The curious lack of type inference support in C# constructors

Yan Cui

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Given a generic class in C# like this one:

public class MyType<T>
{
    public MyType(T value) { }
}

You will need to specify what the type T should be when you call the constructor:

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

The compiler is not able to use type inference to infer the type T to be int.

Now, this is interesting as type inference is supported by any other generic method, so if you were to create a factory class for MyType<T> you could indeed make use of type inference like this:

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

Some argue that it’s not possible because of the possible presence of other overloaded constructors (or for that matter a non-generic version of MyType), but at the end of the day the constructor is a method like any other and the cited ambiguity can also appear with normal generic methods:

public void Foo(short value) {}
public void Foo(int value) {}
public void Foo(long value) {}
public void Foo(double value) {}
public void Foo<T>(T value) {}
…
Foo(42); // which Foo is called? Foo(int) of course!

So amidst all that ambiguity the compiler is able to infer the type for the Foo method, so what’s so special about the constructor? Am I missing something obvious here?

Luckily, as Eric Lippert stated in his answer to my question here, the reason the constructor does not support type inference is a practical one – the benefit of the feature does not out-weight the cost of its implementation and it is some way behind other possible features in terms of priority. Whilst he did say that this feature is on the list, considering that the ‘theme’ of the next C# release (5.0) is rumoured to be meta-programming, there’s a good chance we won’t be seeing type inference in the C# constructor for some time yet!

References:

Jon Skeet’s Brainteasers + Answers

StackOverflow question – why can’t the C# constructor infer type

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