I wrote some VB code that I converted to C# using Sharp Develop IDE.
I defined an Interface, IElement, that is implemented by objects that return XML representations of themselves. Any object that implements this interface should be able to return its TagName and it's XML string representation. To get the it's XML string, it may have to loop through its child/nested collection to get the XML representation of all of its child objects.
Classes that inherit from Element can use the GetXml and GetNestedXml of its base class or choose to override it but the GetNestedXml function would not need to be public because it would only be called from the public GetXml function of derived classes. Therefore, in the original VB version, the scope of the GetNestedXML was set to protected. However, Sharp Develop and I have issues trying to convert this code to C#. Please see the error below.
On a side note, I do realize that there might be better ways to implement this and I would be interested in side suggestions that are easy on flames. :-) Thanks.
Public Interface IElement
ReadOnly Property TagName() As String
ReadOnly Property GetXml(Optional ByVal targetXml As Integer = TargetXmlEnum.All) As String
Function GetNestedXml() As String
End Interface
Public Class Element
Implements IElement
Public ReadOnly Property TagName() As String Implements IElement.TagName
Get
'....
End Get
End Property
Public Overridable ReadOnly Property GetXml(Optional ByVal targetXml As Integer = TargetXmlEnum.All) _
As String Implements IElement.GetXml
Get
'....
End Get
End Property
Protected Overridable Function GetNestedXml() As String Implements IElement.GetNestedXml
'....
End Function
End Class
Converted C# :
public interface IElement
{
string TagName { get; }
string GetXml { get; }
string GetNestedXml();
}
public class Element : IElement
{
public string TagName {
get { //... }
}
public virtual string GetXml
{
get
{
//...
}
}
protected virtual string GetNestedXml()
{
//...
}
}
error:
Error 1 'Smit.SpreadsheetML.Element' does not implement interface member 'Smit.SpreadsheetML.IElement.GetNestedXml()'. 'Smit.SpreadsheetML.Element.GetNestedXml()' cannot implement an interface member because it is not public. D:\Users\Chad\Desktop\SMIT\SMIT.SpreadsheetML.ConvertedToC#\Element.cs 41 24 Smit.SpreadsheetML.Converted
As Interface implementations need to be public or explicit:
change this method
protected virtual string GetNestedXml()
{
//...
}
to
protected virtual string IElement.GetNestedXml()
{
//...
}
Edit
create an Interface like this:
public interface IElement
{
string TagName { get; }
string GetXml { get; }
}
create an abstract base class like this
abstract class ElementBase:IElement
{
public abstract string TagName { get; }
public abstract string GetXml { get; }
protected abstract string GetNestedXml();
}
Impelement your Element class
public class Element : ElementBase
{
public override string TagName {
get { //... }
}
public override string GetXml
{
get
{
//...
}
}
protected override string GetNestedXml()
{
//...
}
}
An interface declared the responsibilities of its all inheriting instances. So you can not use not-public method to implement your interface method.
If it is non-public for any reason else, I suggest that you can use abstract class and use a abstract/virtual method to declare it.
abstract method like this:
public interface IElement
{
string TagName { get; }
string GetXml { get; }
}
public abstract class ElementBase : IElement
{
public string TagName { get; private set; }
public string GetXml { get; private set; }
protected abstract string GetNestedXml();
}
virtual method:
public interface IElement
{
string TagName { get; }
string GetXml { get; }
}
public abstract class ElementBase : IElement
{
public string TagName { get; private set; }
public string GetXml { get; private set; }
protected virtual string GetNestedXml()
{
throw new NotImplementedException();
}
}
The quick way to solve this is to make GetNestedXml public. If you don't want this, you can also declare GetNestedXml as a protected abstract method in an abstract base class. But this means that all classes need to derive from this base class and implement the method. If you want to provide an implementation in the base class, you can also make the method virtual so that the derived classes can but do not necessarily need to override it. In order to achieve this, perform the following steps:
Create a new abstract base class.
Add a protected abstract/virtual implementation of GetNestedXml(). If it is virtual, also provide a method body (and you do not need to make the class abstract).
Remove the method from the interface.
Derive all classes that implement the interface (and want to have the comfort of the basic implementation GetNestedXml) from the base class.
Another way to hide the method would be to implement IElement explicitly, so that the the callers only see it when they access the object using the interface.
