How can I separately declare a getter and a setter for a property?
For exemple, say I want to create the following hierarchy:
interface IReadOnlyFoo
{
string Value { get; }
}
interface IFoo : IReadOnlyFoo
{
string Value { set; }
}
class BasicFoo : IFoo
{
string Value { get; set; }
}
The compiler is compaining because IFoo.Value is hiding IReadOnlyFoo.Value, which is not what I want to do. I want to "merge" the getter and setter declarations.
I've had a look at how the .NET Framwork declares the IReadOnlyList and IList interfaces, but it's done in a different way.
How could I acheive what I want to do ? Can I do that with a property or do I really have to create separate GetValue() and SetValue() methods instead?
When you change your interface definition to
interface IReadOnlyFoo
{
string Value { get; }
}
interface IReadWriteFoo
{
string Value { get; set; }
}
class BasicFoo : IFoo, IReadOnlyFoo
{
public string Value { get; set; }
}
it should work.
When you implement the interface the two members would be merged since you don't have a get method in IFoo.Value.
interface IReadOnlyFoo
{
string Value { get; }
}
interface IFoo : IReadOnlyFoo
{
new string Value { set; }
}
class BasicFoo : IFoo
{
public string Value { get; set; }
}
As long as you are using implicit implementations for the interfaces it would behave as you intended. on the other hand, if you wish to have two different behaviors for the members of the interface then you want to use explicit implementations. You can find an example here
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/interfaces/how-to-explicitly-implement-members-of-two-interfaces
All you have to change in your code is to add a getter to the Value property in the IFoo interface.
Semantically speaking, IFoo is a specific kind of IReadOnlyFoo that adds another capability to it's base type (the Setter of the Value property).
This is the exact definition of inheritance in object oriented programming - a child type is a more specific version of it's base type and is adding capabilities to it.
interface IReadOnlyFoo
{
string Value { get; }
}
interface IFoo : IReadOnlyFoo
{
new string Value { get; set; }
}
class BasicFoo : IFoo
{
public string Value { get; set; }
}
This code is perfectly valid and will give you exactly what you're looking for.
This way, if you have a reference of type IReadOnlyFoo to an instance of BasicFoo, the Value property is indeed readonly, but if your reference type is IFoo it's a read / write property.
You can see a live demo on rextester.
Related
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;}
}
In my abstract base class AbstractType, I have an abstract auto-implemented property Value of unknown type. All my derived classes implement this property with their own types, such string or double. Normally, I know you would just make it AbstractType<T> and have the property be T Value { ... }. However, I don't have the ability to use generics in this case. In AbstractType, I'm trying to implement a method that returns a new derived class from AbstractType, so if I use generics, the caller has to know the type. If I make Value type object, then the caller has to wrap the object to the correct type - very inconvenient type/instance checking.
Here's what my class structure looks like (the method is simplified for the sake of demonstration):
abstract class AbstractType
{
public abstract ??? Value { get; set; }
AbstractType FromValue(int i)
{
if (i == 0)
return new NumberType();
else
return new StringType();
}
}
class NumberType : AbstractType
{
public override double Value { get; set; }
}
class StringType : AbstractType
{
public override string Value { get; set; }
}
Is there any way to do this without using generics?
I want to access a property of a new created object within a generic method, which is constraint by an Interface:
public interface MyInterface
{
int ID { get; set; }
string Name { get; set; }
}
Since the Compiler knows that "T" is of the Type MyInterface it should be possible to access the properties of that inteface:
public T doSomething<T>(String value) where T : MyInterface, new()
{
T entity = new T();
entity.Name = value;
return entity;
}
But it sais: T does not have a definition for 'Name'
If I can use an interface as a constraint here: Why isn't it possible to access its properties?
The code you posted is correct for itself. Maybe you have different versions of your interface (MyInterface in different namespaces)? Check the namespaces / fully qualified names of the interface types. Also the check the assembly versions, if declaring types in another assembly...
public class Foo2 : MyInterface
{
public int ID { get; set; }
public string Name { get; set; }
}
...
var foo = doSomething<Foo2>("test");
Console.WriteLine(foo.Name);
Seems to work as long as your code has the namespace of your interface and concrete class in a using clause. Also, as a matter of convention MyInterface should be IMyInterface.
When I define an interface that contains a write-only property:
public interface IModuleScreenData
{
string Name { set; }
}
and attempt to (naively) implement it explicitly with an intention for the property to also have a publicly available getter:
public class ModuleScreen : IModuleScreenData
{
string IModuleScreenData.Name { get; set; }
}
then I get the following error:
Error 'IModuleScreenData.Name.get' adds an accessor not found in
interface member 'IModuleScreenData.Name'
The error is more or less expected, however, after this alternative syntax:
public class ModuleScreen : IModuleScreenData
{
public string Name { get; IModuleScreenData.set; }
}
has failed to compile, I suppose that what I am trying to do is not really possible. Am I right, or is there some secret sauce syntax after all?
