How can I apply an Interface to a form class
partial class Form1 : Form, InterfaceA
Is this correct?
Basically I would like to implement an Interface on a form.
How To ....
A Form is just a class (that subclasses System.Windows.Forms.Form), so yes - standard syntax is fine, as you have it.
Edit: As to your partial class part of the question, no, you need only declare that you implement the interface once. From MSDN...
If any of the parts are declared abstract, then the entire type is considered abstract. If any of the parts are declared sealed, then the entire type is considered sealed. If any of the parts declare a base type, then the entire type inherits that class.
Remember, there's no magic in forms or partial classes. C#/.Net is one of the few Microsoft projects that's wizardry free - it really does tend to behave the way you think it should.
Yes - a form is just a class at the end of the day
When working with partial classes in C#, either:
any declaration with the ':' operator must specify exactly the same baseclass and interfaces
specifying baseclass and interfaces on one of the declarations will suffice
To make life easier for yourself, add the interface specs in only one place (without checking I suspect this is in the designer class part by default when working with the WinForms designer).
Related
Any ideas of how make internal a base class having a child class of that class public in c#?
In code:
internal class Base { }
public class Child : Base {}
I have a layered architecture and need to expose to other layers (others assemblies) the Child class but not the Base class. I'm using inherit as a way to avoid class composition and all the voile part associated with it.
Any ideas of how to manage this kind of problem?
This cannot be done. You can hide the class by encapsulation, wrapping it and hiding it as implementation.
Base types will always be known.
Instead of making the class internal, you can make all members internal instead. If you do not want Base to be inherited by other assemblies, declare Base's constructor as internal.
This is known C# limitation, however, this is not CLR limitation.
Not the best solution, but it's possible to define new public class which inherits from internal class, using intermediate language.
Also, there might be some languages which already allow you to do this, so there is a chance you don't really need to write IL.
Though, at this point, I don't understand the necessity of Base. Since it's internal, it can't be consumed by others, and polymorpishm is essentially broken. Code reuse with subclassing, but without polymorpishm does not sound good.
Thugh, as I said, imh it is possibe to create a "public" class from "internal class" using IL, after that, yu will be able to consume that "public" class in C# side, eg
public class MyDervClass : MyILPublicClass{}
I have a winforms baseform that contain calls to certain methods that need to be implemented in derived forms. I want to ensure that my derived forms do indeed implement those methods with as much compile time support as possible. We all know the problem of not being able to define a winforms class as abstract if you want to be able to use the designer (see this question).
I created an Interface that contains the method signatures that need to be implemented in a derived form. I then call the Interface methods directly from the base class like such:
((IMyFormInterface)this).SomeInterfaceMethod();
(Note that my base class does not inherit IMyFormInterface. If it did, then derived classes wouldn't be forced to implement it.)
And then I inherit from the Interface in my derived form (which the compiler forces me to implement):
public partial class TestForm : BaseForm, IMyFormInterface
The only thing I (or other users) have to remember is to inherit IMyFormInterface.
Is it acceptable to call interface methods directly like this? My goal in all of this is to be able to get as close as possible to ensuring derived forms implement these "abstract" form methods at compile time, not run time.
That's one way to work around the designer restriction. Another would be to use the designer to build a UserControl, and then let your form base class be abstract and instantiate the control docked to the full client area.
If you use this approach, I'd assert (this is IMyFormInterface) in your constructor to catch that error as early as possible.
I had the same problem with a base control.
Because Designer does not like abstract base control classes I refactored the abstract methods to virtual methods that throw a NotImplementedException.
I don't know which way is the best.
The compiler doesn't seem to mind it so far but I just wanted to double check whether I'm setting myself up for failure in any way by implementing certain methods in my abstract class.
An abstract class usually has one or more abstract method. So yes it can have some method implemented. The goal is to force the user to implement these methods to have an object working. Sometimes abstract classes are used to provide a 'base' implementation of some interfaces, leaving the final user to specify just the key methods. You can also have an abstract class without any abstract method: in this case you are asserting you must derive from that class in order to use it.
It's common to have some implementation in abstract classes.
If there is no implementation at all, consider using an interface instead of an abstract class.
Perfectly fine to implement some methods and leave others abstract.
If all methods had to be abstract, you might as well use an interface for it.
Yes. abstract class cannot be instantiated (you have to instantiate a class that inherits from your abstract class), but it can contains implementations.
it's fine and allowed, an abstract class has at least a member (method/property) not implemented so it cannot be instantiated.
an interface is also called pure abstract class which means it's 100% abstract, so does not allow you to specify any implementation.
keep in mind that there are lots of articles and opinions about never deriving a concrete class from another concrete class but only from abstract ones... at least this was the trend in C++ up to some years ago, then I moved to the C# side, started working more and had no time to keep reading those nice articles... :)
Is it okay for a class to be marked as abstract if it has no abstract members? Even if there is no practical reason for to directly instantiate it? (aside from unit tests)
Yes, it is reasonable and beneficial to mark explicitly as abstract a base class that should not be instantiated -- even in the absence of abstract methods.
It enforces the common guideline to make non-leaf classes abstract.
It prevents other programmers from creating instances of the class. This may make it easier for you to add abstract methods to it later.
Do you want that class to ever have an actual instance? If yes, then don't mark it abstract. If no, then mark it abstract.
Short answer: yes.
Long answer: The abstract keyword marks a class and/or its members as not being useful directly. Why this may be varies from case to case; an abstract class may be too basic to do any real work, or it may have abstract members that are required to exist for other code in this class to work, but cannot be concretely defined at this level. The short of it is that by marking a class abstract, you tell the compiler and other developers not to instantiate this class directly, but instead to inherit from it to create a concrete useful implementation. You can do this even if the class has a working implementation for all its members, if you feel that the class must be inherited to make the best use of that implementation.
If the goal is to make a base class that other classes will extend, it makes sense to make this an abstract class.
If, however, the goal is to make some form of Utility class -- one that has only static members -- the best way to handle this is to give the class a single constructor marked private. That way, the class can not be instantiated, nor subclassed. This sends a clear signal that the only use of the class is to use its static methods. (This is a tip from Effective Java, by Josh Bloch)
Yes, I think so. At least that's what I do from time to time ;-)
Yesterday I thought it would be nice to implement my own Trigger in a WPF app. I created a class MyTrigger which inherited TriggerBase. TriggerBase is a public abstract class. So inheritance isn't a problem. But the constructors inside this class are marked internal. The compiler throws an error because the is no valid constructor. Why does anyone create a public class but marks the constructors as internal?
If you want the class to be visible, but only allow it to be subclassed within your own assembly. The subclasses may have public constuctors themselves - or they may be accessed with a factory.
I can't comment on whether that's a good design decision for TriggerBase in WPF, but it's at least reasonable in some situations.
One reason that I could think of is that the actual creation of new instances would be handled by another public class in the same assembly. This would force that you create the instance through this other class - possibly some sort of a factory pattern implementation.
It's public because it's used as a base class for the triggers that ship with WPF (Trigger, MultiTrigger, EventTrigger, DataTrigger etc). It it wasn't public then you wouldn't be able to flag these classes as public.
The constructors are internal because they don't intend for you to use it yourself. I'd guess you're suppose to derive from one of the classes mentioned above.