The thing is, that when you declare Element implements IElement you say: "Hey, I know how to get my nested XML, and every one can use it! (public...)".
On your class the GetNestedXml is protected, i.e. you are not fulfilling your declaration.
Even if you do an explicit protected implementation:
protected override string IElement.GetNestedXml()
{
//Implementation...
}
Behind the scenes, It will still actually be public.
Related
I have an interface such as this one:
public interface ITestInterface
{
int a { get; set; }
void DoSomething();
}
Some of my classes are deriving from this interface:
public class OneClass : ITestInterface
{
public int a { get; set; }
public void DoSomething()
{
Console.WriteLine(this.a.ToString());
}
}
public class AnotherClass : ITestInterface
{
public int a { get; set; }
public void DoSomething()
{
Console.WriteLine((this.a * 2).ToString());
}
}
Since I now need a (large) common method on all classes derived from my interface, I was trying to provide an additional base class for that:
public class MyBaseClass
{
public void LargeCommonMethod()
{
Console.WriteLine((this.a * 3).ToString()); // no 'a' on base class
}
}
This clearly doesn't work because the base class would also need to implement my interface in order to know about that a field.
I am now asking myself what the best approach would be here:
make MyBaseClass inherit from ITestInterface?
set LargeCommonMethod() to protected and provide all internal data it uses via arguments? (There's actually a lot of these..)
skip the interface all along and replace it with an abstract base class?
...?
C# 8 provides a feature precisely for this scenario.
Your classes all implement an interface
You want to add a method to the interface
You don't want a breaking change to all of the existing classes. If you add a method to the interface all of the classes will break unless you find some way to add the method to all of them. (That includes modifying them all to inherit from a new base class.)
That feature is default interface methods.
You can add your method and a default implementation to the interface:
public interface ITestInterface
{
int a { get; set; }
void DoSomething();
void LargeCommonMethod()
{
Console.WriteLine((this.a * 3).ToString());
}
}
Your existing classes that implement the interface will not break. When cast as the interface, you'll be able to call the method which is defined in the interface. You can still modify any class to provide its own implementation, overriding the interface's default implementation.
For the method to be available the object must be cast as the interface - ITestInterface.
A lot of developers - including myself - found this to be an odd feature. But this is the scenario it's for.
Some documentation
The most common scenario is to safely add members to an interface already released and used by innumerable clients.
If you require a base implementation for a method then an interface is clearly not the way to go.
I would choose an abstract class instead and get rid of the interface. There is no need to complicate the design basically.
The Adapter pattern could fit your Use case, when you want to keep the ITestInterface consistent:
public interface ITestInterface
{
int a { get; set; }
void DoSomething();
}
public class TestInterfaceAdapter : ITestInterface
{
private readonly ITestInterface _testInterface;
public int a {
get => _testInterface.a;
set => _testInterface.a = value;
}
public TestInterfaceAdapter(ITestInterface testInterface)
{
_testInterface = testInterface;
}
public void DoSomething()
{
_testInterface.DoSomething();
}
public void LargeCommonMethod()
{
Console.WriteLine((this.a * 3).ToString());
}
}
public class OneClass : ITestInterface
{
public int a { get; set; }
public void DoSomething()
{
Console.WriteLine(this.a.ToString());
}
}
public class AnotherClass : ITestInterface
{
public int a { get; set; }
public void DoSomething()
{
Console.WriteLine((this.a * 2).ToString());
}
}
I'm having a hard time phrasing the question which is also making it hard for me to search for answers.
