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.
Related
I'm doing and WPF application and I have a ViewModel that I use in several Views and in DataGrids.
Now I have another View that requires an extended or decorated version of that ViewModel. So I decided to go for inheritance in this way:
public class StandardViewModel
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class ExtendedViewModel : StandardViewModel
{
public string Email { get; set; }
}
However, I want to decorate and existing instance of the StandardViewModel. Specifically the selected object in the DataGrid so it can be passed into the other View.
The new View needs access to the properties of both classes (the Email and the FirtsName and LastName)
So I'm thinking of ways to creating a constructor for my ExtendedViewModel.
My idea is to copy the base instance directly.
Is this correct?
And efficient?
Is there any other way of doing it?
public class ExtendedViewModel : StandardViewModel
{
public string Email { get; set; }
public ExtendedViewModel(StandardViewModel base)
{
this = base
}
}
Edit
I'm doing this for not only one but several classes. And they do not have only two properties so I'm trying to avoid copying the values one by one.
Finally I'll avoid using inheritance and I'll create a new class, expose the base class and subscribe to INotifyPropertyChanged as described here and here.
This way I'll be able to have properties that depend on the Base ViewModel updated as done with FullName below.
The resulting ViewModel will look like:
public class ExtendedViewModel
{
public StandardViewModel Base { get; set; }
public string Email { get; set; }
public string FullName {
get => Base.FirstName + Base.LastName;
}
public ExtendedViewModel(StandardViewModel base)
{
Base = base
Base.PropertyChanged += BaseChanged
}
private void BaseChanged(object sender, PropertyChangedEventArgs e)
{
// Here check if FirstName or LastName changed and
RaisePropertyChanged("FullName");
}
}
In the view I will bind directly to Email or to Base.FirstName.
In oversimplified terms a decorator owns an instance of the class it decorates and delegates functionality existing in the owned class to that class, while adding new functionality uncoupled from the owned class instance.
public class StandardViewModel
{
public virtual string FirstName { get; set; }
public virtual string LastName { get; set; }
}
public class ExtendedViewModel : StandardViewModel
{
private StandardViewModel _standard;
public ExtendedViewModel (StandardViewModel standard)
{
if (standard.GetType() != typeof(StandardViewModel )) {
throw new ArgumentException ("Expected a non derived standard view model", nameof(standard));
}
_standard = standard;
}
public string Email { get; set; }
public override string FirstName {
get => _standard.Firstname;
set => _standard.Firstname = value;
}
public override string LastName {
get => _standard.LastName;
set => _standard.LastName = value;
}
}
Is there a way to define my class so that callers of that class would get a compile time error unless they specified every property of the class, with the additional constraint that the parameters pass be named?
Let's say I have this interface:
public interface IPerson
{
string FirstName {get;}
string MiddleName { get; }
string LastName { get;}
}
And these classes:
public class PersonNeedAllProperties : IPerson
{
public string FirstName { get; private set;} // ideally readonly
public string MiddleName { get; private set;}
public string LastName { get; private set;}
public PersonNeedAllProperties(
string firstName,
string lastName,
string middleName)
{
this.FirstName = firstName;
this.MiddleName = middleName;
this.LastName = lastName;
}
}
public class PersonCanNamePropertiesButNotSadlyNotRequired : IPerson
{
public string FirstName { get; set;} // ideally readonly
public string MiddleName { get; set;}
public string LastName { get; set;}
}
Then the problems with these imho are the following:
var p1 = new PersonNeedAllProperties("lastName", "firstName", "middle");
// woops wrong order because not named.
var p2 = new PersonCanNamePropertiesButNotSadlyNotRequired()
{
FirstName = "firstName",
LastName = "lastName"
}; // woops forgot all about the middlename.
Is there not a way to get a simple POCO object with an initalization similar to person two but that requires that all the properties be named?
Here is the fiddle of the above: https://dotnetfiddle.net/gUk8eT#
To answer your question No, this is not possible.
You ask to be able to force two things:
a) force all properties be set
b) force properties to be named.
These things can be done on their own but they cannot be combined because the methods for doing so are diametrically opposed as described below.
a) To force all properties be set you need to take them as inputs in a constructor.
b) To force properties to be set by name they cannot be specified in the constructor.
Side note: It is possible to provide named parameters to a constructor but it is not possible to force these i.e var person = new Person(firstName: 'John', lastName: 'Doe', middleName: 'A')
The closest you can get is to mark your properties readonly (private set) and have them only be set from within your constructor as you have done in your PersonNeedAllProperties class.
Something that might be a viable alternative is called the Builder Pattern.
