I have an interface that declares some properties (shortened to Id only in the example) with only a get method. Classes implementing this interface do not have to provide a public setter for this property:
public interface IMetadataColumns
{
Int32 Id { get; }
}
Now I need another interface with the same properties, but now they must all be writable as well. So I added a new interface inheriting from the old one where each property also has a setter:
public interface IMetadataColumnsWritable : IMetadataColumns
{
Int32 Id { get; set; }
}
Visual Studio now underlines this and warns me to use the new keyword if hiding the old properties was intended.
What shall I do now? I have classes implementing the IMetadataColumns interface which need some of the properties to be read-only, but I also have other classes where exactly those same properties must be writable as well.
I guess hiding a property sounds somehow not like the way to go...
When interfaces are involved, the new keyword doesn't mean you are hiding the property in the same sense as with classes.
Your class implementing IMetadataColumnsWritable will only have one Id, regardless of whether you cast it to the base IMetadataColumns interface or not (unless you add an explicit implementation for the readonly property - but this is not needed and would only allow for errors in this case).
In other words, you might have:
// this interface is public, and it allows everyone to read the Id
public interface IMetadataColumns
{
int Id { get; }
}
// this one is writeable, but it's internal
internal interface IMetadataColumnsWritable : IMetadataColumns
{
// we need to use 'new' here, but this doesn't mean
// you will end up with two getters
new int Id { get; set; }
}
// internal implementation is writeable, but you can pass it
// to other assemblies through the readonly IMetadataColumns
// interface
internal class MetadataColumns : IMetadataColumnsWritable
{
public int Id { get; set; }
}
So, unless you explicitly implement the readonly property as a separate one, you will have only a single property, whichever interface you use to access it:
var columns = new MetadataColumns { Id = 5 };
var x = (columns as IMetadataColumns).Id;
var y = (columns as IMetadataColumnsWritable).Id;
Debug.Assert(x == y);
Hiding a member inside the class using the new keyword, on the other hand, is what I would not recommend in most cases, because it basically creates a completely new member, which just happens to be called the same as a different member in the base class.
You should indeed use the new keyword here. In your class implementation, you have to implement both properties (since you are not really hiding the property). If they have different return types, at least one of them has to be implemented explicitly.
public class X : IMetadataColumnsWritable
{
public int Id
{
get;
set;
}
// only necessary if you have to differentiate on the implementation.
int IMetadataColumns.Id
{
get
{
return this.Id; // this will call the public `Id` property
}
}
}
Does using explicit implementation of the getter only version forwarding to a public version work?
int IMetadataColumns.Id {
get {
return this.Id;
}
}
public Id {
get { … }
set { … }
}
You might find the public version does need to be new, but the explicit version will be picked up by references of type IMetadataColumns.
Related
is there any diffrence between this 3 auto properties ?
interface MyInterface {
public int p1 { get; set; }
public int p2 { get; }
public int p3 { set; }
}
also why we can write this code in an interface but not in a class ?
public int p { get; }
For the same reason you can write this in an interface:
interface IFace {
void Test();
}
Also, your interface is invalid, as public isn't valid in an interface. The point being, different things are legal in interfaces and classes.
When you do public int P1 { get; set; } in a class, that turns into a auto property. However, you can't do public int P1 { get; }, because what would you want that to mean? Should it always return 0? There is no way to set it. So if you want a read only property you have to define the getter yourself like this:
int _p1;
public int P1 {
get { return _p1; }
}
Also. Another way to achieve more or less the same is this:
public int P1 { get; private set; }
There are differences between those properties. Firstly, you should remove the public modifier from your declaration. Secondly, by putting get or set within the block you define what properties in derived classes should look like. For example, public int p1 { get; set; } requires getter and setter in a derived class, public int p2 { get; } only getter, and public int p3 { set; } requires only setter to be implemented.
You can't use access modifiers inside interfaces because interfaces are guidelines for other developers that force them to go in a certain direction when developing the implementing classes.
Look at this post for more information about that.
