In C#, if a class or a method is not marked as sealed or virtual, what is it?

Question

In other words, what's the default (if nothing is specified)? I'm guessing the default is virtual, because you can use the "new" keyword to override a base method even when the base method has no virtual specified. If that's the case then why do we even need a Virtual option? Couldn't we just use Sealed when we do need to prevent further inheritance?

Answer 1

C# methods are sealed by default -- you cannot override them without the virtual keyword.

The new keyword hides the method in the base class.

Here's what I mean by hiding:

public class HidingA
{
    public string Get()
    {
        return "A";
    }
}

public class HidingB : HidingA
{
    public new string Get()
    {
        return "B";
    }
}

HidingA a = new HidingA();
HidingB b = new HidingB();

Console.WriteLine(a.Get()); // "A"
Console.WriteLine(b.Get()); // "B"

HidingA c = new HidingB();
Console.WriteLine(c.Get()); // "A". Since we're calling this instance of "B" an "A",    
                            //we're using the "A" implementation.

Now, the Virtual version!

public class VirtualA
{
    public virtual string Get()
    {
        return "A";
    }
}

public class VirtualB : VirtualA
{
    public override string Get()
    {
        return "B";
    }
}
VirtualA a = new VirtualA();
VirtualB b = new VirtualB();

Console.WriteLine(a.Get()); // "A"
Console.WriteLine(b.Get()); // "B"

VirtualA c = new VirtualB();
Console.WriteLine(c.Get()); // "B". We overrode VirtualB.Get, so it's using the 
                            // "B" implementation of the method

So if we make a method that takes HidingA as a parameter and pass it an instance of a HidingB , we're going to get the HidingA implementation of the Get method.

MSDN: http://msdn.microsoft.com/en-us/library/6fawty39.aspx

Classes are open for inheritance unless the sealed keyword is specified.

Answer 2

new creates a new method (not overriding it), where as an overridden one would replace it, hence being virtual . As stated above by DanielM, methods are sealed by default.

This is why we need virtual:

   class Foo
   {
       public void Baz() { Assert("Foo.Baz"); }
   }

   class ChildOfFoo : Foo
   {
       public new void Baz() { Assert("ChildOfFoo.Baz"); }
   }

   Foo foo = new ChildOfFoo();
   foo.Baz(); // Foo.Baz