c# inheritance and casting down inheritance hierarchy

Question

I'd like to create a new object of type B from an existing object of type A. B inherits from A. I'd like to ensure that all the property values in the object of type A are copied to the object of type B. What's the best method for achieving this?

class A
{
     public int Foo{get; set;}
     public int Bar{get; set;}
}

class B : A
{
    public int Hello{get; set;}
}


class MyApp{
    public A getA(){
         return new A(){ Foo = 1, Bar = 3 };
    }

    public B getB(){
         A myA = getA();
         B myB = myA as B; //invalid, but this would be a very easy way to copy over the property values!
         myB.Hello = 5;
         return myB;
    }

    public B getBAlternative(){
         A myA = getA();
         B myB = new B();

         //copy over myA's property values to myB
         //is there a better way of doing the below, as it could get very tiresome for large numbers of properties
         myB.Foo = myA.Foo;
         myB.Bar = myA.Bar;

         myB.Hello = 5;
         return myB;
    }
}

score 4 · Accepted Answer · edited May 23 '17 at 12:04

class A {
    public int Foo { get; set; }
    public int Bar { get; set; }

    public A() { }

    public A(A a) {
        Foo = a.Foo;
        Bar = a.Bar;
    }
}

class B : A {
    public int Hello { get; set; }

    public B()
        : base() {

    }

    public B(A a)
        : base(a) {

    }
}

EDIT

If you don't want to copy each property, you can make A and B serializable and serialize your instance of A (say to a Stream, not a file) and initialize your instance of B with it. But I warn you, this is disgusting and has quite a bit of overhead:

A a = new A() {
    Bar = 1,
    Foo = 3
};

System.Xml.Serialization.XmlSerializer xs = new System.Xml.Serialization.XmlSerializer(typeof(A));
System.IO.Stream s = new System.IO.MemoryStream();
xs.Serialize(s, a);

string serial = System.Text.ASCIIEncoding.ASCII.GetString(ReadToEnd(s));
serial = serial.Replace("<A xmlns", "<B xmlns");
serial = serial.Replace("</A>", "</B>");

s.SetLength(0);
byte[] bytes = System.Text.ASCIIEncoding.ASCII.GetBytes(serial);
s.Write(bytes, 0, bytes.Length);
s.Position = 0;

xs = new System.Xml.Serialization.XmlSerializer(typeof(B));
B b = xs.Deserialize(s) as B;

You can get more information on ReadToEnd here.

Thanks for adding the Serializer code. I'd probably also be able to use reflection to loop through all the members and copy the values across, but that also comes with a lot of overhead. — Iain Sproat, Jan 27 '11 at 14:48

score 3 · Answer 2 · answered Jan 27 '11 at 11:33

3

You could define an explicit casting operator for B:

   public static explicit operator B(A a) 
   {
     B b = new B();
     b.Foo = a.Foo;
     b.Bar = a.Bar;
     b.Hello = 5;
     return b;
   }

so then you can do:

B myB = (B)myA;

answered Jan 27 '11 at 11:33

Nick Jones

6,413
2
18
18

This is a nice refactoring of the code - I like that B is then responsible for its casting from A. I could also create a constructor for B that accepted an object of type A. Unfortunately it still requires moving over the value for each property in turn. – Iain Sproat Jan 27 '11 at 11:38
That can be so misleading though...there's no need to override the casting operator... – bitxwise Jan 27 '11 at 11:38
I believe any implementation would require copying each value over, as moving from one class to it's subclass requires a new memory allocation. Gave a +1, but I also think that this could be difficult to follow for other developers. Personally I'd provide an overloaded constructor to take the base class. – Rob Jan 27 '11 at 11:39

score 0 · Answer 3 · answered Jan 27 '11 at 11:33

0

This is not possible in an OO mechanism but AutoMapper has been created for this very usage.

answered Jan 27 '11 at 11:33

Aliostad

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c# inheritance and casting down inheritance hierarchy

3 Answers3