How to cast a base class to a derived class

Question

Why can't I cast a base class to a derived class? Also, why doesn't the compiler catch this?

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            Parent p = new Parent();
            Child c = (Child)p;

        }
    }

    class Parent
    {
        public string Data { get; set; }
    }

    class Child : Parent
    {
        public string OtherDate { get; set; }
    }
}

Pretend it works. What does `c.OtherDate` do? – Blorgbeard Oct 30 '13 at 23:44 — Blorgbeard, Oct 30 '13 at 23:44

score 0 · Accepted Answer · edited Oct 31 '13 at 00:38

0

p is an instance of Parent, so you cannot tell the runtime to interpret it as one.

The compiler does not catch it because code like this

 Parent p = new Child();
 Child c = (Child)p;

and compilers do not do the static code analysis needed to catch it. Reasons for not checking it are:

It is time consuming
It can only catch some of the instances of the error.

edited Oct 31 '13 at 00:38

Lee Taylor

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answered Oct 30 '13 at 23:44

SJuan76

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How to cast a base class to a derived class

1 Answers1