Keep in mind interface does NOT contain any implementation data. When you add property in an interface, it merely says that a class implementing this interface needs to have said property with get, set or both methods, depending on what you wrote. So any class implementing your interface has to implement (or have auto-generated) p1 property with get and set method, p2 with get method, and p3 with set method. Interface doesn't care whether these will be auto-generated or your own custom implementations, they just have to be in an implementing class.
Therefore, you can write
int p { get; }
in an interface as all it does is telling that any class implementing this interface has to have property p with getter, again, not caring about its actual implementation - you could write a getter that does some computations, returns some constant, etc. OTOH in a class writing the same would mean that you want a property with auto-generated backing field, except since it would have no setter, you couldn't actually change its value, so it would always have its default value 0.
And as noted, you cannot write access modifiers in an interface, as all interface members are implicitly public.
For example, suppose I want an ICar interface and that all implementations will contain the field Year. Does this mean that every implementation has to separately declare Year? Wouldn't it be nicer to simply define this in the interface?
Though many of the other answers are correct at the semantic level, I find it interesting to also approach these sorts of questions from the implementation details level.
An interface can be thought of as a collection of slots, which contain methods. When a class implements an interface, the class is required to tell the runtime how to fill in all the required slots. When you say
interface IFoo { void M(); }
class Foo : IFoo { public void M() { ... } }
the class says "when you create an instance of me, stuff a reference to Foo.M in the slot for IFoo.M.
Then when you do a call:
IFoo ifoo = new Foo();
ifoo.M();
the compiler generates code that says "ask the object what method is in the slot for IFoo.M, and call that method.
If an interface is a collection of slots that contain methods, then some of those slots can also contain the get and set methods of a property, the get and set methods of an indexer, and the add and remove methods of an event. But a field is not a method. There's no "slot" associated with a field that you can then "fill in" with a reference to the field location. And therefore, interfaces can define methods, properties, indexers and events, but not fields.
Interfaces in C# are intended to define the contract that a class will adhere to - not a particular implementation.
In that spirit, C# interfaces do allow properties to be defined - which the caller must supply an implementation for:
interface ICar
{
int Year { get; set; }
}
Implementing classes can use auto-properties to simplify implementation, if there's no special logic associated with the property:
class Automobile : ICar
{
public int Year { get; set; } // automatically implemented
}
Declare it as a property:
interface ICar {
int Year { get; set; }
}
Eric Lippert nailed it, I'll use a different way to say what he said. All of the members of an interface are virtual and they all need to be overridden by a class that inherits the interface. You don't explicitly write the virtual keyword in the interface declaration, nor use the override keyword in the class, they are implied.
The virtual keyword is implemented in .NET with methods and a so-called v-table, an array of method pointers. The override keyword fills the v-table slot with a different method pointer, overwriting the one produced by the base class. Properties, events and indexers are implemented as methods under the hood. But fields are not. Interfaces can therefore not contain fields.
Why not just have a Year property, which is perfectly fine?
Interfaces don't contain fields because fields represent a specific implementation of data representation, and exposing them would break encapsulation. Thus having an interface with a field would effectively be coding to an implementation instead of an interface, which is a curious paradox for an interface to have!
For instance, part of your Year specification might require that it be invalid for ICar implementers to allow assignment to a Year which is later than the current year + 1 or before 1900. There's no way to say that if you had exposed Year fields -- far better to use properties instead to do the work here.
The short answer is yes, every implementing type will have to create its own backing variable. This is because an interface is analogous to a contract. All it can do is specify particular publicly accessible pieces of code that an implementing type must make available; it cannot contain any code itself.
Consider this scenario using what you suggest:
public interface InterfaceOne
{
int myBackingVariable;
int MyProperty { get { return myBackingVariable; } }
}
public interface InterfaceTwo
{
int myBackingVariable;
int MyProperty { get { return myBackingVariable; } }
}
public class MyClass : InterfaceOne, InterfaceTwo { }
We have a couple of problems here:
Because all members of an interface are--by definition--public, our backing variable is now exposed to anyone using the interface
Which myBackingVariable will MyClass use?
