c# - who is who or creating objects from extended class -

try understand power of oop

create 3 classes

class {      public string foo()     {         return "a";     } }  class b:a {      public string foo1()     {         return "a";     } }  class program {    static void main(string[] args)     {         a = new b(); //can use method         b b = new b(); //both         aa = new a(); //just         string result = b.foo();         string n = ((a)b).foo().tostring();     } } 

currently try understand difference between a = new b(); , a = new a(); - try use - can use same method classes - see pic

also try understand difference beetwen

b b = new b(); ((a)b).foo(); 

and

a = new b();  b.foo(); 

and

a =(А) new b(); , a = new a(); 

also try find tutorial explanation of main principles of oop, still have question. understanding

the simplest difference is:

a = new b(); b b = (b) a;  // works! 

a = new a(); b b = (b) a;  // compiler error 

even if assign b instance a-typed variable it's still instance of b class, can cast b.

but that's simple example. real differences pop out when class have virtual methods

class {     public virtual void foo()     {         console.writeline("a : foo();");     } } 

that overriden

class b : {     public override void foo()     {         console.writeline("b : foo();");     } } 

and/or hidden:

class c : {     public new void foo()     {         console.writeline("c : foo();");     } } 

within derived classes. class declarations you'll following results:

{     a = new a();     b b = new b();     c c = new c();      a.foo();  // prints : foo();     b.foo();  // prints b : foo();     c.foo();  // prints c : foo(); }  {     a = new a();  // prints : foo();     b = new b();  // prints b : foo();     c = new c();  // prints : foo();      a.foo();     b.foo();     c.foo(); } 

that's more interesting, isn't it? should read more overriding , hiding methods , class inheritance in general..