This will have an extra class that is responsible for constructing your object, but you still wouldn't be able to get a compile time error, only runtime.
There's a horrible horrible way to almost enforce this. Not by name but by type. i'm not recommending it. but there is a horrible way to do it. did i mention it is horrible?
"any problem in computer science can be solved by adding another layer of indirection" - Wheeler?
Make each of the properties their own distinct type. now you can't pass them in the wrong order or the compiler will tell you that you passed an invalid type.
public class FirstName
{
string Value { get; set; }
}
public class MiddleName
{
string Value { get; set; }
}
public class LastName
{
string Value { get; set; }
}
public interface IPerson
{
FirstName FirstName {get;}
MiddleName MiddleName { get; }
LastName LastName { get;}
}
public class PersonNeedAllProperties : IPerson
{
public FirstName FirstName { get; private set;} // ideally readonly
public MiddleName MiddleName { get; private set;}
public LastName LastName { get; private set;}
public PersonNeedAllProperties(
FirstName firstName,
MiddleName lastName,
LastName middleName)
{
this.FirstName = firstName;
this.MiddleName = middleName;
this.LastName = lastName;
}
}
This can be achieved with a nested class I guess:
public interface IPerson
{
string FirstName { get; }
string MiddleName { get; }
string LastName { get; }
}
public class Person : IPerson
{
public string FirstName { get; private set; }
public string MiddleName { get; private set; }
public string LastName { get; private set; }
//Make sure the parameterless constructor is private
private Person() { }
private Person(string first, string middle, string last)
{
this.FirstName = first;
this.MiddleName = middle;
this.LastName = last;
}
public class Builder
{
private Person person = new Person();
public Builder WithFirstName(string first)
{
person.FirstName = first;
return this;
}
public Builder WithMiddleName(string middle)
{
person.MiddleName = middle;
return this;
}
public Builder WithLastName(string last)
{
person.LastName = last;
return this;
}
public IPerson Build()
{
if (person.FirstName != null
&& person.MiddleName != null
&& person.LastName != null)
return person;
throw new Exception("Cannot build person because reasons...");
}
}
}
Then use it as follows:
var person = new Person.Builder()
.WithFirstName("Rob")
.WithMiddleName("<I have no middle name>")
.WithLastName("Janssen")
.Build();
You can use the With... methods in any order you like:
var person = new Person.Builder()
.WithLastName("Janssen")
.WithFirstName("Rob")
.WithMiddleName("<I have no middle name>")
.Build();
Fiddle here
Edit: Crap; I din't notice the other requirement of compile time errors.
This does, however, 'force' you to set all 3 required fields and 'forces' you to "name" the field using the With... methods which makes it hard to confuse the values and also allows you to specify the values in the order you desire. It also prevents you from instantiating a Person yourself and also allows you to have your private setters (e.g. 'read only' properties) and keeps your interface intact. The only thing missing here is compile time errors; you won't get one here. Maybe Code Contracts can help in that department, but that would at least require some 'team coordination' (e.g. everyone needs to install an extension and have static checking enabled); and I'm not 100% sure if it can be done with code contracts either.
No, interfaces aren't designed to behave that way.
An interface assure you that the classes who implements it will have all the elements declared in the interface. But doesn't make any statements on how they are going to be implemented.
You can accomplish what you want with an abstract class with a custom constructor.
public abstract class Person
{
public abstract string FirstName {get; protected set; }
public abstract string MiddleName { get; protected set; }
public abstract string LastName { get; protected set; }
public Person(string firstName, string middleName, string lastName){
FirstName = firstName;
MiddleName = middleName;
LastName = lastName;
}
}
public class PersonNeedAllProperties : Person
{
public override string FirstName{get; protected set;}
public override string MiddleName{get; protected set;}
public override string LastName{get; protected set;}
public PersonNeedAllProperties(
string firstName,
string lastName,
string middleName)
: base(firstName, lastName, middleName)
{
}
}
The custom constructor force you to complete the names in the child classes.
You can combine this answer with the one of John Gardner using classes instead of strings for the names (firstName, middleName, lastName) to simulate the named parameters.
You need to be aware of the limitations of this method, in C# you cannot inherit from more than one class, so you need to keep this restriction in mind if you decide to implement this solution.
I am struggling with understanding C# interfaces. I understand that they allow for multiple inheritance. I'm trying to code for a scenario: Employees can view their own data, but not others. Managers can view and edit Employee information, if the Employee reports to that Manager.
