I have the following base class:
public class OAuthRefreshToken {
//Other properties..
public int UserId { get; set; }
public virtual OAuthUser User { get; set; }
}
And this derived class:
public abstract class OAuthRefreshToken<U> : OAuthRefreshToken
where U : OAuthUser {
public virtual U User { get; set; }
}
What I want is to override the User property from the OAuthRefreshToken base class with the one in my derived class.
I thought of adding the override keyword:
public override virtual U User { get; set; }
But this throws a compilation error as it is not allowed.
If I leave the code like that (without override) a warning appears saying I'm hiding inherited member (which I intend to do).
It tells me to use override if hiding is intended...
And then we are on a nice loop where the warning tells you do something and the compiler tells you not to do it.
Of course, I'll listen to the compiler, but how can I fix the warning? I don't like building my project and have a nice bunch of warnings appear.
Thank you for your time.
You can't change the type of the property when overriding it. If you want to change the type, you need to hide the method instead of overriding it. If you want to override it instead of hiding it, you'll need to maintain the same type that the base class uses.
The warning should tell you what you can do - something like this (emphasis mine):
'OAuthRefreshToken.User' hides inherited member 'OAuthRefreshToken.User'.
To make the current member override that implementation, add the override keyword.
Otherwise add the new keyword.
However, presumably you want the two properties to do be about the same value - so you need to also override the property from the base class.
Now, there's a problem because you can't have two properties with the same name but different types declared in the same type... so I'd suggest using a different name for the new property - at which point you don't need to declare it with new because it doesn't hide anything.
public abstract class OAuthRefreshToken<U> : OAuthRefreshToken where U : OAuthUser
{
public override OAuthUser User
{
get { return TypedUser; }
set { TypedUser = (U) value; }
}
public virtual U TypedUser { get; set; }
}
I suspect there may be alternatives where you go back to hiding the property if you introduce an intermediate class, but that's almost certainly going to be even uglier.
Related
I'm not really sure what looks better or when do I really use in abstract classes and properties, or when to use non abstract properties. I'll try to make a simple example. Let's say I have this:
abstract class Human
{
public GenderType Gender { get; set; }
public string Name { get; set; }
public Date Born { get; set; }
public bool IsNerd { get; set; }
abstract public void Speak();
abstract public void Sleep();
abstract public void AnoyingPeopleOnStackOverflow();
//... so on
}
class Peter : Human
{
//Peter is special, he got a second name
//But thats all, everything else is the same as like on other humans
public string SecondName { get; set; }
//...override abstract stuff
}
Is this alright? As I understood, I don't have to use an abstract property if I dont want to override it. And in this situation it would be ok, just the methods like Speak, Sleep and so on should be abstract.
Now, if this is ok, when would or should I use an abstract property?
Use an abstract property when you have no default implementation and when derived classes must implement it.
Use a virtual property when you have an implementation in the base class but want to allow overriding.
Use the override keyword to override a member. Mark the member as sealed override if it should not be overridden again.
Don't mark the property as abstract or virtual if you don't want it to be overridden.
Use the new keyword to hide a non-abstract, non-virtual member (this is rarely a good idea).
How to: Define Abstract Properties
I find that abstract properties often occur in a design which implies that they will have type-specific logic and/or side effects. You are basically saying, "here is a data point that all subclasses must have, but I don't know how to implement it". However, properties which contain a large amount of logic and/or cause side effects may not be desirable. This is an important consideration, though there is no fixed right/wrong way to do it.
See:
Should Properties have Side Effects
CA1024: Use properties where appropriate
Personally, I find that I use abstract methods frequently but abstract properties rarely.
I know what I want them to do, I don't care how they do it: Interface.
I know what I want them to do, I don't care how they do some of it, but I've firm ideas on how they'll (or at least most of them) do other bits: Abstract class.
I know what I want them to do, and how most of them will do it: Concrete class with virtual members.
You can have other cases such as e.g. an abstract class with no abstract members (you can't have an instance of one, but what functionality it offers, it offers completely), but they're rarer and normally come about because a particular hierarchy offers itself cleanly and blatantly to a given problem.
(Incidentally, I wouldn't think of a Peter as a type of Human, but of each peter as an instance of human who happens to be called Peter. It's not really fair to pick on example code in this way, but when you're thinking about this sort of issue it's more pertinent than usual).
Abstract members are simply virtual members that you have to override. You use this for something that has to be implemented, but can't be implemented in the base class.
If you want to make a virtual property, and want that it has to be overridden in the class that inherits your class, then you would make it an abstract property.