The most common approach taken is to declare the interface and a barebones abstract class that implements it. This allows you the flexibility of either inheriting from the abstract class and getting the implementation for free, or explicitly implementing the interface and being allowed to inherit from another class. It works something like this:
public interface IMyInterface
{
int MyProperty { get; set; }
}
public abstract class MyInterfaceBase : IMyInterface
{
int myProperty;
public int MyProperty
{
get { return myProperty; }
set { myProperty = value; }
}
}
Others have given the 'Why', so I'll just add that your interface can define a Control; if you wrap it in a property:
public interface IView {
Control Year { get; }
}
public Form : IView {
public Control Year { get { return uxYear; } } //numeric text box or whatever
}
A lot has been said already, but to make it simple, here's my take.
Interfaces are intended to have method contracts to be implemented by the consumers or classes and not to have fields to store values.
You may argue that then why properties are allowed? So the simple answer is - properties are internally defined as methods only.
Interfaces do not contain any implementation.
Define an interface with a property.
Further you can implement that interface in any class and use this class going forward.
If required you can have this property defined as virtual in the class so that you can modify its behaviour.
Beginning with C# 8.0, an interface may define a default implementation for members, including properties. Defining a default implementation for a property in an interface is rare because interfaces may not define instance data fields.
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/interface-properties
interface IEmployee
{
string Name
{
get;
set;
}
int Counter
{
get;
}
}
public class Employee : IEmployee
{
public static int numberOfEmployees;
private string _name;
public string Name // read-write instance property
{
get => _name;
set => _name = value;
}
private int _counter;
public int Counter // read-only instance property
{
get => _counter;
}
// constructor
public Employee() => _counter = ++numberOfEmployees;
}
For this you can have a Car base class that implement the year field, and all other implementations can inheritance from it.
An interface defines public instance properties and methods. Fields are typically private, or at the most protected, internal or protected internal (the term "field" is typically not used for anything public).
As stated by other replies you can define a base class and define a protected property which will be accessible by all inheritors.
One oddity is that an interface can in fact be defined as internal but it limits the usefulness of the interface, and it is typically used to define internal functionality that is not used by other external code.
I have some classes with common properties, however, I cannot make them derive from a base type (LINQ-to-SQL limitations).
I would like to treat them as if they had a base type, but not by using Reflection (performance is critical).
For example:
public class User
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle
{
public int Id { get; set; }
public string Label { get; set; }
}
In this case I would be happy if I had the Id property available, regardless of the type I'm holding.
Is there any way in C# to to something similar to this:
public static int GetId<T>(T entity) where T // has an int property 'Id'
{
return entity.Id;
}
I guess I could have used dynamic, however, I'm looking for a way to restrict the code in compile time from using this method for an object that has no Id property.
You can use interfaces:
public interface IHasId
{
int Id { get; }
}
public class User : IHasId { ... }
public class Vehicle : IHasId { ... }
public static int GetId<T>(T entity) where T : IHasId
{
return entity.Id;
}
However, if you are not able to modify the classes to add the interface, you won't be able to do this. No compile-time checks will verify that a property exists on T. You'd have to use reflection - which is slow and obviously not ideal.
There is no way to guarantee a type has a given member without constraining to a common base type or interface. One way to work around this limitation is to use a lambda to access the value
public static int Use<T>(T value, Func<T, int> getIdFunc) {
int id = getIdFunc(value);
...
}
Use(new User(), u => u.Id);
Use(new Vehicle(), v => v.Id);
You can create an interface with the common properties and make your classes implement it:
public interface IEntity
{
int Id { get; set; }
}
public class User : IEntity
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle : IEntity
{
public int Id { get; set; }
public string Label { get; set; }
}
public static int GetId<T>(T entity) where T : IEntity
{
return entity.Id;
}
You could simplify GetId like this:
public static int GetId(IEntity entity)
{
return entity.Id;
}
The other answers mentioning the interface approach are certainly good, but I want to tailor the response to your situation involving Linq-to-SQL.
But first, to address the question title as asked
Can C# constraints be used without a base type?
Generally, the answer is no. Specifically, you can use struct, class, or new() as constraints, and those are not technically base types, and they do give some guidance on how the type can be used. That doesn't quite rise to the level of what you wish to do, which is to limit a method to types that have a certain property. For that, you will need to constrain to a specific interface or base class.