So I start with an abstract class, since everyone is a User and takes vacation the same.
public abstract class User {
string _id;
string _firstName;
string _lastName;
double _salaryAmount;
double _bonusAmount;
int _vacationBalance;
public void TakeVacation(int hours) {
_vacationBalance -= hours;
}
//do I implement this here?
/*
public int SalaryAmount{ }
*/
}
Should I use an interface to define IEditable and IViewable?
Could I do something like this?
public interface IEditable {
double SalaryAmount { get; set; }
}
public class Employee : User, IEditable {
public double SalaryAmount {
get {
return base._salaryAmount;
}
set {
base._salaryAmount = value;
}
}
}
//Will I need a ReadOnlyEmployee or something?
I'm just not sure how to block a Manager from editing a User, or how to write an interface for this type of situation.
I'm not sure I'd say that Interfaces allow multiple inheritance (not in the true C++ form). An interface provides an abstraction which allows multiple implementations to behave consistently via a predefined set of methods/properties.
Often this is used as a general abstraction, for example say you want to get some data. You can create an Interface that surfaces a method that returns a Stream. Then based on the implementation of the object you can return the stream from a file or websocket or pipe, etc. without the consumer of the interface knowing/caring where it's coming from.
In your example you don't have to do anything in the User class, because it doesn't implement the IEditable interface, only the Employee class does.
Here's what I think you are looking for. This uses Explicit interface syntax (IEditable.Name) because the base method name and the interface name are the same but need to act differently.
public abstract class User {
public String Id { get; protected set; }
public String FirstName { get; protected set; }
public String LastName { get; protected set; }
// ...
public Double SalaryAmount { get; protected set; }
// ...
public Int32 VacationBalance { get; protected set; }
public void TakeVacation(int hours) {
VacationBalance -= hours;
}
}
public interface IEditable {
Double SalaryAmount { get; set; }
}
public class Employee: User, IEditable {
#region IEditable Members
double IEditable.SalaryAmount {
get { return base.SalaryAmount; }
set { base.SalaryAmount = value; }
}
#endregion
}
class Program {
static void Main(string[] args) {
var emp = new Employee();
emp.SalaryAmount = 3; // ERROR!
((IEditable) emp).SalaryAmount = 3; // GOOD!
}
}
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Closed 11 years ago.
Consider this:
public class interface Person : IPerson
{
int ID { get; protected set; }
string FirstName { get; set; }
string LastName { get; set; }
string FullName { get { return FirstName + " " + LastName; } }
}
And this:
public class StubPerson : IPerson
{
int ID { get { return 0; protected set { } }
string FirstName { get { return "Test" } set { } }
string LastName { get { return "User" } set { } }
string FullName { get { return FirstName + " " + LastName; } }
}
Usage:
IPerson iperson = new Person();
Or:
IPerson ipersonStub = new StubPerson();
Or:
IPerson ipersonMock = mocks.CreateMock<IPerson>();
So in effect we are declaring the IPerson interface and the Person class at the same time:
public class interface Person : IPerson
Do you think it would be useful to have this kind of support in .NET/C#?
Edit:
Due to mass confusion I think I need to clarify the proposed purpose:
Without this feature you would have to write:
interface IPerson
{
int ID { get; }
string FirstName { get; set; }
string LastName { get; set; }
string FullName { get; }
}
as well as this:
public class Person : IPerson
{
int ID { get; protected set; }
string FirstName { get; set; }
string LastName { get; set; }
string FullName { get { return FirstName + " " + LastName; } }
}
I'm not proposing any semantic change at all.
Let me see if I am understand what you're asking:
Why can't we declare an interface:
interface IPerson
{
string Name {get;set;}
int ID {get;set;}
}
And classes which implement that interface will inherit its properties without having to re-declare them:
class Person : IPerson { }
//person now has properties Name and ID
The reason you can't do this is even though the text of your interface code and your class code are very similar, they mean very different things. The interface simply says "implementor will have a string Name with getter and setter". It is the class which says "return private field when getter for name is invoked." Even if you use the auto-property shortcut to let the compiler implement that logic, it is still logic, which belongs in the class. Just because:
string Name {get;set;}
looks the same in an interface and in a class, it does not mean even remotely the same thing.
It would be very dangerous for the compiler to implement arbitrary logic to fulfill your contracts for you, instead of complaining at compile time that you haven't implemented them. It could introduce bugs very difficult to track down. Having compilers fall back to default behavior when no behavior is defined is a very, very bad idea.
I considered the same sort of thing a while ago, particularly for use in the case where you only have one production implementation of an interface, but you want to mock it out for testing. At the moment it ends up being a bit like the .c/.h files of yore.
