I am looking to 'extending' an interface by providing set accessors to properties in that interface. The interface looks something like this:
interface IUser
{
string UserName
{
get;
}
}
I want something like this:
interface IMutableUser : IUser
{
string UserName
{
get;
set;
}
}
I need the inheritence. I cannot copy the body of IUser into IMutableUser and add the set accessors.
Is this possible in C#? If so, how can it be accomplished?
I don't see any reason why what you have posted shouldn't work? Just did a quick test and it compiles alright, but gives a warning about hiding. This can be fixed by adding the new keyword, like this:
public interface IMutableUser : IUser
{
new string Username { get; set; }
}
An alternative would be to add explicit set methods; eg:
public interface IMutableUser : IUser
{
void SetUsername(string value);
}
Of course, I'd prefer to use setters, but if it's not possible, I guess you do what you have to.
You could use an abstract class:
interface IUser
{
string UserName
{
get;
}
}
abstract class MutableUser : IUser
{
public virtual string UserName
{
get;
set;
}
}
Another possibility is to have this:
interface IUser
{
string UserName
{
get;
}
}
interface IMutableUser
{
string UserName
{
get;
set;
}
}
class User : IUser, IMutableUser
{
public string UserName { get; set; }
}
You can "override" properties in an interface by explicitly implementing the interfaces. Chris' answer is likely all you'll need for the scenario you've outlined, but consider a slightly more complex scenario, where you need a getter/setter on your class, but the interface only defines the getter. You can get around this by doing the following:
public class MyUser : IUser
{
IUser.MyProperty { get { return "something"; } }
public MyProperty { get; set; }
}
By explicitly implementing IUser.MyProperty, you satisfy the contract. However, by providing public MyProperty, the API for your object will never show the explicit interface version, and will always use MyProperty with the get/set.
Related
I am having trouble understanding the proper use of base and this within an inherited get method. I have an interface IMatchModel:
public interface IMatchModel
{
int TypeId { get; }
DateTime DataDate { get; set; }
string TypeName { get; set; }
}
And a base model class TradeModel:
public class TradeModel
{
public long TradeId { get; set; }
public DateTime DataDate { get; set; }
public string TradeName { get; set; }
}
Then I have a class that inherits from TradeModel and implements IMatchModel. I am currently using the following method:
public class TradeMatchModel : TradeModel, IMatchModel
{
public int TypeId { get { return 1; } }
public string TypeName
{
get
{
return base.TradeName;
}
set
{
base.TradeName = value;
}
}
}
The TradeModel class is used within a function that operates on all of its attributes. IMatchModel is used in a function that only needs the attributes contained in the interface. The code works properly, but I still feel like I don't quite understand if it is best to be using base over this. Is the use of base in this context incorrect?
The only time you need to use base is when you are inside a overridden virtual method and you need to call the base implementation of the method you are currently overriding. All other times you can use this..
Also this. is generally not needed unless you have a name conflict between a field or property in the class and a name of a variable or a parameter. 99% of the time you can just leave off the this. and do return TradeName;
I was just coding a simple C# interface, and I put a property in it without thinking it through too far. For example:
public interface IMyInterface
{
string Name { get; set; }
object[][] Data { get; set;
}
I realized that I'm a little confused with properties when applied to interfaces and abstract base classes. In a normal class, this syntax would generate the accessor and mutator for a hidden string member that it generated behind the scenes.
Interfaces shouldn't be able to have data members. So, does this syntax do something different in that case?
What about for abstract classes? If I put this same syntax in the abstract base and the derived class, would both end up with a hidden member?
Interfaces shouldn't be able to have data members.
Those are properties, and those are allowed:
An interface contains only the signatures of methods, properties, events or indexers.
See also c# properties on Interface.
As for your second question:
If I put this same syntax in the abstract base and the derived class, would both end up with a hidden member?
Yes. You can prevent that by marking the property virtual on the base class and override on the derived class.
The property declaration in the interface is completely separate from the implementation. Thus you can implement it using automatic properties
private class MyImpl : IMyInterface
{
public string Name { get; set; }
public object[][] Data { get; set; }
}
or declare your own backing field
private class MyImplBacked : IMyInterface
{
private string _name;
public string Name
{
get
{
return _name;
}
set
{
_name = value;
}
}
public object[][] Data { get; set; }
}
Same scenario in abstract classes
public abstract class MyAbstractClass
{
public abstract string Name { get; set; }
public abstract object[][] Data { get; set; }
}
private class MyImpl : MyAbstractClass
{
public override string Name { get; set; }
public override object[][] Data { get; set; }
}
private class MyImplBacked : MyAbstractClass
{
private string _name;
public override string Name
{
get
{
return _name;
}
set
{
_name = value;
}
}
public override object[][] Data { get; set; }
}
Interfaces shouldn't be able to have data members. So, does this syntax do something different in that case?
Technically it's not a data member - it's a get/set method pair that has an underlying data member. There's no implementation.