Here's a contrived scenario that mimics what I'd like to do:
void Main()
{
Console.WriteLine(TestClassA.MyPropertyName());
Console.WriteLine(TestClassB.MyPropertyName());
var speaker = new TestSpeaker();
speaker.Speak<TestClassA>();
speaker.Speak<TestClassB>();
}
public class TestSpeaker {
public void Speak<T>() where T : BaseClass<T> {
Console.WriteLine(/* I want to call T.MyPropertyName() here */);
}
}
public class TestClassA : BaseClass<TestClassA> {
public string Name { get; set; }
}
public class TestClassB : BaseClass<TestClassB> {
public string OtherPropertyName { get; set; }
}
public abstract class BaseClass<T> {
public static string MyPropertyName(){
return typeof(T).GetProperties().Single().Name;
}
}
The Console right now would read:
Name
OtherPropertyName
I'd like to replace my commented out code so that it would read:
Name
OtherPropertyName
Name
OtherPropertyName
if you change your Writeline to
Console.WriteLine(BaseClass<T>.MyPropertyName());
you will get what you want
Why use a static function in a base class to retrieve information about a derived class? In any case, you could implement a member function to wrap the static call:
public static string MyStaticFunction() => return "whatever";
public string MyMemberFunction() => MyStaticFunction();
But in your scenario, perhaps you should simply declare an abstract property (or function) meant to return the value you're looking for and override it in derived classes:
Base:
public abstract string MyPropertyName { get; }
Derived:
public override string MyPropertyName => nameof(OtherPropertyName); // or more complex logic
And yet another possible solution would be to pass the information to the base class's constructor as a string (or property expression should you be so inclined):
Base:
public string MyPropertyName { get; init; }
public BaseClass(string propertyName)
{
MyPropertyName = propertyName; // maybe validate that the property exists
}
Derived:
public MyTestClassB() : BaseClass(nameof(OtherPropertyName)) {}
This is class design question.
I have main abstract class
public abstract class AbstractBlockRule
{
public long Id{get;set;}
public abstract List<IRestriction> Restrictions {get;};
}
public interface IRestriction{}
public interface IRestriction<T>:IRestriction where T:struct
{
T Limit {get;}
}
public TimeRestriction:IRestriction<TimeSpan>
{
public TimeSpan Limit{get;set;}
}
public AgeRestriction:IRestriction<int>
{
public int Limit{get;set;}
}
public class BlockRule:AbstractBlockRule
{
public virtual List<IRestriction> Restrictions {get;set;}
}
BlockRule rule=new BlockRule();
TimeRestriction t=new TimeRestriction();
AgeRestriction a=new AgeRestriction();
rule.Restrictions.Add(t);
rule.Restrictions.Add(a);
I have to use non-generic Interface IRestriction just to avoid specifying generic type T in main abstract class. I'm very new to generics. Can some one let me know how to better design this thing?
Your approach is typical (for example, IEnumerable<T> implements IEnumerable like this). If you want to provide maximum utility to consumers of your code, it would be nice to provide a non-generic accessor on the non-generic interface, then hide it in the generic implementation. For example:
public abstract class AbstractBlockRule
{
public long Id{get;set;}
public abstract List<IRestriction> Restrictions { get; set; }
}
public interface IRestriction
{
object Limit { get; }
}
public interface IRestriction<T> : IRestriction
where T:struct
{
// hide IRestriction.Limit
new T Limit {get;}
}
public abstract class RestrictionBase<T> : IRestriction<T>
where T:struct
{
// explicit implementation
object IRestriction.Limit
{
get { return Limit; }
}
// override when required
public virtual T Limit { get; set; }
}
public class TimeRestriction : RestrictionBase<TimeSpan>
{
}
public class AgeRestriction : RestrictionBase<TimeSpan>
{
}
public class BlockRule : AbstractBlockRule
{
public override List<IRestriction> Restrictions { get; set; }
}
I also showed using a base restriction class here, but it is not required.
The runtime treats IRestriction<TimeSpan> and IRestriction<int> as different distinct classes (they even have their own set of static variables). In your case the only classes common to both IRestriction<TimeSpan> and IRestriction<int> in the inheritance hierarchy are IRestriction and object.
So indeed, having a list of IRestriction is the only sensible way to go.
As a side note: you have a property Limit in there that you might want to access regardless of whether you're dealing with an IRestriction<TimeSpan> or IRestriction<int>. What I would do in this case is to define another property object Limit { get; } on IRestriction, and hide it in the actual implementation. Like this:
public interface IRestriction
{
object Limit { get; }
}
public interface IRestriction<T> : IRestriction
where T : struct
{
new T Limit { get; set; }
}
public class TimeRestriction : IRestriction<TimeSpan>
{
public TimeSpan Limit { get; set; }
// Explicit interface member:
// This is hidden from IntelliSense
// unless you cast to IRestriction.
object IRestriction.Limit
{
get
{
// Note: boxing happens here.
return (object)Limit;
}
}
}
This way you can access Limit as object on all your IRestriction when you don't care what type it is. For example:
foreach(IRestriction restriction in this.Restrictions)
{
Console.WriteLine(restriction.Limit);
}
Interfaces are contracts that need to be followed by the entity that implements the contract.
