Instantiate a class and assign it to an interface variable

http://stackoverflow.com/questions/1176416/implementing-ilist-interface

I as looking at the implementation of the IList interface using the above linke. The private member of the class has been declared as

        private readonly IList<T> _list = new List<T>();

I don't quite understand how we can instantiate a class List<T> and assign it to an interface variable _list.My understanding was that an interface was a contract of methods that need to be implemented when a class derives from it. What exactly is happening here? It compiles without problems.

 

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