C# List: why you cannot do `List foo = new List();`

Question

If you have an Interface IFoo and a class Bar : IFoo, why can you do the following:

List foo = new List();          


        

Answer 1

The reason is that C# does not support co- and contravariance for generics in C# 3.0 or earlier releases. This is being implemented in C# 4.0, so you'll be able to do the following:

IEnumerable foo = new List();

Note that in C# 4.0, you can cast to IEnumerable, but you won't be be able cast to List. The reason is due to type safety, if you were able to cast a List to List you would be able to add other IFoo implementors to the list, breaking type safety.

For more background on covariance and contravariance in C#, Eric Lippert has a nice blog series.