You have created two contract with the same name IRestriction
As far as I can see, what you are basically may need is a flag for classes that can be restricted, which should implement the IRestriction non-generic interface.
The second interface seems to be restrictable objects that also contain a limit property.
Hence the definition of the second IRestriction interface can be ILimitRestriction or whatever name suits your business needs.
Hence ILimitRestriction can inherit from IRestriction which would mark classes inheriting ILimitRestriction still objects of IRestriction
public abstract class AbstractBlockRule
{
public long Id{get;set;}
public abstract List<IRestriction> Restrictions {get;};
}
public interface IRestriction{}
public interface IRestrictionWithLimit<T>:IRestriction where T:struct
{
T Limit {get;}
}
public TimeRestriction:IRestrictionWithLimit<TimeSpan>
{
public TimeSpan Limit{get;set;}
}
public AgeRestriction:IRestrictionWithLimit<int>
{
public int Limit{get;set;}
}
public class BlockRule:AbstractBlockRule
{
public virtual List<IRestriction> Restrictions {get;set;}
}
I have a following interface:
interface IName
{
string Name { get; }
}
And some base class BaseClass. Childs of this class may implement IName interface, but not all of them do.
If a Child implements IName, I would also like to override ToString() method, exactly the same way for all cases, as follows:
public override string ToString()
{
return Name;
}
It seems that a good place for overriding ToString() would be in IName interface, but i believe that it is not possible in C#.
Implementing ToString() in every class seems a bad idea too, because it's a lot of code redundancy (and a waste of time).
What is a proper solution for a case like this?
I'd suggest to make a second base class as such:
public abstract class BaseClass
{
// your base class implementation
}
public abstract class NamedBaseClass : BaseClass, IName
{
public string Name { get; set;}
public override string ToString()
{
return Name;
}
}
this way, if you want a child to implement both BaseClass and IName, then you should inherit from NamedBaseClass.
According to your words that "some might implement IName and some not", then BaseClass should not implement IName, but you should still have some sort of a base implementation. this is my solution.
EDIT:
to make a single class which returns the name and has nothing to do with BaseClass, then you can make an unrelated abstract implementation just for that:
public abstract class NameStringClass : IName
{
public string Name { get; set; }
public override string ToString()
{
return Name;
}
}
Consider cFoo as the child class which has IName. And cBar parent class, and cBaz as a class which doesnt implement IName.
public interface IName
{
string Name { get; }
}
public class CBaz : CBar
{
}
public class CFoo : CBar, IName
{
public CFoo(string name)
{
Name = name;
}
public string Name { get; }
}
abstract public class CBar
{
public override string ToString()
{
if (this is IName)
{
var temp = (IName) (this);
return temp.Name;
}
else
{
return base.ToString();
}
}
}
[Old Answer]
Note that an interface is essentially empty. You need to think of an interface as bearing more of the meaning of contract, implying that the person whom implements this interface as requiring to implement a property/method/field/etc of such signature.
For implementation specific tasks which may be shared commonly by many classes, an abstract class is more suitable.
The moment you need to resolve some kind of logic, you must go through a gateway of implementation, some implementation must occur, interfaces are essentially empty templates. The most loose way of implementing this is via an abstract class which contains both the name property and the ToString override, from which you then inherit from for all your subsequent classes.
At the same time you can consider an abstract class for your CBar class. As well as calling the base method base.ToString().
[End of Old Answer]
I want to create an abstract base class for all paramter-type classes to inherit from in my application. All paramters will have name, ID and required properties.
All parameters will have their properties set from XML via the SetProperties method (the XmlParser class shown below is just for demonstration).
Since all parameters will have the same 3 properties, I wanted the base class to set those properties but have the inheriting class extend the SetProperties method to set the additional properties it contains.
I was thinking of something like overriding events on a Control.