What about for abstract classes? If I put this same syntax in the abstract base and the derived class, would both end up with a hidden member?
If the class is abstract and the property is virtual then yes, you will be overriding an auto-implemented property with another auto-implemented property (which is pointless).
If the class is abstract and the property is NOT virtual then you still have two implementations, but the base class is hiding the parent implementation rather than overriding it (which is still pointless if they're both auto-implemented).
If the property is abstract then the abstract class won't have an implementation. You'll have to implement the get/set in your concrete class (which could be auto-implemented bot doesn't have to be).
I'm trying to figure out a good way to approach this. I have a Customer class which implements the ICustomer interface. This interface has a number of properties in it:
public interface ICustomer
{
string FirstName {get; set;}
string LastName {get; set;}
}
I only want certain classes to have the ability to set those properties however; namely, those classes in the project. So I thought about making the setter internal:
public class Customer : ICustomer
{
string FirstName {get; internal set;}
string LastName {get; internal set;}
}
I'd like to mark that setter as internal in the interface however, so there's no chance someone implements ICustomer and someone outside the assembly modifies those properties. Is there a good way to do this?
The properties in the interface should be read only. It's acceptable for the concrete class that implements the interface to have a setter even if none is defined in the interface.
public interface ICustomer
{
string FirstName { get; }
string LastName { get; }
}
public class Customer : ICustomer
{
public string FirstName { get; internal set; }
public string LastName { get; internal set; }
}
If it's really important that the setter be exposed through an interface, rather than having the interface being entirely read-only, you can use something like this:
public interface IReadCustomer
{
string FirstName { get; }
string LastName { get; }
}
internal interface IWriteCustomer
{
string FirstName { set; }
string LastName { set; }
}
internal interface IReadWriteCustomer : IReadCustomer, IWriteCustomer
{ }
public class Customer : IReadWriteCustomer
{
private string _firstName;
private string _lastName;
public string FirstName
{
get { return _firstName; }
internal set { _firstName = value; }
}
public string LastName
{
get { return _lastName; }
internal set { _lastName = value; }
}
string IReadCustomer.FirstName
{
get { return FirstName; }
}
string IReadCustomer.LastName
{
get { return LastName; }
}
string IWriteCustomer.FirstName
{
set { FirstName = value; }
}
string IWriteCustomer.LastName
{
set { LastName = value; }
}
}
I'd like to mark that setter as internal in the interface however, so there's no chance someone implements ICustomer and someone outside the assembly modifies those properties. Is there a good way to do this?
No. Property members are always public, unfortunately. Additionally, messing around with access levels on properties where part of it is specified on the interface gets painful, IIRC. What you can do is this:
public interface ICustomer
{
string FirstName { get; }
string SecondName { get; }
}
internal interface ICustomerWithSetMethods : ICustomer
{
void SetFirstName(string name);
void SetLastName(string name);
}
public class Customer : ICustomerWithSetMethods
Then from the outside it'll look like Customer only implements ICustomer, but from inside your code will see that it implements ICustomerWithSetMethods.
Unfortunately that doesn't play nicely if your API needs to declare any public methods where you'd really like to just declare a return type of ICustomer, but you'll actually know that it's always ICustomerWithSetMethods.
Assuming you still want to allow multiple implementations, you could potentially go for an abstract class instead:
public abstract class CustomerBase
{
public abstract string FirstName { get; }
public abstract string LastName { get; }
internal abstract void SetFirstName(string name);
internal abstract void SetLastName(string name);
}
Now we have the slight oddity that no-one outside the assembly can extend your CustomerBase, because there are abstract methods they'd have to override that they can't even see - but it does mean you can use CustomerBase everywhere in your API.
This is the approach we took in Noda Time for calendar systems in the end - I blogged about it when I first came up with the plan. I generally prefer interfaces to abstract classes, but the benefit here was significant.
I want to create a class that can take different types of value in a property. I am trying to do this using polymorphism, but I am not still learning how to do this properly, hence my request for advice.
I have a base class and two classes that inherit from it:
public abstract class BaseClass
{
public string Name { get; set; }
public Unit Unit { get; set; }
}
public class DerivedClassFloat : BaseClass
{
public float Value { get; set; }
public override string ToString()
{
return Value.ToString();
}
}
public class DerivedClassString : BaseClass
{
public string Value { get; set; }
public override string ToString()
{
return Value;
}
}
All is good, I can create a List and add different specialized subclasses. My problem comes when I need change the values of the items in my list:
foreach (var item in ListOfBaseClasses)
{
if(item is DerivedClassFloat)
((DerivedClassFloat) item).Value = float.NaN;
if (item is DerivedClassString)
((DerivedClassString) item).Value = string.Empty;
}
According to what I have read, that looks like a code smell. Is there a better way to access the value property of my derived classes based on the type I am trying to assign?
What about when you want to create the right subclass based on the value?
