I need to define a static property or method in certain classes of my bussiness logic, to explicity determine which classes are cacheables in Session or Cache of ASP.NET service. I'm thinking, static property or method in the interface would be perfect, but C# 4.0 doesn't support this.
All a need is be able to evaluate in a generic manager which classes are cacheables and, if they are, at what level: session (user) or cache (application).
Now I'm trying with a empty interface with T parameter to evaluate, but, maybe exists a better approach?? Thanks.
public interface ICacheable<T>
{
}
public class Country : ICacheable<CacheApplication>
{
}
public class Department : ICacheable<CacheUser>
{
}
public class Gestor<T>
{
// ...
if (typeof(T) is ICacheable<CacheApplication>)
{
}
// ...
}
How about using a custom attribute? Your classes then would look something like this:
[Cacheable(Level = CacheLevels.Application)]
public class Country { }
[Cacheable(Level = CacheLevels.User)]
public class Department { }
You can read here on how to create your own custom attribute and then access its value by using reflection.
You cant define static interfaces, for one thing, you cant make instances of static classes so you cant substitute them for others with the same base class.
You might be better off having a singleton instance of one class and using interfaces as normal. You could enforce one and one-only instance through a factory pattern too.
Related
I have a factory object ChallengeManager to generate instances of a Challenge object for a game I'm building. There are many challenges. The constructors for each Challenge class derivation are different, however there is a common interface among them, defined in the base class.
When I call manager.CreateChallenge(), it returns an instance of Challenge, which is one of the derived types.
Ideally, I would like to keep the code for the object construction inside the derived class itself, so all the code related to that object is co-located. Example:
class Challenge {}
class ChallengeA : Challenge {
public static Challenge MakeChallenge() {
return new ChallengeA();
}
}
class ChallengeB : Challenge {
public static Challenge MakeChallenge() {
return new ChallengeB();
}
}
Now, my ChallengeManager.CreateChallenge() call only needs to decide the class to call MakeChallenge() on. The implementation of the construction is contained by the class itself.
Using this paradigm, every derived class must define a static MakeChallenge() method. However, since the method is a static one, I am not able to make use of an Interface here, requiring it.
It's not a big deal, since I can easily remember to add the correct method signature to each derived class. However, I am wondering if there is a more elegant design I should consider.
I really like the pattern you are describing and use it often. The way I like to do it is:
abstract class Challenge
{
private Challenge() {}
private class ChallengeA : Challenge
{
public ChallengeA() { ... }
}
private class ChallengeB : Challenge
{
public ChallengeB() { ... }
}
public static Challenge MakeA()
{
return new ChallengeA();
}
public static Challenge MakeB()
{
return new ChallengeB();
}
}
This pattern has many nice properties. No one can make a new Challenge because it is abstract. No one can make a derived class because Challenge's default ctor is private. No one can get at ChallengeA or ChallengeB because they are private. You define the interface to Challenge and that is the only interface that the client needs to understand.
When the client wants an A, they ask Challenge for one, and they get it. They don't need to worry about the fact that behind the scenes, A is implemented by ChallengeA. They just get a Challenge that they can use.
You're "decentralizing" the factory, such that each subclass is responsible for creating itself.
More commonly you would have a central factory that would know about the possible subtypes and how to construct them (often enough, simply by creating a new instance and returning that instance typed as a common interface or common base class). That approach avoids the issue you currently have. I also see no benefit to your current approach. You are currently gaining no encapsulation or code reuse over the more typical implementation of a factory.
For additional reference, have a look at
http://www.oodesign.com/factory-pattern.html
Not necessarily the answer you are looking for but...
You can use following implementation, if you can move away from static method per class.
using System;
public class Test
{
public static void Main()
{
var c1 = ChallengeManager.CreateChallenge();
var c2 = ChallengeManager.CreateChallenge();
//var c = ChallengeManager.CreateChallenge<Challenage>(); // This statement won't compile
}
}
public class ChallengeManager
{
public static Challenage CreateChallenge()
{
// identify which challenge to instantiate. e.g. Challenage1
var c = CreateChallenge<Challenage1>();
return c;
}
private static Challenage CreateChallenge<T>() where T: Challenage, new()
{
return new T();
}
}
public abstract class Challenage{}
public class Challenage1: Challenage{}
public class Challenage2: Challenage{}
Explanation:
I have an abstract class like so:
public abstract class Serializer<T> where T : new()
{
T obj { get; set; }
public string ToXML()
{
// return string XML
}
}
And another class that inherits this abstract class:
public class Account : Serializer<Account>
{
// Code
// I don't want to have to implement the methods of
// the inherited class/interface.
}
I want to access it like such:
Account account = new Account();
Console.WriteLine(account.ToXML());
Question:
Can I do this and pass the account to the property obj so the ToXML can perform its task of converting the object to a string?
Serializer s = new Serializer();
s.ToXML(account);
I'd prefer to have each object inherit the Serialize class and all its methods, and just be able to know that without editing anything but adding the inheritance of the class that I can now access these methods.