You can do this:
public class ModuleScreen : IModuleScreenData
{
string IModuleScreenData.Name
{
set { Name = value; }
}
public string Name { get; private set; }
}
On a side note, I generally wouldn't recommend set-only properties. A method may work better to express the intention.
You can't change the how the interface is implemented in the inheriting class. That is the whole point.. if you need to do something new with a property you could make a new property that references the inherited properties specific implementation. Interfaces are there so you can conform to a specified standard for object inheritance.
UPDATE:
On second thought.. you should be able to just do this.... this will compile fine:
public class ModuleScreen : IModuleScreenData
{
public string Name { get; set; }
}
I'm writing a TemplateEngine that will allow me to use my own markup in text based files. I'm wanting to add controls as plugins as the application matures. Currently i've got a structure like the following:
interface IControl
string Id
object Value
class Label : IControl
string Id
string Value
class Repeater : IControl
string Id
List<IControl> Value
Now you'll see the strange part right away in the Repeater class with the Value property. I was hoping that having the Value type as object in the interface would allow me the flexibility to expand the controls as i go along. The compiler doesn't like this and for good reason i guess.
Bottom line: I'm trying to get all control classes to implement the same interface but have different types for the Value property.
Does anyone have any suggestions how to accomplish this?
Note: Please don't go into suggesting things like use Spark View Engine for templating. There is a reason i'm creating extra work for myself.
Normally the Repeater would implement something different, like an IItemsControl for example.
EDIT 1
(removed for brevity)
EDIT 2
Ah okay, you can always use explicit interface implementation of course:
interface IControl
{
string Id { get; set; }
object Value { get; set; }
}
class Label : IControl
{
public string Id { get; set; }
public string Value { get; set; }
object IControl.Value
{
get { return this.Value; }
set { this.Value = (string)value; }
}
}
class Repeater : IControl
{
public string Id { get; set; }
public IList<IControl> Value { get; set; }
object IControl.Value
{
get { return this.Value; }
set { this.Value = (IList<IControl>)value; }
}
}
you could also use generics:
interface IControl<T>
{
string ID{get;set;}
T Value{get;set;}
}
class SomeControl : IControl<string>
{
public string ID{get;set}
public string Value{get;set;}
}
class SomeOtherControl : IControl<int>
{
public string ID{get;set}
public int Value{get;set;}
}
I like this better than the explicit interface idea if it's just one return value that needs to change. However, I think if you had several properties that each would return a different type, you wouldn't want to have IControl. At least, I wouldn't. In that case I would recommend the explicit interfaces.
Of course, this wouldn't work if you didn't have access to the source of IControl.
Edit: had a typo. Fixed
No, the compiler doesn't allow same name fields to be of different data types other than what is defined in the interface in derived classes.
The properties (since no fields are allowed in interface) should be implemented in the deriving classes and they need to have same data type. So, you cannot probably do it with properties without explicit declaration.
However, if you make Value to be returned by a function, then it works, but you need to check the return type because the return types should match for the function, otherwise you will get error that interface's function was not implemented.
interface IControl
{
object Value();
}
class A : IControl
{
string m_value = string.Empty;
public object Value() { return m_value; }
};
class B : IControl
{
List<IControl> m_value = new List<IControl>();
public object Value() { return m_value; }
};
....
object o = new B().Value();
if (o is List<IControl>)
MessageBox.Show("List");
[Update]
You have to be careful if explicitly defining the body of the properties. Having one name for two properties would be dangerous if implementation is not done carefully.
These two properties if contain different definition, it would be unexplainable for the final use of the interface and classes.
public IList<IControl> Value
object IControl.Value
See this example:
...
class Repeater : IControl
{
List<IControl> m_Value = new List<IControl>();
public IList<IControl> Value
{
get { return this.m_Value; }
set { this.m_Value = (IList<IControl>)value; }
}
object IControl.Value
{
get
{
return this.m_Value;
}
set
{
this.m_Value = new List<IControl>();
this.m_Value.Add(new Label());
this.m_Value.AddRange((List<IControl>)value);
}
}
}
...
Repeater b = new Repeater();
IControl i = b;
List<IControl> list = new List<IControl>();
list.Add(new Repeater());
i.Value = list;
You can observe that the list container in Repeater will have different values when data is added via IControl (because of the explicit definition of IContainer.Value).