This is a common discussion I see on the internet[0][1][2][3]. Paraphrased below

You shouldn't need to know whether the client is using an interface or an implementation class. List represents a list, and a User represents a user - not an IUser. Another problem with using I, is that (oddly) we use it to communication the implementation decision of using an interface. Usually we don't want implementation decisions expressed so loudly, because that makes them hard to change.

[0]http://programmers.stackexchange.com/questions/117348/should...

[1]http://stackoverflow.com/questions/541912/interface-naming-i...

[2]http://stackoverflow.com/questions/5816951/prefixing-interfa...

[3]https://blogs.msdn.microsoft.com/brada/2004/02/03/why-do-int...