On another note, I feel like inheriting a class violates the is-a and can-do principles between choosing an interface or a class, but I don't want to override all the methods, when I already have the code written to do it generically in a class (i.e., I don't want to implement the interface). Is there a way to inherit the methods of an interface like a class (no implementing/overriding).
Try to return this:
T obj { get { return (T)this; } }
But, this means that the child class has to provide itself as a type parameter, it's the curiously recurring "template" pattern... To be sure though, you don't necessarily need to know the type of the object at compile time to serialize it to XML (if you use the usual serializers), so accessing this within the serializer method would be OK, without the need for that type parameter and that property.
I'd personally prefer a more hands off approach to an abstract base class (using the XmlSerializer):
public interface MXmlSerializable { }
public static class XmlSerializable {
public static string ToXml(this MXmlSerializable self) {
if (self == null) throw new ArgumentNullException();
var serializer = new XmlSerializer(self.GetType());
using (var writer = new StringWriter()) {
serializer.Serialize(writer, self);
return writer.GetStringBuilder().ToString();
}
}
}
M stands for mixin. It's effectively a marker interface with an extension method. Use it like this:
public class Account : MXmlSerializable {
...
}
...
Account account = new Account();
...
string accountXml = account.ToXml();
Your construct hardly makes sense. There's no clear point for the Serializer<T> instance to point to itself. You can easily use this and cast it to T. Also, unless your ToXML method implements some really generic XML serialization algorithm (like processing the current instance via reflection), you should make it virtual and place the specific implementations in Serializer<T>'s ancestors.
Also, I would also object your approach to inheritance. If your Account class is in fact a single purpose account serializer, then name it so (AccountSerializer). If not, and Account represents an actual account, then yes, from an independent reader's point of view you are mixing two primary concepts: a business object (Account) and some technical mechanism (Serializer<T>).
If you have a general-purpose serialization algorith, why don't you just have a separate, non-abstract Serializer<T> class, accepting T instances in ToXML()? You will end up with better separation of concerns.
Is there a way to inherit the methods of an interface like a class (no implementing/overriding).
No. Interfaces are interfaces, not classes than embed particular code.
What about just doing it like this;
public string ToXML()
{
convert `this` to xml directly
// return string XML
}
Why do you need the property to return a reference to the class itself.
With the above structure you can do the following, without even needing the T property
Account account = new Account();
Console.WriteLine(account.ToXML());
If your code in the Abstract class will not change just to serialize the object, this approach doesn't make sense, Serialization you do not need to know the object type (Especially if your using xml which means building a easy to access string).
If you want the Serialization only available to certain objects make an interface i.e
public interface ISerialize
{
}
public class Account : ISerialize
{
}
and then create an Extension method
public static class ExtenstionMethods
{
public static string ToXml(this ISerialize obj)
{
// code to build xml string
}
}
This way you can do what you want to do because Account is of the Interface and the ExtensionMethods will only work on that Interface, thus you only need the code in the ExtensionMethods class and then include the namespace wherever you want to use the "ToXml()" etc
Account account = new Account();
Console.WriteLine(account.ToXML());
I have a class that serves as a data model. I'll simplify it like this:
public class DataModel
{
public bool IsDataModelActive {get; internal set;}
}
So the programmers who will use my DLL will only see what they need without risking to break anything.
On the flipside, I want to use DI in my classes that use DataModel. So I have to create an interface:
public interface IDataModel
{
bool IsDataModelActive {get;}
}
And so I inject this in my class:
public class Class1
{
IDataModel dataModel;
public Class1(IDataModel dataModel)
{
this.dataModel = dataModel
}
}
In this case, it is impossible for me to access the setter of dataModel.IsDataModelActive.
The best patch I've found is to cast after receiving the injection:
public class Class1
{
DataModel dataModel;
public Class1(IDataModel dataModel)
{
this.dataModel = (DataModel)dataModel
}
}
This helps for the unit tests, but it kind of break the whole concept of DI. What if I have another class that implements IDataModel?
What strategy do you/would you use?
If the interface IDataModel doesn't allow you to access the setter from outside of the DataModel, then (hopefully not surprisingly) you shouldn't need to access the setter from outside of the DataModel. Either the DataModel itself or something that can see the setter should be responsible for setting that value.
Is Class1 publicly visible - i.e. exposed to the consumers? If not, then create another internal interface IDataModelInt : IDataModel and provide setter there. Use the second one for your internal work.
C# .Net 4.0
I'd like to know how I can have a class which can only be instantiated from one single place. An example:
I've got a Provider class. This class exposes a method called GetData. When GetData is called, the Provider will instanciate a Data class, populate and return it. The Data class cannot be instanciated by anybody different then the Provider, so the only way to access the data will be through the Provider. Once GetData is called and a caller has received the Data class instance, he should be able to access properties/methods of this class.
How can this be done? Is there a pattern for this sort of problem? A short sample would be highly appreciated. Thanks in advance!
It sounds like you are looking for the factory pattern:
The factory pattern is a creational
design pattern used in software
development to encapsulate the
processes involved in the creation of
objects.
