C# inheritance. Derived class from Base class

The base problem you have is, that you have to construct an instance of type B (which contains of cause the properties of type A). Your approach to clone an A instance won't work, because that gives you an instance of type A, which you can't convert to B.

I would write constructors for class A and B which takes a parameter of type A. The constructor of class B just passes the value to its base class A. The constructor of class A knows how to copy the fields to itself:

class A {
    public A(A copyMe) {
        s1 = copyMe.s1;
        ...
    }

class B : A {

    public B(A aInstance) : base(aInstance) {
    }

}

Use it this way:

A a = new A();
a.s1 = "...";

B b = new B(a);

EDIT

When you don't want to have to change the constructor of A when adding new fields or props, you could use reflection to copy the properties. Either use a custom attribute to decorate what you want to copy, or copy just all props/fields of A:

public A (A copyMe) {
    Type t = copyMe.GetType();
    foreach (FieldInfo fieldInf in t.GetFields())
    {
        fieldInf.SetValue(this, fieldInf.GetValue(copyMe));
    }
    foreach (PropertyInfo propInf in t.GetProperties())
    {
        propInf.SetValue(this, propInf.GetValue(copyMe));
    }
}

I havn't tried the code, but the point should become clear.

You could create a generic clone method in class A:

     public T Clone<T>() where T : A, new() {
          return new T() { a = this.a, b = this.b};
     }

Or if you want to make the cloning extendable:

     public T Clone<T>() where T : A, new() {
          var result = new T();
          this.CopyTo(result);
          return result;
     }

     protected virtual void CopyTo(A other) {
          other.a = this.a;
          other.b = this.b;
     }

You use it like this:

     A  a = new A();
     // do stuff with a
     // Create a B based on A:
     B b = a.Clone<B>();

Please note: in your example, both the new A(), and the MemberwiseClone will create a new object of type A.

If you do not want to code the copy method yourself, you could look at a tool like AutoMapper.

After playing around and reading everything I could get my eyes on, both of the above solutions by GvS and Jan work. However, the end result that I wanted to achieve is not to be forced to write out each member in the Copy methods.

Why: a) If the class is edited and another object is added, the copy method will have to be updated. If someone else updates the class, they may forget to do this.

b) There may be a lot of members, and assigning them may be time consuming.

c) It just doesn't "feel" right. (Probably because I am very lazy).

Fortunately, I am not the only one with the same thoughts. Found a very very easy solution via the ValueInjector. (it has been discussed on these boards a lot).

After getting the dll (http://valueinjecter.codeplex.com/documentation)

The code becomes:

A a = new A();
a.s1 = "...";


B b = new B();
b.InjectFrom(a);

That's it :)

Obviously you would have to include:

using Omu.ValueInjecter;

And not forget to add it to the references.