I have an interface IEntity
public interface IEntity{
bool Validate();
}
And I have a class Employee which implements this interface
public class Employee : IEntity{
public bool Validate(){ return true; }
}
Now if I have the following code
Employee emp1 = new Employee();
IEntity ent1 = (IEntity)emp1; // Is this a boxing conversion?
If it is not a boxing conversion then how does the cast work?
No, it's not.
Your instance of Employee is already a Reference Type. Reference Types are stored on the Heap so it doesn't need to be boxed/unboxed.
Boxing only occurs when you store a Value Type on the Heap or, in MSDN language, you could say:
Boxing is an implicit conversion of a Value Types (C# Reference) to the type object or to any interface type implemented by this value type. Boxing a value type allocates an object instance on the heap and copies the value into the new object.
Boxing means converting a value type to object. You are converting a reference type to another reference type, so this is not a boxing conversion.
In your example above, no but sometimes yes.
Boxing is the process of "boxing" a value type into a referenceable object; a reference type. In your example above, Employee is already a reference type, so it is not boxed when you cast it to IEntity.
However, had Employee been a value type, such as a struct (instead of a class), then yes.
No.
Because emp1 is a reference type.
Boxing occurs when a value type is converted to an object, or an interface type.
No, since Employee is a class, which is a reference type rather than a value type.
From MSDN:
Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing extracts the value type from the object.
The aforementioned MSDN link has further examples that should help clarify the topic.