Abstracts: can I do better than this?

MollyTheCoder

I know I'm missing something simple. I have a base class, Device, and a bunch of derived classes Dev1, Dev2... Depending on the type of device active at the moment, I have an abstract method for each derived type. Deciding which to use leads me to:

if (SelectedDevice.GetType() == typeof(Dev1))
{
SetDevice((Dev1)SelectedDevice);
}
else if (SelectedDevice.GetType() == typeof(Dev2))
{
SetDevice((Dev2)SelectedDevice);
}
else if ...

It works, but it just feels ugly. Molly

Luc Pattyn

rather than comparing types explicitly, you could use the is keyword. and what is wrong with a simple SetDevice(SelectedDevice);? or perhaps SelectedDevice.Set()? :)

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realJSOP

You could establish an enum and then have an appropriate property in each class. Setting/getting a property probably has less impact on performance than casting.

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DaveyM69

I've had this problem in the past. What I decided on was to have an enum and pass the type of the device to the base classes constructor. Something like...

public enum DeviceType
{
TypeA,
TypeB
}

public abstract class DeviceBase
{
private DeviceType deviceType;

internal DeviceBase(DeviceType deviceType)
{
    this.deviceType = deviceType;
}

public DeviceType DeviceType
{
    get { return deviceType; }
}

}

public class DeviceTypeA : DeviceBase
{
public DeviceTypeA()
: base(DeviceType.TypeA)
{ }
}

public class DeviceTypeB : DeviceBase
{
public DeviceTypeB()
: base(DeviceType.TypeB)
{ }
}

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Why are you using VB6? Do you hate yourself? (Christian Graus)

MollyTheCoder

My apologies for using the term 'abstract' a little too loosely. The SetDevice methods control display within a form, and are not part of the Device class. I started with SetDevice(SelectedDevice); but the compiler kicks that back because it can't be resolved at compile time. Molly

Paulo Zemek

Why can't you do something like: SelectedDevice.Set(this) Which Set being virtual (abstract), so the right one is called without the IF and also, if newer classes appear, you don't need to change the "Deciding" code.

Lost User

MollyTheCoder wrote:

I have a base class, Device, and a bunch of derived classes Dev1, Dev2

SetDevice((Device)SelectedDevice);

This one?

I are Troll :suss:

PIEBALDconsult

Is the casting required at all? Even if SetDevice is overloaded for each type there shouldn't be a problem should there? And if not, then you still don't need to cast to each type. Edit: I just tried it and I see the problem. More information about SetDevice would be helpful. And, as Luc said, use is rather than comparing types. Edit: Or perhaps SetDevice should handle determining which type of device it was passed:

private static void
SetDev
(
DevBase Dev
)
{
System.Console.WriteLine ( "DevBase" ) ;

if ( Dev is Dev1 ) SetDev ( (Dev1) Dev ) ;
else if ( Dev is Dev2 ) SetDev ( (Dev2) Dev ) ;

return ;

}

modified on Wednesday, December 30, 2009 2:01 PM

MollyTheCoder · modified on Wednesday, December 30, 2009 2:01 PM

I have a form with device information for for each type of device. Most of the information is similar, so I'm sharing a single form, using SetDevice() to setup fields and controls specific to each derived device type. (right now, all SetDevice methods are inside the form, not the Device classes). SelectedDevice is instantiated as a Device, then cast to whichever derived class is appropriate. The cast is necessary because at compile-time, SelectedDevice is just a Device, but at run-time will have a specific derived type.

PIEBALDconsult

I wonder whether or not dynamics in C# 4 will do it.

MollyTheCoder · modified on Wednesday, December 30, 2009 2:01 PM

PIEBALDconsult wrote:

Or perhaps SetDevice should handle determining which type of device it was passed.

Wouldn't that mean creating a big SetDevice to handle all types, and just move the same code into SetDevice? Even if the code is a little less ugly thanks to is. Molly

pbalaga

Afaik, the 'is' keyword is not fully equal to x.GetType() == typeof(y) . Let's say I have: class A class B:A (B inheriting from A) Then following (pseudo-)code:

b = new B();
b is A

returns true and

b = new B();
b.GetType() == typeof(A)

returns false. The latter option checks for an exact type match.

Lost User

Paul89 wrote:

b = new B();
b.GetType() == typeof(A);

The above test the immediate types A.GetType() != B.GetType() && B.GetType() != A.GetType() for a better understanding try this:

b = new B();
a = new A();

typeof(A).ToString();
typeof(B).toString();

a.GetType().toString();
b.GetType().toString();

However this works like the is keyword: B.IsSubclassOf(A) returns true -or- A.IsAssignableFrom(B) returns true On the is key word: 'An is expression evaluates to true if the provided expression is non-null, and the provided object can be cast to the provided type without causing an exception to be thrown.'

modified on Wednesday, December 30, 2009 2:42 PM

MollyTheCoder

Thanks! I think is works for the code I've got today, but you probably saved me from myself on some future project. Molly

petercrab

looks fine to me.

PIEBALDconsult

Or maybe not! :laugh:

MollyTheCoder

Who goes 400 pages deep (as of today) into a forum just to rub it in? :sigh:

PIEBALDconsult

That'd be me. :-D But I took a shortcut -- I saw on your profile that you had only posted a few questions, I checked those few. I didn't actually know I'd find this.