and other possible areas that we could put that effort. And ideally it has to fit in well to whatever the "theme" is of the release.

Furthermore, as you correctly note, you can get the benefits of this feature without actually having the feature itself, by using a factory pattern. The existence of easy workarounds makes it less likely that a feature will ever be implemented.

This feature has been on the list of possible features for a long time now. It's never been anywhere near high enough on the list to actually get implemented.

UPDATE March 2015

The proposed feature made it close enough to the top of the list for C# 6 to be specified and designed, but was then cut.

Answer 2

public class MyType<T> 
{ 
   private readonly T field; 
   public MyType(T value) { field = value; } 
} 

they can, there is no need to tell the constructor 'what T is' again, seeing as you have already done that in the class decleration.

also your factory is incorrect, you need to have public class MyTypeFactory<T> not just public class MyTypeFactory - unless you declare the factory inside the MyType class

Edit for update:

Well, is 42 a long, a short, an int, or something else?

Let's say you have the following

class Base
{
   public virtual void DoStuff() { Console.WriteLine("Base"); }
}

class Foo : Base
{
   public override void DoStuff() { Console.WriteLine("Foo");  }
}

Then you did this

var c = new Foo();

var myType = new MyType(c);

Would you expect foo to be used, or base? We need to tell the compiler what to use in place of T

When you really wanted to on type base

Hence the

var myType = new MyType<Base>(c);

Answer 3

The main reason generic type inference can't work on constructors like you wish is because the class "MyType" doesn't even exist when all you've declared is "MyType<T>". Remember it is legal to have both:

public class MyType<T> {
}

and

public class MyType {
}

Both would be legal. How would you disambiguate your syntax if you did in fact declare both, and both of them declared a conflicting constructor.

Answer 4

Although this has been answered many times before, I feel I need to clarify one thing: C# supports generic type inference on constructors. The problem is, it doesn't support neither adding generic parameters to constructors nor type generic type inference. Wanting to infer generic type argument of the type itself is basically the same as requiring Foo.Bar(0) to infer to Foo<int>.Bar(0).

Answer 5

The constructor needs to have the same generic specification as the class itself. Otherwise it would be impossible to know if the int in your example would relate to the class or to the constructor.

var obj = new MyType<int>(42);

Would that be class MyType<T> with constructor MyType(int) or class MyType with constructor MyType<T>(T)?