Should interfaces define properties?

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Interfaces, as defined by MSDN "contain only the signatures of methods, delegates or events." However, since properties are no more than syntactic sugar for a get and set method, they are also allowed in interfaces. My question is - is there any situation where defining properties in an interface is appropriate or should we stick to the scenarios described by MSDN?

I think properties are perfectly acceptable in interfaces.

As you said, they really are a get, set, or get and set method. Many interfaces in the Framework define properties, such as IAsyncResult and IWebProxy.

Why does C# allow properties in interfaces?, A property is just a get method and a set method. Since interfaces are just a list of methods you have to implement, it's natural that interfaces can have them. Many interfaces have more than one method signature and even properties (which are just convenient forms of getter and setter methods).? Interfaces can also define properties, events, and indexers. Here’s the definition for IDisposable, in case you were wondering:

Should interfaces define properties?, I think properties are perfectly acceptable in interfaces. As you said, they really are a get, set, or get and set method. Many interfaces in the  Yes, An interface should define properties when it really in need. Please suppose that. There is a IUser interface that has defined a property "Name" then you can use it without worry about if the object didn't implement the property.

Yes, An interface should define properties when it really in need. Please suppose that. There is a IUser interface that has defined a property "Name" then you can use it without worry about if the object didn't implement the property.

public void main()
{
    IUser u = User.GetUser("id");
    string name = u.Name;
}

C# Interface, In C#, an interface can be defined using the interface keyword. An interface can contain declarations of methods, properties, indexers, and events. However  An interface has the following properties: An interface is typically like an abstract base class with only abstract members. Any class or struct that implements the interface must implement all its members. Optionally, an interface may define default implementations for some or all of its members. An interface can't be instantiated directly.

It is completely legitimate to define properties in interfaces. A fairly good and detailed explanation as to why you can use properties but cannot use fields is given in the answer here: Why Can's C Sharp Interfaces Contain Fields

You'll want to bear in mind that any properties defined in an interface will always be public. That is to say, if you define a getter and a setter within the interface, then both the getter and the setter must be public. You can, if you wish, define only the getter in the interface and then define a private setter in the implementing class. It's definitely an area to work carefully in.

Similarly, Java does not allow for instance variables in its interfaces. However, it does allow variables to be declared, and these will be treated as static & readonly. The convention in Java is to write getMyVariable() and setMyVariable() methods if you require them in the implementing class. C# essentially just allows for a cleaner syntax.

Interface Attributes != Class Attributes, NET MVC team, we occasionally get questions about attributes and how they apply from interfaces to classes, like: I have an interface IFoo defined like this: public interface Interface implementation can take two forms: implicit and explicit. Interfaces have the following properties − An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface. Each method in an interface is also implicitly abstract, so the abstract keyword is not needed. Methods in an interface are implicitly public. Example

Interfaces, They can have properties but these need to be abstract or to provide accessor implementations. An interface is defined using the keyword interface. interface  Interfaces are capable of describing the wide range of shapes that JavaScript objects can take. In addition to describing an object with properties, interfaces are also capable of describing function types. To describe a function type with an interface, we give the interface a call signature.

C# 8 Interfaces: Properties and Default Implementation, We can create the object with no problems. But if we try to set the BadMember property, bad things happen, specifically we get a Stack Overflow:. 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.

.NET Interface-based Programming, An abstract class can have non-public (protected or private) methods and properties, even if they are all abstract. In an interface, by definition, all members are  Learn: Interface in java, this article will explain about interfaces in java and their properties. Submitted by Amit Shukla, on June 10, 2017 . In Java programming interfaces are just a like class which contains methods with empty implementation and constant variables.

Comments
  • Thank you, Reed. In this case, should these properties be of elementary (non user defined) types? The reason I ask is because if you for example define it as a domain type, you may end up with a circular reference.
  • @Otávio Décio: Keep in mind the purpose of an interface. An interface defines a contract. If a property is required for the contract, then it's fine. You shouldn't end up with circular references, since the properties shouldn't be types that are directly implementing the interface, but rather either basic, framework types, or types that, themselves, form part of the "contract". IWebProxy (linked above) is a good example - it has a Credentials property, which is of ICredentials. That's part of the contract, but essential a "user defined" type still (for that lib).
  • just to make sure - properties defined in interfaces should either be non user defined concrete types or interface types (user defined or not).
  • @Otávio Décio: I don't have a problem with user defined concrete types, either - but I think care needs to be maintained in this case...