Interfaces can't be instantiated but is this an exception

Question

I'm very surprised after seeing that I actually have to Instantiate an Interface to use the Word Interoop in C#.

Microsoft.Office.Interop.Word.Application word = new Microsoft.Office.Interop.Word.Application();

The Microsoft.Office.Interop.Word.Application according to what its XML documentation says is an interface:

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How is it possible, has visual studio confused it with something else? Or what allows this interface to be instantiated?

Related: [Please explain why I am able to instantiate the "Application" interface in Excel VSTO](http://stackoverflow.com/q/11039064) — Cody Gray - on strike, May 20 '14 at 15:00
See http://stackoverflow.com/questions/1303717/newing-up-interfaces — Kris Vandermotten, May 20 '14 at 15:02

score 14 · Accepted Answer · edited May 23 '17 at 11:50

14

It's because it's a COM interface. COM interfaces - and only COM interfaces - can be instantiated directly. The runtime will create an instance of the real type behind the scenes. Personally I think it's a bit ugly (and I can't find any references to it in the C# spec) but there we go.

You can actually fake this so that the C# compiler will believe your interface is a COM type without getting COM involved properly... see my related question for details.

edited May 23 '17 at 11:50

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answered May 20 '14 at 14:58

Jon Skeet

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You are a real C# king from my perspective – JAX May 20 '14 at 14:59

Yair Nevet · Answer 2 · 2014-05-21T06:56:57.337

You can't instantiate an interface in c# by definition and Application is not a regular .NET interface, but a COM interface. From MSDN:

This is a .NET interface derived from a COM coclass that is required by managed code for interoperability with the corresponding COM object.

Now, you're probably asking your self how come?

well, when the C# compiler identifies that a System.Runtime.InteropServices.CoClass attribute is decorating a interface, it would automatically use Activator.CreateInstance on the provided type from the CoClass property, for example:

[System.Runtime.InteropServices.CoClass(typeof(Test))] - So a new instance of Test will be created.

Interfaces can't be instantiated but is this an exception

2 Answers2