c# upcasting is always allowed

Question

In Java,

"Up-casting is casting to a supertype, while downcasting is casting to a subtype. Supercasting is always allowed, but subcasting involves a type check and can throw a ClassCastException."

(What is the difference between up-casting and down-casting with respect to class variable)

Is upcasting also always allowed in C#?

Answer 1

yes, up-casting is allowed :-)

Casting and Type Conversions (C# Programming Guide)

Answer 2

OOP principles state that you can always upcast; however, unlike Java with very restricted number of primitive classes, .Net implementation allows to declare struct types, some of them are weird counter-examples with boxing:

 TypedReference reference = new TypedReference();

 // Compile time error here! Even if Object is the base type for all types
 Object o = (Object)reference;

Technically, TypedReference is an Object:

 Object
   ValueType
     TypedReference

you can easily check it:

 Console.Write(typeof(TypedReference).BaseType.BaseType == typeof(Object) 
   ? "TypedReference derived from Object via ValueType" 
   : "Very strange");

but in order to be represented as Object instance (via cast) it should be boxed which can't be done.

Answer 3

Yes, it is allowed, since a subclass is a particularization of the ancestor class.

Example:

Let us consider the case when we have a class called Bird, another called Sparrow and a third one Eagle. Sparrow and Eagle are inherited from Bird. Sparrows differ from Eagles greatly, but they are Birds. So, if you want to have a Collection of Birds for some reason, then you can have Eagle and Sparrow objects in that Collection at the same time, since they are still Birds, if only specific Birds.