Creating objects from an interface in C#

Question

It is possible, given only an interface, to create an object from this?

Something like:

var obj = new IWidget();

(I know this code isn't right - VS stays cannot create an instance of IWidget)

I'm in a context where my project has references to the interfaces, and I want to create concrete objects and return them from a method - but I can't figure out how to create the objects purely from the interfaces.

Answer 1

You can't create an object from an interface. You can create an object from a class that uses that interface.

For example:

IList<string> x = new IList<string>();

will not work.

IList<string> x = new List<string>();

will.

Interfaces cannot be created, only objects that use the interface can be created.

Answer 2

That code is actually correct for COM objects, due to compiler wizardry. Normally it's not though.

The problem is, how is the compiler meant to know which implementation to create? There could be any number around. You might want to consider having a factory for the interface, or possibly using dependency injection (e.g. with Spring.NET or Castle Windsor).