For your specific use case, you mention Linq-to-SQL. If you are working from models that are generated for you, then you should have options to modify those classes without modifying the generated model class files directly.
You probably have something like
// code generated by tool
// Customer.cs
public partial class Customer // : EntityBaseClasses, interfaces, etc
{
public int ID
{
get { /* implementation */ }
set { /* implementation */ }
}
}
And other similar files for things such as Accounts or Orders or things of that nature. If you are writing code that wishes to take advantage of the commonly available ID property, you can take utilize the partial in the partial class to define a second class file to introduce a common interface type to these models.
public interface IIdentifiableEntity
{
int ID { get; }
}
And the beauty here is that using it is easy, because the implementation already exists in your generated models. You just have to declare it, and you can declare it in another file.
public partial class Customer : IIdentifiableEntity { }
public partial class Account : IIdentifiableEntity { }
// etc.
This approach has proven valuable for me when using a repository pattern, and wishing to define a general GetById method without having to repeat the same boilerplate in repository after repository. I can constrain the method/class to the interface, and get GetById for "free."
Either you need to make both classes implement an interface with the properties you need, and use that in the generic constraint, or you write separate methods for each type. That's the only way you'll get compile-time safety.
I read other threads like this but they didn't work for me.
I got two classes:
public class ClassA
{
public string _shouldBeInteger;
public string _shouldBeBool;
public string _shouldBeDateTime;
}
public class ClassB : ClassA
{
public int? shouldBeInteger
{
get { return (_shouldBeInteger != null) ? Convert.ToInt32(Convert.ToDouble(_shouldBeInteger)) : new int?(); }
set { _shouldBeInteger = Convert.ToString(value); }
}
//... same thing with datetime etc.
}
If I now create a new object of ClassB I get
_shouldBeInteger, _shouldBeBool, _shouldBeDateTime;
shouldBeInteger,shouldBeBool,shouldBeDateTime
But I want to hide the _variables to the User.
Setting them private in ClassB will override them, but I need to access them in order to parse there string values.
Update
There is a ClassC filling ClassAs' values, which mainly is the reason why they have to be writeable. There is no way for me to change the way that works, but I'm fully in Control of ClassA and ClassB
ClassC //not changeAble for me
{
//infomagic filling values of ClassA
}
Setting ClassA variables to private won't work, because programmer of ClassA produced it in a strange way.
Solution
Because ClassA needs to be writeable, but not readable to other classes than inheritated, I finally got this:
ClassA
{
public string _shouldBeInteger { protected get; set; }
//and so on
}
which causes ClassB to work with theese properties, without giving them outside.
Intellisense still shows them, but you might consider using:
[EditorBrowsable(EditorBrowsableState.Never)]
to solve that.
Thanks to all.
I think you can solve your problem using:
public class ClassA
{
protected string _shouldBeInteger;
protected string _shouldBeBool;
protected string _shouldBeDateTime;
}
so those variables are accessible to derived classes but not to user.
EDITED after user update:
I don't know if this could be a vali solution for you, but try:
public class ClassB : ClassA
{
public new int? _shouldBeInteger
{
get { return (base._shouldBeInteger != null) ?
Convert.ToInt32(Convert.ToDouble(base._shouldBeInteger)) :
new int?(); }
set { base._shouldBeInteger = Convert.ToString(value); }
}
}
Inheritance can't hide the members as you would think. The new modifier exists to "hide" a base member, but that doesn't play nice when talking to base types.
http://msdn.microsoft.com/en-us/library/435f1dw2.aspx
You can either change the access level of the fields (the preferred way) or you can wrap the class instead of inheriting from it and provide simple pass-through methods to delegate to the wrapped class. This is called the Adapter Pattern:
public class ClassB
{
private ClassA _wrappedClass;
}
Just as an aside, your public fields are following the naming convention commonly used for private fields.
The required access level for derived classes is protected. If the members are used publicly but in the same assembly you can use protected internal. If the members are used publicly by other assemblies... I'd suggest refactoring.
