C#: new versus override-Collection of common programming errors

The new modifier instructs the compiler to use your child class implementation instead of the parent class implementation. Any code that is not referencing your class but the parent class will use the parent class implementation.

public class Base
{
    public virtual void DoIt()
    {
    }
}

public class Derived : Base
{
    public override void DoIt()
    {
    }
}

Base b = new Derived();
b.DoIt();

will call Derived.DoIt if that overrides Base.DoIt.

The override modifier may be used on virtual methods and must be used on abstract methods. This indicates for the compiler to use the last defined implementation of a method. Even if the method is called on a reference to the base class it will use the implementation overriding it.

public class Base
{
    public virtual void DoIt()
    {
    }
}

public class Derived : Base
{
    public new void DoIt()
    {
    }
}


Base b = new Derived();
Derived d = new Derived();
b.DoIt();
d.DoIt();

Will first call Base.DoIt, then Derived.DoIt. They’re effectively two entirely separate methods which happen to have the same name, rather than the derived method overriding the base method.