BaseClass newClass = null;
if (phenotype is DerivedClassFloat)
newClass = new DerivedClassFloat(){Value = 12.2};
if (phenotype is DerivedClassString)
newClass = new DerivedClassString(){Value = "Hello"};
I read about overriding virtual methods, but that works if I want to process the value, not to add or change it … maybe I am missing something?
I should make this more concrete, my apologies, I am not used to post question in this great site.
I need a property that is made of a list of attributes. Each attribute has a name and a value, but the value can be of different types. For example:
public class Organism
{
public string Name { get; set; }
public List<Attribute> Attributes { get; set; }
}
public class Attribute
{
public string AttributeName { get; set; }
public object AttributeValue { get; set; }
}
For a given organism I can have several attributes holding different value types. I wanted to avoid using the object type so that I don’t have to cast to the right type. I though property polymorphism was the solution to handle this case elegantly, but then I found myself using If ..Then which didn’t seem too different from casting in the first place.
If in your particular case you want to reset Value, you can define an abstract ResetValue method in the base class, which will be implemented by the derives classes.
As for your second case, you should check out Creational Design Patterns, and specifically the Factory and Prototype design patterns.
You can use generics to define the type and the implementing subclass will set the Value type to the type constraint:
public abstract class BaseClass<T>
{
public string Name { get; set; }
public Unit Unit { get; set; }
public T Value { get; set; }
public override string ToString()
{
return Value.ToString();
}
}
public class DerivedFloat : BaseClass<float> {}
public class DerivedString : BaseClass<string> {}
You can use Generics for this particular case:
public abstract class BaseClass<T>
{
public string Name { get; set; }
public Unit Unit { get; set; }
public T Value { get; set; }
}
public class DerivedClassFloat : BaseClass<float>
{
public override string ToString()
{
return Value.ToString();
}
}
public class DerivedClassString : BaseClass<string>
{
public override string ToString()
{
return Value;
}
}
Polymorphic behaviour works on abstraction. Based on what your trying to do, you can reduce code smell to moving as much of your variability in code to base classess.
i would suggest is instead of property write method like as follows. You can something like as follows.
public void setValue(string val, Type type);//move this to your base class
Class MyValue{
private string strVal;
private int intVal;
//constructor
MyValue(string val, Type type){
//check the type enum here and set the values accordingly
}
}
then when set values
foreach (var item in ListOfBaseClasses)
{
item.setValue = MyValue("",Type.INT);
}
I'm not quite sure what you are trying to achieve with this approach - the Value properties are not of the same type, there is also no Value property on the base class which suggests that other types derived from the base class might not have it at all.
If all of your classes require a Value property, then maybe it should be of the most general type object - you could put it onto the base class, but that would require casting the values in the derived classes.
But then you could have a NullObject to represent an absence of value that you could assign to the Value property for every derived class.
You can use the abstract factory pattern. Consider this example:
// Base class
class Button
{
protected Button()
{
}
public string Name { get; set; }
}
// Factory interface
public interface ButtonFactory
{
Button CreateButton();
}
// And the concrete classes
class WindowsButton : Button
{
// ...
}
class WindowsButtonFactory : ButtonFactory
{
public Button CreateButton()
{
return new WindowsButton();
}
}
class MacButton : Button
{
// ...
}
class MacButtonFactory : ButtonFactory
{
public Button CreateButton()
{
return new MacButton();
}
}
Furthermore, you can combine the abstract factory pattern with the strategy pattern to encapsulate the custom behaviors that change with type.
I would like to force a set of classes to define three fields (of type string).
In an abstract class, I get that fields cannot be abstract and in an interface, I get an error saying that an interface cannot contain a field.
Is there no way to do this or am I not understanding this correctly? I'd rather not use methods because for some weird reason, the parentheses annoy me.
You can use properties for that:
interface MyInterface {
string Prop1 { get; set; }
string Prop2 { get; set; }
string Prop3 { get; set; }
}
Interface or abstract members force derived classes to provide code.
Fields don't have code.
You should use a property, which can be used like a field, but has code.
You can use Properties instead of fields:
// works similarly for Interfaces too
abstract class MyAbstractClass { public virtual string MyProperty1 { get; set; } }
class MyConcreteClass : MyAbstractClass { }
Then you can access MyProperty1 from any instance derived from MyAbstractClass:
MyAbstractClass obj1 = new MyConcreteClass;
obj1.MyProperty1 = "abcd";
Like everyone else says, use properties instead of fields, but you can do something like I interpreted in the comments as follows for read-only members:
abstract public class Base
{
abstract public string Foo { get; }
abstract public string Bar { get; }
abstract public string Baz { get; }
}
public class Derived : Base
{
public override string Foo { get { return "foo"; } }
public override string Bar { get { return "bar"; } }
public override string Baz { get { return "baz"; } }
}
If you want the fields to be modifiable later, you'll have to either use automatic properties or declare concrete backing fields and getter/setter pairs for each property.