The problem is that you declared the fields public in the base class. In order not to violate the polymorphic nature of inheritance, anything public in the base class must be public in all derived classes as well. If you could change that, you could never be sure that a ClassB could be passed to something expecting a ClassA.
Therefore, as other people have suggested, you probably want the base class fields to be declared protected, which is like private except derived classes can see them.
However if you do need to access them via an actual instance of ClassA, you could declare them private and give them virtual public properties which the derived class can then override. This at least allows the derived class to change their behaviour, but it still can't actually hide them.
If that also doesn't fit, then it's probably worth considering using composition instead of inheritance because the substitution principle is actually getting in your way, and that's an inheritance fundamental.
If you don't have control over ClassA, you'll need to create a wrapper/adapter class like so:
public class ClassB
{
private readonly _classA = new ClassA();
public int? shouldBeInteger
{
get
{
return (this._classA._shouldBeInteger != null)
? Convert.ToInt32(Convert.ToDouble(this._classA._shouldBeInteger))
: new int?();
}
set
{
this._classA._shouldBeInteger = Convert.ToString(value);
}
}
}
public class ClassB
{
private int shouldBeInteger;
public int ShouldBeInteger
{
get { return shouldBeInteger; }
set { shouldBeInteger = value; }
}
}
OR
public class ClassB
{
public int ShouldBeInteger{ get; set; }
}
In both of this case ShouldBeInteger will be accesible outside the class.
In first case there were a private field, which cannot be accesible outside the class,
values to private filed can be set through the public field.
In second case the compiler automatically create a private backing field and do the same
process as above. This is auto implemented property.
Hope this may help you.
I have a class like the below, I want to override the set value of "School,Country..etc.." property when some one sets a value , i don't want to change the student class but i need to do it in the base class and use it as a generic method
public class Student : BaseClass
{
public String School { get; set; }
public String Country{ get; set; }
}
ie:
When some one sets
Student.School="Harvard",
I need to store it as
Student.School="Harvard my custom value";
Note:
Basically calling OnPropertyChanged in base class rather than the main class.
If you want to do it with aspects, then try Postsharp
Basically you cannot override a non-virtual property. You can hide it by other property with the same name in the derived class, but this won't give you the desired effect if some other code accesses your object by the reference to the base class.
public class Student : BaseClass
{
private string _school
public string School
{
get { return _school; }
set
{
if(value == "Harvard")
value = "Harvard custom";
_school = value;
}
}
public String Country{ get; set; }
}
is that what you mean?
If the School property is in the BaseClass then you can either use the new keyword, or if you control the BaseClass, then you can add the virtual keyword to the School property there, and override it in the Student class.
This is just not doable by solely modifying BaseClass. Think about it this way: If it were possible to "annotate" automatic properties that easily, then we wouldn't need all those <rant>useless tons of</rant> manual property implementations for data model classes that implement INotifyPropertyChanged (same for DependencyProperties).
You need to provide hooks in your subclasses that your base class can use. Implementing PropertyChanged, which you already mentioned, is one possible solution, another one would be a simple method call:
public class Student : BaseClass
{
private string _school;
public String School
{
get { return _school; }
set {
_school = value;
DoMoreChanges(ref _school); // DoMoreChanges is defined in BaseClass
}
}
public String Country{ get; set; }
}
If you have lots of subclasses that need this, you can either use Visual Studio Code Snippets to create the code or T4 templates.
Since your base class does not have those properties you will not be able to modify them from within the base class using standard OOD patterns or principles.
Now if you move the properties to your base class either as normal properties or virtual properties you can modify what you do in the set block of the properties to do extra work.
However if you cannot move these to the base class, and you cannot modify the Student class, as you seem to imply in you question, then you could encapsulate the student class within a new class like StudentProxy or something and then have it expose similar properties that will then call into the real student class how you want.
For example:
public class StudentProxy
{
private Student _student;
public StudentProxy(Student student)
{
this._student = student;
}
public String School
{
get { return _student.School; }
set
{
_student.School = value + " my custom value";
}
}
public String Country
{
get { return _student.Country; }
set
{
_student.Country = value + " my custom value";
}
}
}