Using a collection to store generics whose type implement a common interface

Question

I have a type:

ExcelSheet<T>

And I have Some types that implement an interface:

IAddress
public class Instructor :  IAddress
public class Student : IAddress

I would like to do the following.

....
ExcelSheet<Instructor> instructorSheet = GetSheet<Instructor>(); 
ExcelSheet<Student> student = GetSheet<Student>();

List<ExcelSheet<IAddress>> sheetsWithAddress = new List<ExcelSheet<IAddress>>
    {
        instructorSheet,
        student
    }

As written this is not possible. I'm using c# 4.0 Is there a way for this to work?
Is this a bad idea?

Answer 1

Yes, you can. This is covariance and it's a new feature in C# 4.0.

Read more here:

• http://msdn.microsoft.com/en-us/library/dd799517.aspx

Answer 2

If you constrain the T in ExcelSheet to IAddress it will work.

class ExcelSheet<T> where T : IAddress