Using Target-Typed New with the params keyword

[brandon-irl]

Can someone tell me why I can't do this?

        static void Main(string[] args)
        {
            DoSomething(new DateTime()); // This is fine
            DoSomething(new ());         // ERROR: The type 'DateTime[]' may not be used as the target type of new()
        }

        private static void DoSomething(params DateTime[] dates)
        {
            // TODO
        }

[333fred]

Let me answer your question with a question: were you expecting that to give you a new array, or were you expecting that to give you a new DateTime? Because the compiler inferred the former (and has no choice but to infer the former), and to create a new array you need an array creation expression, not a constructor.

[brandon-irl]

Makes sense, but then how does DoSomething(new DateTime()); create a new array, and why can't new() do the same?

[333fred]

how does DoSomething(new DateTime()); create a new array

This is, in general, how the params keyword words. When the compiler sees a params keyword, it allows you to optionally put a series of expressions with the element type of the params array. new DateTime() has the same type as the element type of the params array, so it's allowed. new() doesn't have a type, it's inferred from context. When you provide a single element, the compiler has to figure out what you mean, and here new() could either refer to the element type of the params array, or it could refer to the array type itself. C# says that the compiler must always prefer the params array type itself, not the element type, so new() is target-typed to DateTime[], not DateTime.

[brandon-irl]

I see makes sense. I assumed that knowing that new() can't be used with array target types, the compiler would use the element type and not prefer the params array type. For anyone else reading this, DoSomething(new (), new()); works and illustrates the answer.

[333fred]

It is theoretically something we could change with a language proposal, I believe, but I'm not certain whether we explicitly decided against it during new() design. @alrz or @jcouv, do either of you remember if this fell out, or if it was an intentional decision?

[alrz]

Maybe the error is misleading. In a similar situation you get an ambiguous call, since new() does not contribute to overload resolution.

using System;
class C {   
    // effectively we have:
    private static void DoSomething(DateTime[] dates) {}
    private static void DoSomething(DateTime dates) {}  
    static void Main() {
        DoSomething(new()); // error CS0121: The call is ambiguous
    }
}

In params case, afaik we choose the expanded form based on the type of the arguments, but since new() doesn't have a type, it's resolved to the method as-is.