C# create instance of the derived class in the base class

Question

I have the following set up:

public abstract class A
{
 public void f()
 {
 //Want to make an instance of B or C here
 //A bOrC = new ?
 }
 public abstract void f2();
}
public class B : A { public override void f2(){} }
public class C : A { public override void f2(){} }

Is this possible? If so how?

Edit: bOrC needs to be the type of the particular derived class f() is called from

• Marc Gravell: Sure, you can new B() or new C() - is there something specific in your context that makes this not possible?
• Jon Egerton: I think the point is that an instance of B or C is required, but which of these types is not known at the point of the call.
• Jon Egerton: Is the f() method to make a copy of the current instance or something - ie to make a new instance of the child type?
• gsharp: can you be a bit more specific?
• Stormenet: Perhaps this helps: stackoverflow.com/questions/1368643/…

Answer 1

I can think of two ways to solve this issue. One uses generics and the other just requires an abstract method. First the simple one.

public abstract class A
{
 public void f()
 {
 A bOrC = newInstance();
 }
 public abstract void f2();
 protected abstract A newInstance();
}
public class B : A {
 public override void f2(){}
 public override A newInstance(){
 return new B();
 }
}
public class C : A {
 public override void f2(){}
 public override A newInstance(){
 return new C();
 }
}

And now with generics

public abstract class A<T> where T : A, new()
{
 public void f()
 {
 A bOrC = new T();
 }
 public abstract void f2();
}
public class B : A<B> {
 public override void f2(){}
}
public class C : A<C> {
 public override void f2(){}
}

• Clivest: Thanks. I had thought of your first method, but wondered if there was a neater way of doing it using things like this.GetType()

Answer 2

You can use Activator.CreateInstance(this.GetType());

• Clivest: It wasn't me that down voted, but I would then need to cast the object it returns into B or C without knowing which one. It doesn't solve my problem
• Ken Wayne VanderLinde: You can just cast it to A, and you'll have everything you would be able to do the other way as well.

Answer 3

This is not possible, and would lead to some weird consequences if it was. However, there is an easy work around rendering code that is easy to read.

public abstract class A
{
 public void f()
 {
 //Want to make an instance of B or C here
 //A bOrC = new ?
 A bOrC = Create();
 }
 public abstract void f2();
 public abstract A Create();
}
public class B : A {
 public override void f2(){}
 public override A Create() { return new B(); }
}
public class C : A {
 public override void f2(){}
 public override A Create() { return new C(); }
}