If you for example have an animal class, its ability to breathe would not be possible to detemine just from the information that it's an animal, but it's something that is pretty crucial:
public abstract class Animal {
public abstract bool CanBreathe { get; }
}
For a fish and a dog the implementation would be different:
public class Dog : Animal {
public override bool CanBreathe { get { return !IsUnderWater; } }
}
public class Fish : Animal {
public override bool CanBreathe { get { return IsUnderWater; } }
}
Use abstract when all sub-classes have to implement the method/property. If there's no need for each and every sub-class to implement it, then don't use it.
As for your example, if SecondName is not required for each person, then there's no need to make an abstract property in the base class. If on the other hand, every person does need a second name, then make it an abstract property.
Example of correct usage of an abstract property:
public class Car
{
public abstract string Manufacturer { get; }
}
public class Odyssey : Car
{
public override string Manufacturer
{
get
{
return "Honda";
}
}
}
public class Camry : Car
{
public override string Manufacturer
{
get
{
return "Toyota";
}
}
}
Making Maker abstract is correct because every car has a manufacturer and needs to be able to tell the user who that maker is.
An abstract property would be used where you want the class to always expose the property, but where you can't pin down the implemetation of that property - leaving it up to/forcing the inheriting class to do so.
There's an example here, where the abstract class is named Shape, and it exposes an abstract Area property. You can't implement the Area property in the base class, as the formula for area will change for each type of shape. All shapes have an area (of some sort), so all shapes should expose the property.
Your implementation itself looks just fine. Was trying to think of a sensible example of an abstract property for a Human, but couldn't think of anything reasonable.
I have an interface 'IBase' that specifies a nullable int. A later interface 'IDerived' hides the nullable int and 'redefines' it as non-nullable.
interface IBase
{
int? Redefineable { get; set; }
}
interface IDerived : IBase
{
new int Redefineable { get; set; }
}
The class that implements these interfaces must explicitly implement the hidden property, however it's private so the client can't see it.
class TheClass : IDerived
{
public int Redefineable { get; set; }
int? IBase.Redefineable { get; set; }
}
However, even though it's a private property, I can still access it through the IBase interface!
var o = new TheClass();
o.Redefineable = 1; // ok
var hack = o as IBase;
hack.Redefineable = null; // uh!
This seems like some kind of violation of C# access modifiers, but either way it isn't really what I had in mind for redefining (not just hiding) a property. It's correct in the sense that it does what you're asking, get an IBase interface which has a nullable int but this is non-intuitive to the client who could then modify the wrong version of the property.
What I really want, is that if the client accesses IBase.Redefinable, then it behaves as if it's accessing the IDerived.Redefinable property, the 'real' property of TheClass. That way it's actually redefined, as in back through the hierarchy.
class TheClass : IDerived
{
public int Redefineable { get; set; }
int? IBase.Redefineable {
get {
// redirect to redefined property
return this.Redefineable;
}
set
{
// stop client setting it to null
if (!value.HasValue)
throw new InvalidOperationException();
// redirect to redefined property
this.Redefineable = value.Value;
}
}
}
This just feels like a hack, almost as if I'm missing something, so I want to ask if anyone knows a better/alternative way to implement re-definable properties?
However, even though it's a private property, I can still access it through the IBase interface!
It's not a private property. It's just a property using explicit interface implementation. That means it's public through the interface, but only available through the interface. Explicit interface implementation is mostly designed to make it feasible to implement "contradictory" interfaces, as well as being used to "discourage" (but not prohibit) the use of some interface methods. It's not meant to give the impression that the members don't exist at all.
Fundamentally, it sounds like you shouldn't be using inheritance here - if you don't want something to be able to act as an IBase, you shouldn't inherit from IBase.
I am having a hard time implementing a property in C# that only has a getter in the abstract base class, but where I need to introduce a setter in one of the derived classes.
Update: For a shorter explanation of a generalized example of this question, see this question. The selected answer has explained why this is currently impossible to do in C#, however, in my mind no satisfactory solution has yet been provided.
An overview of my class diagram is shown below:
My objective is that the two classes TextElementStatic and TextElementReferenceSource should have a Text property with both getters and setters, while the class TextElementReferenceTarget should have a Text property with only a getter. I'm constantly using ITextElement while referencing all of these objects, and I need to ensure that the ITextElement interface only has a getter. Also, the base class TextElement implements a lot of common code, so all classes need to inherit from that class.