Here is a sample of what I was thinking, although it does not work.
abstract class ParameterBase
{
protected string ParameterName;
protected bool IsRequired;
protected int ParameterId;
public abstract void SetProperties(string xml)
{
this.ParameterName = XmlParser.GetParameterName(xml);
this.IsRequired = XmlParser.GetIsRequired(xml);
this.ParameterId = XmlParser.GetParameterId(xml);
}
}
abstract class Parameter1 : ParameterBase
{
private string _value;
public string ParameterName
{
get { return base.ParameterName; }
}
public bool IsRequired
{
get { return base.IsRequired; }
}
public int ParameterId
{
get { return base.ParameterId; }
}
public string Value
{
get { return _value; }
}
public Parameter1()
{
}
public override void SetProperties(string xml)
{
base.SetProperties(xml);
_value = XmlParser.GetValue(xml);
}
}
I would do simply, like this:
abstract class ParameterBase
{
protected string ParameterName;
protected bool IsRequired;
protected int ParameterId;
public abstract void SetProperties(string xml);
}
and derived one:
public class Parameter1 : ParameterBase
{
public override void SetProperties(string sml)
{
//set properties including those ones of parent
}
}
It's easy and clearer to manage in this way. Move common properties in separate base class it's good, but persistance management (Save/Load), leave to children. They should know how to do that.
Code provided has a couple of problems:
abstract method can not have body
you have strange public override void SetValues(string xml) which I think should be
public override void SetProperties(string xml)
There are four issues with your code that I can see:
You are redefining the 3 shared properties, and you're trying to name them the same as an existing field. This is not allowed. The simplest way is to implement the properties in the base class in the same way you implemented Value in the inheriting class: with backing fields. In C# 3.0 and above (Visual Studio 2008 and up), you can use auto-implemented properties with a private setter. This will make the compiler create the backing fields for you. For example:
public string ParameterName { get; private set; }
You are declaring the SetProperties method as abstract. This should be virtual. abstract means that the subclass must define the entire implementation. That's not the case here.
In your derived class, you override SetValues but the method is called SetProperties.
You are declaring Parameter1 as abstract. You can't instantiate abstract classes, so you would have to inherit a class from Parameter1 as well in order to use it. I'm guessing you would just want to remove the abstract qualifier.
I would make the common Base class properties Public with protected setters, then you can access them from any derived classes, no use duplicating code!
protected string ParameterName { get; protected set; };
protected bool IsRequired { get; protected set; };
protected int ParameterId { get; protected set; };
You are making it too complicated. The first three properties can be inhertied from the base class:
public abstract class ParameterBase
{
public string ParameterName { get; private set; }
public bool IsRequired { get; private set; }
public int ParameterId { get; private set; }
public virtual void SetProperties(string xml)
{
ParameterName = XmlParser.GetParameterName(xml);
IsRequired = XmlParser.GetIsRequired(xml);
ParameterId = XmlParser.GetParameterId(xml);
}
}
public class Parameter1 : ParameterBase
{
public string Value { get; private set; }
public override void SetProperties(string xml)
{
base.SetProperties(xml);
Value = XmlParser.GetValue(xml);
}
}
Also note that an abstract method cannot have a body, instead it is terminated by a semicolon:
public abstract void SetProperties(string xml);
You must delacre it as virtual if you want to give it a base implementation.
(And you must override SetProperties, not SetValue.)
As noted, don't over think it. I'd declare the abstract parameter class so that it has a single constructor (protected) that takes the mandatory three properties ( Name, IsRequired, and ID). This means that every concrete subtype must construct it properly.
Then, I'd have an abstract factory method, CreateInstance(), that every concrete subtype must implement, returning an instance of AbstractParameter. Why? Read up on the Liskov Substitution Principle. In the Real World, of course, it might make more sense to, rather than use a factory method, separate the concern of how to creating parameter instances from the concerns of being a parameter, by moving the construction logic into its own factory class (AbstractParameterFactory?).
I might note, though, that you're missing an essential property that all parameters will have: a Value. You might consider making your abstract parameter's base class generic.
Anyway, here's my AbstractParameter class:
public abstract class AbstractParameter
{
public string Name { get ; protected set ; }
public bool IsRequired { get ; protected set ; }
public int ID { get ; protected set ; }
protected AbstractParameter( string name , bool isRequired , int id )
{
this.Name = name;
this.IsRequired = isRequired;
this.ID = id;
this.Value = default(T) ;
return;
}
public abstract AbstractParameter CreateInstance( string xml ) ;
}
A concrete parameter class that derives from AbstractParameter might then look something like this:
public class ConcreteParameter : AbstractParameter
{
public ConcreteParameter( string name , bool isRequired , int id ) : base( name , isRequired , id )
{
return ;
}
public override AbstractParameter CreateInstance( string xml )
{
string name = XmlParser.GetName();
bool required = XmlParser.GetIsRequired();
int id = XmlParser.GetID();
ConcreteParameter instance = new ConcreteParameter( name , required , id );
return instance;
}
}