I suspect in the end that the benefits of it are outweighed by the extra complexity both in the language and then reading the code afterwards. I'd still be interested in seeing it explored more thoroughly though. Even then, there are other things way higher on my priority list - better support for immutability being at the top :)
I believe Eiffel does something like this on .NET, in order to support multiple inheritance. A class declaration automatically produces a corresponding interface. When the class type is referred to, the compiler mostly emits a reference to the interface type instead. The main exception is in constructor expressions of course.
Well I think the other answers will help you understand the use of the interface to abstract logic in different concrete classes, I also think you can accomplish something similar to what you want using the refactoring tools built into VS.
Define your class...
public class Person
{
public int ID { get; protected set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string FullName { get { return FirstName + " " + LastName; } }
}
Then right click, select Refactor -> Extract Interface.
This will create a separate file containing the interface for the definition of the class, you could then mold the interface and implementing classes accordingly.
Extracted Interface:
interface IPerson
{
string FirstName { get; set; }
string FullName { get; }
int ID { get; }
string LastName { get; set; }
}
I guess I am missing the point - what are you accomplishing by mixing a class and an interface together? What problem are you solving with this approach?
This:
IPerson iperson = new Person();
is already legal in C#.
Edit: For clarification - the code above is legal given the following:
interface IPerson { }
class Person : IPerson { }
I'd at least like Visual Studio to implement my properties from an interface as auto properties if I request it to do so.
Unfortunately this option doesn't and I have to deal with Not Implemented Exception stubs
No, because you would be forced to expose all public members of an interface. Try ReSharper, and never worry about this again.
Resharper can provide this functionality, e.g.,
You can write your Person class first.
You can extract your interface by pulling members up to the IPerson interface.
Consequently you can have Visual Studio auto-generate implementation stubs for you.
UPDATE
Anyway, let's expound on interfaces first, citing the code you provided in your question:
public class interface Person : IPerson
{
int ID { get; protected set; }
string FirstName { get; set; }
string LastName { get; set; }
string FullName { get { return FirstName + " " + LastName; } }
}
You have to understand that an Interface is not an Abstract Class. An interface is merely a contract, a blueprint of sorts, which means that it will tell an object what to expect in another object without really caring about how it is implemented.
An Abstract Class, on the other hand, can contain snippets of functionality that can be inherited and overridden.
In the case above, your "interface" is invalid because:
you couldn't declare scope constraints on interfaces (public, private, protected, internal) as that is an implementation detail
you couldn't declare a default implementation (e.g., your FullName property) because again, that is an implementation detail
It appears to me what you really really want is an abstract class, e.g.,
public abstract class BasePerson
{
public abstract int ID { get; protected set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public virtual string FullName { get { return FirstName + " " + LastName; } }
}
I'm just guessing, but maybe that's what you really need.
UPDATE 2
Okay, I think I'm getting at what you want to happen, so what you want is to be able to write this:
public interface IPerson
{
int ID { get; set; }
string FirstName { get; set; }
string LastName { get; set; }
string FullName { get; }
}
And then for your implementation only need to write this:
public class Person : IPerson
{
public int ID { get; protected set; }
public string FullName { get { return FirstName + " " + LastName; } }
}
Without needing to specify the FirstName and LastName properties.
First problem that we need to tackle is the fact that interfaces don't allow access delimiters in its implementation: what would happen is that the properties would inherit the default access delimiter, which is private.
Second is the fact that while in our eyes string FirstName { get; set; } in an interface and public string FirstName { get; set; } in a class are the same, they are actually not:
in an interface, the property definition will indicate that the method signatures for the getter and/or setter methods will be available for all classes implementing that interface.
in a class, the property definition will instruct the CLR to create an anonymous object which will hold the value of the said Property.
Subtle difference for the programmer, worlds apart for the compiler.
Lastly, when you do specify that you are implementing an interface, Visual Studio does perform synctactic magic that automatically makes those property stubs for you.
I think a better abstraction for this is a trait, or, as I've described here, a role. This is like an interface with code. Your example could be coded like this:
public role RPerson {
int ID { get; protected set; }
string FirstName { get; set; }
string LastName { get; set; }
string FullName { get { return FirstName + " " + LastName; } }
}
public class Person : RPerson { }
public class StubPerson : RPerson {
int ID { get { return 0; protected set { } }
string FirstName { get { return "Test" } set { } }
string LastName { get { return "User" } set { } }
string FullName { get { return FirstName + " " + LastName; } }
}
// ...
RPerson rperson = new Person();
RPerson rpersonStub = new StubPerson();
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.