My current code looks like this:
Interface: ITextElement
public interface ITextElement
{
string Text { get; }
}
Interface: ITextElementUpdatable
public interface ITextElementUpdatable : ITextElement
{
new string Text { get; set; }
}
Abstract class: TextElement (This is where my problem is, explained below)
public abstract class TextElement : ITextElement
{
// I want to mark this 'abstract', but that causes my problem
public virtual string Text
{
get
{
// NOTE: This should never be called
Debug.Fail("Called virtual Text getter that should never be called");
return default(string);
}
}
}
Abstract class: TextElementUpdatable
public abstract class TextElementUpdatable : TextElement, ITextElementUpdatable
{
// Should have both a getter and a setter
public new virtual string Text { get; set; }
}
Class: TextElementStatic
public class TextElementStatic : TextElementUpdatable
{
// Should have both a getter and a setter
// No Text property declaration
// Inherits Text property from TextElementUpdatable
}
Class: TextElementReferenceSource
public class TextElementReferenceSource : TextElementUpdatable
{
// Should have both a getter and a setter
public override string Text
{
get { return _internalobject.Text; }
set { _internalobject.Text = value; }
}
}
Class: TextElementReferenceTarget
public class TextElementReferenceTarget : TextElement
{
// Should ONLY have a getter
public override string Text
{
get { return _internalobject.Text; }
}
}
So, my issue is: I really want to declare the Text property in the base class TextElement abstract, because it should always be implemented in the derived classes (both TextElementUpdatable, TextElementReferenceSource and TextElementReferenceTarget implements this property). However, if I try to convert the property to public abstract string Text { get; }, then I receive an error in TextElementUpdatable specifying that
TextElementUpdatable.Text hides the inherited property TextElement.Text
Further, if I change the property in TextElementUpdatable from new to override the error message is replaced by:
Cannot override because TextElement.Text does not have an overridable set accessor
Now, I could go back to TextElement and change the property to public virtual string Text { get; private set; } and call it a day, since that method should never be called anyway (which is basically the solution I have now). However, if I or someone create another derived class later on, I want to force me/them to implement the Text-property, hence I would rather mark it abstract than provide a virtual implementation.
Any suggestions on how I can do this the right way - even if it should involve a lot of refactoring?
I know that I could separate the two objectives her, providing one inherited Text property with only a getter, and then introduce a SetText() method in the ITextElementUpdatable interface. However, I'm wondering whether it is possible to find a good solution with properties only.
Another similar question, but without any answers I've been able to use: C# - What should I do when every inherited class needs getter from base class, but setter only for ONE inherited class
It is really an exciting desing problem, but.. You have to use the new keyword what is not a good practice. Try to avoid them.
Of course, property names can be the same in the interfaces, but if both implemented by a class (and one of the props defined without a setter), we have to implement them explicitelly. We have to accept that these properties "conflict".
You could introduce abstract methods:
public abstract class TextElement : ITextElement
{
public string Text { get { return GetText(); } }
protected abstract string GetText();
}
public abstract class TextElementUpdatable : TextElement, ITextElementUpdatable
{
string ITextElementUpdatable.Text
{
get { return GetText(); }
set { SetText(value); }
}
protected abstract void SetText(string text);
}
It can be a bit confusing that you use the same property in your hierarchy with different meanings. Maybe the implementation of ITextElement.get_Text and ITextElementUpdatable.get_Text will diverge later - the interfaces define two independent behavior, and we do not use basic types all the time, like string.
So my suggestion is that you should have a property in ITextElement for read only purpose, and another property in ITextElementUpdatable with different name. In this manner, your methods can be defined as abstract, of course.
When I implement an interface for the first time into a class I want either resharper 6 or visual studio 2010 to implement my properties as auto implemented properties and not put in the default value of throw new NonImplementedException();. How can I do this? For example:
public interface IEmployee
{
// want this to stay just like this when implemented into class
ID { get; set; }
}
public class Employee : IEmployee
{
// I do not want the throw new NonImplemented exception
// I want it to just appear as an auto implemented property
// as I defined it in the interface
public int ID
{
get
{
throw new NotImplementedException();
}
set
{
throw new NotImplementedException();
}
}
}
Because this happens all the time, I am finding myself having to constantly refactor and manually remove those throw new UnImplimented() exceptions and manually make the properties be auto implemented... a pain! After all, I defined it as an auto implemented property in my interface.
Any help or advice much appreciated! Thanks.
Note: your R# keyboard shortcuts may differ, I am using the Resharper 2.x keyboard schema.
If you declare the interface on the class and then use Alt+Enter and select “Implement members”:
Then you will get the default implementation, which happens to be throwing NotImplementedException, unless you change that.
But if you ignore the suggested course of action and instead use Alt+Insert to open the Generate menu, you can select “Missing members”:
This will open Generate window, where you can select to implement the property (or properties) as auto-implemented:
That will do exactly what you want:
class Employee : IEmployee
{
public int Id { get; set; }
}
After all, I defined it as an auto implemented property in my interface.
No, you didn't. You declared it as a property without an implementation. That's all you can do in an interface: you're just saying that classes implementing the interface must provide the concrete implementations of such properties.
Personally I would be wary of having too many writable properties within interfaces - if this is something you find "happens all the time" I wonder whether you're using interfaces where possibly abstract classes would be more appropriate.
