Question

I found an explanation What is a covariant return type? but I am no smart to understand it all.

I think Covariant return type theoritically is the type returned by a function that has the same signature with a built-in base class function whose return type is different.

class Base{
   TypeX func( int i ){return typex;} // builtin function
};
class Derived:Base{
   TypeY func(int i){return typey;}
}

Is my understanding of this so-called covariant return type correct ? [the term really confuses me.]

Answer 1

Unlike java, covariant return types are not supported in C#. I believe this is due to the implementation of C# properties, if covariant return types were allowed, the following would be possible:

class TypeX { }

class TypeY : TypeX { }

class Base
{
    public virtual TypeX Prop { get; set; }
}

class Derived : Base
{
    public override TypeY Prop { get; set; }
}

Derived derived = new Derived();
derived.Prop = new TypeY(); // Valid

Base @base = derived;
@base.Prop = new TypeX(); // Invalid - As this would be using the derived property which should be of TypeY

See Eric Lippert’s answer for more information.