C#•3y ago

Confused about is-a relationship (static and dynamic types) [Answered]

C#
// Testing
BaseClass newObject = new BaseClass();
BaseClass newObjectB = new DerivedClass(209.21312f);

C#
// Testing
BaseClass newObject = new BaseClass();
BaseClass newObjectB = new DerivedClass(209.21312f);

What's with the syntax on the second line? And why would we ever use the syntax on the second line if it limits us to using only the members in DerivedClass? Like is

C#
BaseClass newObjectB = new DerivedClass(209.21312f);

C#
BaseClass newObjectB = new DerivedClass(209.21312f);

The same as

C#
BaseClass newObjectB = new BaseClass(209.21312f)

C#
BaseClass newObjectB = new BaseClass(209.21312f)

45 Replies

Pobiega•3y ago

Is it fair to rephrase the question as "Why would you ever want to use a parent type instead of the actual type for a variable"?

Bobby BobOP•3y ago

Yeah I guess

Pobiega•3y ago

Well, for a local variable you basicly don't.

Bobby BobOP•3y ago

I think that's the question I'm trying to get at.... I think

Pobiega•3y ago

but for return types, or class properties, you do

Bobby BobOP•3y ago

I don't understand, wdym by basically don't? return types and class properties in what context? Like types and properties for the baseclass or derived class? I still don't understand the syntax and purpose of such a weird way to initialize an object. Why can't you just do BaseClass newObjectB = new BaseClass(209.21312f) instead of BaseClass newObjectB = new DerivedClass(209.21312f); ? Wouldn't there be zero differences between these code? Except for it's syntax, like functionally no difference?

Pobiega•3y ago

There is a ton of difference You see, the type you give after new is the "actual" type while the type of the variable is the "apparent" type 1 sec, lemme write some sample code

Bobby BobOP•3y ago

🤔

Pobiega•3y ago

public class BaseClass
{
    public float Value { get; }

    public BaseClass(float value) => Value = value;

    public virtual void DoSomething() => Console.WriteLine(Value);
}

public class DerivedClass : BaseClass
{
    public DerivedClass(float value) : base(value)
    {
    }

    public override void DoSomething() => Console.WriteLine(Value * 3);
}

public class BaseClass
{
    public float Value { get; }

    public BaseClass(float value) => Value = value;

    public virtual void DoSomething() => Console.WriteLine(Value);
}

public class DerivedClass : BaseClass
{
    public DerivedClass(float value) : base(value)
    {
    }

    public override void DoSomething() => Console.WriteLine(Value * 3);
}

if I now do

BaseClass item = new DerivedClass(100f);

item.DoSomething();

BaseClass item = new DerivedClass(100f);

item.DoSomething();

what do you think is printed?

Bobby BobOP•3y ago

mhmmmmm 🤔 I think this will be executed Console.WriteLine(Value); Instead of Console.WriteLine(Value * 3);

MODiX•3y ago

Pobiega#2671

REPL Result: Success

public class BaseClass
{
    public float Value { get; }

    public BaseClass(float value) => Value = value;

    public virtual void DoSomething() => Console.WriteLine(Value);
}

public class DerivedClass : BaseClass
{
    public DerivedClass(float value) : base(value)
    {
    }

    public override void DoSomething() => Console.WriteLine(Value * 3);
}

BaseClass item = new DerivedClass(100f);

item.DoSomething();

public class BaseClass
{
    public float Value { get; }

    public BaseClass(float value) => Value = value;

    public virtual void DoSomething() => Console.WriteLine(Value);
}

public class DerivedClass : BaseClass
{
    public DerivedClass(float value) : base(value)
    {
    }

    public override void DoSomething() => Console.WriteLine(Value * 3);
}

BaseClass item = new DerivedClass(100f);

item.DoSomething();

Console Output

Compile: 691.196ms | Execution: 54.397ms | React with ❌ to remove this embed.

Bobby BobOP•3y ago

So it would probably be 100 as the output wait what on earth??? I don't get it, what's going on?

Pobiega•3y ago

Well, the item is actually a DerivedClass instance its just stored in a BaseClass variable

Bobby BobOP•3y ago

Wait hold on I'm even more confused How can it store a reference type inside a reference type? Is it like a pointer of a pointer in memory ?

Pobiega•3y ago

this isnt C/C++, stop thinking about pointers 😛 DerivedClass inherits BaseClass, ye? that means that a DerivedClass is a BaseClass. It can just ALSO do other things

Bobby BobOP•3y ago

It helps me sometimes to think of it this way cuz most of what I apply in practice to learn is what I've learned in my first advanced programming class So wouldn't it make sense to just declare it as DerivedClass item = new DerivedClass

Pobiega•3y ago

in this case, absolutely thats what I meant with

Well, for a local variable you basicly don't.

however, there are situations where this "trick" is really useful as we saw, the actual class decides what code gets executed (when using virtual/override) So imagine a system that receives messages with a "key" property and a body of data now, there are hundreds of different message types each of them needs to be processed differently, and we want to code this up in a nice way

Bobby BobOP•3y ago

When u say messages, r u talking about one of those HTTP Get/Request thingy magik?

