How interface can be instantiate this way

Question

from a url i saw people can instantiate interface like this way

class Program
{
    static void Main(string[] args)
    {
        var foo = new IFoo(1);
        foo.Do();
    }
}

[
    ComImport, 
    Guid("C906C002-B214-40d7-8941-F223868B39A5"), 
    CoClass(typeof(FooImpl))
]
public interface IFoo
{
    void Do();
}

public class FooImpl : IFoo
{
    private readonly int i;

    public FooImpl(int i)
    {
        this.i = i;
    }

    public void Do()
    {
        Console.WriteLine(i);   
    }
}

how it is possible to write like this var foo = new IFoo(1); looking for guidance. thanks

It isn't. I suggest stopping asking all those interface question and read some material about interfaces and polymorphism in C#. — Samuel, Feb 20 '14 at 09:04
see http://stackoverflow.com/questions/1303717/newing-up-interfaces?rq=1 — rt2800, Feb 20 '14 at 09:07
Even if possible, this would just make the whole `interface` idea useless, the big question is: Why would you *ever* want to do this? — Jite, Feb 20 '14 at 09:09
@Thomas: did you follow the [documentation guidelines](http://msdn.microsoft.com/en-us/library/hh708954.aspx) about adding a reference to a COM object? — Steve B, Feb 20 '14 at 09:13
@dcastro This was indeed new to me. Thomas please accept my apology on the first sentence. I've learnt something new today. Beside COM specific attributes this is not a common approach when working with interfaces. And I still recommend concentrating on base interface concepts introduction. — Samuel, Feb 20 '14 at 09:33
Looks like he's asking about the code from [Instantiating interfaces - Ayande](http://ayende.com/blog/4121/instantiating-interfaces). — Sam, Feb 20 '14 at 09:34

dcastro · Accepted Answer · 2014-02-20T09:28:29.367

That's just how COM works. You've declared FooImpl to be IFoo's coclass. new IFoo(1); will be compiled to new FooImpl(1);

According to §17.5 of the C# specification, attributes under the System.Runtime.InteropServices namespace may break all the rules. This is specific to Microsoft's C# implementation.

Marc Gravell and Jon Skeet have really good blog posts about this: Who says you can’t instantiate an interface? and Faking COM to fool the C# compiler

How interface can be instantiate this way

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