In terms of your exact question: I don't know whether it's possible to change the default implementation either VS or R# provides for interfaces - but I would resist making those changes anyway, to be honest.
EDIT: Under R# options, "Code Generation", you can choose between throwing an exception, returning a default value, or giving uncompilable code. It's possible that this will do what you want. Give it a go, but I'd still strongly urge you to think carefully before going down this path.
An interface is not meant to specify how the methods will be implemented so there is no way around it using the interface. One solution would be to make an abstract base class with the auto-implemented properties and inherit that class instead of directly implementing the interface.
Here's a quick workaround that I found in VS2015. Mark your class as abstract then implement the interface abstractly. This adds the auto properties for you then you just replace the "abstract " with "" in your file. Then you can remove the abstract keyword from your class.
In the case of VS2015 I'm using a find and replace macro as a workaround:
Find:
(\r|\n| |\t)+\{(\r|\n| |\t)+get(\r|\n| |\t)+\{(\r|\n| |\t)+throw(\r|\n| |\t)+new(\r|\n| |\t)+NotImplementedException\(\);(\r|\n| |\t)+\}(\r|\n| |\t)+set(\r|\n| |\t)+\{(\r|\n| |\t)+throw(\r|\n| |\t)+new(\r|\n| |\t)+NotImplementedException\(\);(\r|\n| |\t)+\}(\r|\n| |\t)+\}
replace:
{get;set;}
In addition to Jon's answer... if you really want to change this (out of box) behavior of Visual Studio and creating auto properties when implement interface, you can try one of following...
Use T4 for code generation - http://msdn.microsoft.com/en-us/library/bb126445.aspx
Write a custom plugin for Visual Studio using VS SDK achieve this
It's not related to this question completely, however, a different approach when you have multiple of such properties in an Interface, instead of interface you can have a class and refer to that class as a return type in your main class. You can create a class and refer to that class in your main class. Example
public class Asset
{
public int AssetTrackingID { get; set; }
public Category AssetCategoryInfo { get; set; }
public Manufacturer AssetManufacturerInfo { get; set; }
public ManufacturerModel AssetModelInfo { get; set; }
public Status AssetStatusInfo { get; set; }
public EmployeeDetails AssetEmployeeInfo { get; set; }
public string AssetNumber { get; set; }
public string SerialNumber { get; set; }
public DateTime? AcquiredDate { get; set; }
}
I'm working with Telerik RadScheduleView and have implemented the IAppointment class into a child 'Job' class that I have defined. The 'Start' and 'End' (DateTimes) properties are found in the IAppointment class (of which I inherit). Navigating to the 'Public Virtual' method (in the IAppointment class) shows me a { get; set; } but I can't change it to say, for example, display a MessageBox on setting a new value to Start or End. It says MetaData in the tab, is this just something I am not able to edit? Is there a way I can override this access method somehow??
The name IAppointment would indicate to me that it is not a class you are inheriting, but an interface you're implementing - however, I'm not familiar with Telerik products and their naming conventions might just be weird, so, taking what you say at face value, yes you ought to be able to override a property defined as virtual.
If, for example, we have the following class defined somewhere, but accessible, so that we may inherit, and which exposes a virtual member:
public class A
{
public virtual int J { get; set; }
}
Then we can inherit and override - we may still access the base implementation, but also "inject" our own, if required:
public class B : A
{
public override int J
{
get
{
return base.J;
}
set
{
base.J = value;
}
}
}
But I can't for the life of me imagine why you'd want to show a message box from within the logic of property accessors, and can't stress enough that you shouldn't.
As Mr Disappointment mentioned I'd expect IAppointment to be an interface.
If it is indeed a class you could use the new modifier and do something along the lines of this.
public class Job : IAppointment
{
new public DateTime End
{
get
{
//get the value directly from the base class
return base.End;
}
set
{
//display your messagebox here
//then pass the value to the base class
base.End = value;
}
}
}
you can write a property in your derived class which will eventually get and set the propety of base class no need to override.
First of all, your question is confusing. IAppointment isn't a class, its an interface.
Is this your situation?
There is an interface called IAppointment with 2 properties namely Start and End. ( http://www.telerik.com/help/wpf/t_telerik_windows_controls_scheduleview_iappointment.html )
There is a base implementation named AppointmentBase (or the more derived, Appointment). (http://www.telerik.com/help/wpf/t_telerik_windows_controls_scheduleview_appointmentbase.html)
You are building your own class which is inheriting from the AppointmentBase.
You want the setter of AppointmentBase.End to show a messageBox but you dont know how to add this logic.
In that case i have good news.
AppointmentBase only servers virtual members so you can easily override the Set property
class Job : AppointmentBase
{
public override DateTime End
{
get { return base.End; }
set
{
MessageBox.Show("Unbelievable!");
base.End = value;
}
}
}