Pobiega•3y ago

sure why not its just some generic message at this point

Bobby BobOP•3y ago

ok, got it I'm still not clear on the virtual/override keyword. What's the virtual keyword?

Pobiega•3y ago

virtual makes the method overridable

Bobby BobOP•3y ago

If I have a virtual keyword put in place, does that mean all invokation on that virtual method is actually invoking it's derivedclass substitute of that method"?

Pobiega•3y ago

yup assuming the calling type is a derived class yes

Bobby BobOP•3y ago

oh, so every invokation to a virtual method is always an invokation to it's overriden counterpart

Pobiega•3y ago

yeah, unless the actual type is the baseclass

Bobby BobOP•3y ago

So in the same sense derivedclass can use a method from the baseclass, a baseclass can use a method from it's derivedclass right?

Pobiega•3y ago

BaseClass base = new BaseClass(); base.DoSomething(); would still invoke the base method yeah, as long as its virtual

Bobby BobOP•3y ago

WOuld it still invoke the base method if that method is a virtual method? Does a virtual keyword just basically forces the CLR to execute the overriden method instead of the original method instead?

Pobiega•3y ago

yes - no derived class is specified or taken into account here note that the "actual" type is BaseClass

Bobby BobOP•3y ago

Bobby BobOP•3y ago

I'm trying to wrap my head around this. So if you assign a static type to a variable, at the start of runtime, memory of that exact static type is allocated? But this memory changes during runtime when it comes across the dynamic type? Is this more to do with memory efficiency rather than something functional?

Pobiega•3y ago

This isn't at all related to "is-a" relationships and derived classes

Bobby BobOP•3y ago

Wait hold on, that still wouldn't make sense because can't I make the static and reference type the exact same if I want to give it a accurate memory allocation? oh....

Pobiega•3y ago

if you mark a field as static, it just means that instance is shared across all instances of the class it "belongs" to the class, rather than an instance of the class

Bobby BobOP•3y ago

oh I thought static could also refer to the type declared on the left hand side of the statement

Pobiega•3y ago

static string whatever = "hello"; thats a static field. note that using "variable" here is a bit vague no difference, they are the same we always use the lowercase one thou the lowercase is an alias for the uppercase one but its an alias that can't ever be subverted unlike String which could be changed and strings are reference types

Bobby BobOP•3y ago

I still don't really understand.... can I still invoke the base method even if that method has the virtual keyword in it? I'm trying to imagine like what would be the application of doing this weird way of initializing objects

Kouhai•3y ago

Just to clarify something, dynamic is very different 😅

Bobby BobOP•3y ago

Ah ty 😄

Pobiega•3y ago

Like I said, you wouldn't do it for a local variable its more commonly used with return types

Bobby BobOP•3y ago

What's a local variable in this context? Is this something like a variable declared inside of a method?

Pobiega•3y ago

yes, exactly that

public interface IMessageHandler
{
    IMessage Handle(string body);
}

public interface IMessage
{
}

public class BobbyMessageHandler : IMessageHandler
{
    public IMessage Handle(string body)
    {
        return new BobbyMessage();
    }
}

public class BobbyMessage : IMessage
{
}

public interface IMessageHandler
{
    IMessage Handle(string body);
}

public interface IMessage
{
}

public class BobbyMessageHandler : IMessageHandler
{
    public IMessage Handle(string body)
    {
        return new BobbyMessage();
    }
}

public class BobbyMessage : IMessage
{
}

look at this for example BobbyMessageHandler returns a BobbyMessage, but the interface just specifies an IMessage. This is because

public class PobiegaMessageHandler : IMessageHandler
{
    public IMessage Handle(string body)
    {
        return new PobiegaMessage();
    }
}

public class PobiegaMessage : IMessage
{
}

public class PobiegaMessageHandler : IMessageHandler
{
    public IMessage Handle(string body)
    {
        return new PobiegaMessage();
    }
}

public class PobiegaMessage : IMessage
{
}

I also want a message handler 🙂 the whole purpose of base classes / interfaces is to allow abstractions over what the actual return type is combined with virtual/override (or just interfaces), you can let implementing/derived classes decide what should happen

Bobby BobOP•3y ago

I don't know what an interface is, this code is a bit advanced for me

Kouhai•3y ago

To simply interfaces, an interface is basically a contract ensures any type implementing it HAVE to define certain methods/properties in the case of

public interface IMessageHandler
{
    IMessage Handle(string body);
}

public interface IMessageHandler
{
    IMessage Handle(string body);
}

A type that implements IMessageHandler needs to define method IMessage Handle(string body);

Accord•3y ago

✅ This post has been marked as answered!

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Confused about is-a relationship (static and dynamic types) [Answered]

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