Basically your Provider class is the factory that controlls the creation of instances of the Data class.
One thing you could do control this would be to place these two types in their own assembly and make the constructor for Data be internal but the class itself public. This would mean that anyone who references the assembly would be forced to use the Provider class to create instances of Data (unless they used reflection, of course).
Another solution would be to create an interface IData, and declare the Provider.GetData method to return IData instead of Data. Then you can have your Data class nested inside Provider. This way your Data class cannot be instantiated even by classes in the same assembly. Here is an example:
public interface IData
{
// properties and methods
}
public class Provider
{
public IData GetData()
{
return new Data();
}
private class Data : IData
{
// your implementation
}
}
Here is an example of what Andrew described:
public class Data
{
internal Data()
{
// internal constructor cannot be called from outside the assembly
}
// properties, fields and methods
}
public class Provider
{
public Data GetData()
{
return new Data();
}
}
public class Provider
{
protected Provider()
{
}
public static Provider CreateNewProvider()
{
return new Provider();
}
}
I recently ran into a problem where it seems I need a 'static abstract' method. I know why it is impossible, but how can I work around this limitation?
For example I have an abstract class which has a description string. Since this string is common for all instances, it is marked as static, but I want to require that all classes derived from this class provide their own Description property so I marked it as abstract:
abstract class AbstractBase
{
...
public static abstract string Description{get;}
...
}
It won't compile of course. I thought of using interfaces but interfaces may not contain static method signatures.
Should I make it simply non-static, and always get an instance to get that class specific information?
Any ideas?
You can't.
The place to do this is with Attributes.
Eg
[Name("FooClass")]
class Foo
{
}
If you don't mind deferring to implementations to sensibly implement the Description property, you can simply do
public abstract string ClassDescription {get; }
// ClassDescription is more intention-revealing than Description
And implementing classes would do something like this:
static string classDescription="My Description for this class";
override string ClassDescription { get { return classDescription; } }
Then, your classes are required to follow the contract of having a description, but you leave it to them to do it sensibly. There's no way of specifying an implementation in an object-oriented fashion (except through cruel, fragile hacks).
However, in my mind this Description is class metadata, so I would prefer to use the attribute mechanism as others have described. If you are particularly worried about multiple uses of reflection, create an object which reflects over the attribute that you're concerned with, and store a dictionary between the Type and the Description. That will minimize the reflection (other than run time type inspection, which isn't all that bad). The dictionary can be stored as a member of whatever class that typically needs this information, or, if clients across the domain require it, via a singleton or context object.
If it is static, there is only one instance of the variable, I don't see how inheritance would make sense if we could do what you want to accomplish with static vars in derived classes. Personally I think you are going to far to try to avoid a instance var.
Why not just the classic way?
abstract class AbstractBase
{
protected string _Description = "I am boring abstract default value";
}
class Foo : AbstractBase {
public Foo() {
_Description = "I am foo!";
}
}
Combining static and abstract is somewhat meaningless, yes. The idea behind static is one need not present an instance of the class in order to use the member in question; however with abstract, one expects an instance to be of a derived class that provides a concrete implementation.
I can see why you'd want this sort of combination, but the fact is the only effect would be to deny the implementation use of 'this' or any non-static members. That is, the parent class would dictate a restriction in the implementation of the derived class, even though there's no underlying difference between calling an abstract or 'static abstract' member (as both would need a concrete instance to figure out what implementation to use)
A possible workaround is to define a Singleton of your derived class in your base class with the help of Generics.
import System;
public abstract class AbstractBase<T>
where T : AbstractBase<T>, new()
{
private static T _instance = new T();
public abstract string Description { get; }
public static string GetDescription()
{
return _instance.Description;
}
}
public class DerivedClass : AbstractBase<DerivedClass>
{
public override string Description => "This is the derived Class";
}
class Program
{
static void Main(string[] args)
{
Console.WriteLine(DerivedClass.GetDescription());
Console.ReadKey();
}
}
The trick is to tell your AbstractBase<T> some details about how DerivedClass is implemented:
It is newable with where T: new() so it can create a Singleton instance
It derives from itself with where T : AbstractBase<T> so it knows that there will be a implementation of Description
This way _instance contains the Description field which can be called in the static Method GetDescription().
This forces you to overwrite Descriptionin your DerivedClass and allows you to call its value with DerivedClass.GetDescription()
It's not static if it has to be called on an instance.
If you're not calling it on an instance, then there's no polymorphism at play (i.e. ChildA.Description is completely unrelated to ChildB.Description as far as the language is concerned).
You can...
In the abstract class...
protected abstract InWindow WindowInstance { get; set; }
In the derived class...
private static InWindow _instance;
protected override InWindow WindowInstance
{
get => _instance;
set => _instance = value;
}
You could make the "abstract" base method throw an Exception, so then a developer is "warned" if he tries to invoke this method on a child class without overriding.
The downside is that one might extend the class and not use this method. Then refer to other answers provided.