In earlier versions of C# IEnumerable was defined like this: public interface IEnumerable<T> : IEnumerable Since C# 4 the definition is: public interface IEnumerable<out T> : IEnumerable Is it just to make the annoying casts in LINQ expressions go away? Won't this introduce the same problems like with string[] <: object[] (broken array variance) in C#? How was the addition of the covariance done from a compatibility point of view? Will earlier code still work on later versions of .NET or is recompilation necessary here? What about the other way around? Was previous code using this interface strictly invariant in all cases or is it possible that certain use cases will behave different now?

Why was IEnumerable<T> made covariant in C# 4?

In earlier versions of C# IEnumerable was defined like this:

public interface IEnumerable<T> : IEnumerable

Since C# 4 the definition is:

public interface IEnumerable<out T> : IEnumerable

Is it just to make the annoying casts in LINQ expressions go away?
Won't this introduce the same problems like with string[] <: object[] (broken array variance) in C#?
How was the addition of the covariance done from a compatibility point of view? Will earlier code still work on later versions of .NET or is recompilation necessary here? What about the other way around?
Was previous code using this interface strictly invariant in all cases or is it possible that certain use cases will behave different now?