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Auto implemented Properties in C# Interfaces

I am referring to the documentation of Microsoft - https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/auto-implemented-properties . They state that auto implemented properties are basically properties without body, when there is no additional logic needed inside a get or set. so int Myproperty1 {get;set;} is an auto implemented property. This Documentation also states below points
Statement1:
"You can't declare auto-implemented properties in interfaces. Auto-implemented properties declare a private instance backing field, and interfaces may not declare instance fields."
But i can declare auto implemented property like below in an interface
public MyInterface { int Myproperty1 {get;set;} . Is this not conflicting above statement that we cant declare auto implemented properties in Interface.
Microsoft documentation then says:
statement2:
"Declaring a property in an interface without defining a body declares a property with accessors that must be implemented by each type that implements that interface."
I fail to understand what is declaring a property without body , is it not auto implemented property, if it is then is the first statement not incorrect?
IMPORTANT EDIT TO THE QUESTION: I apologize, I had posted the question with this link by mistake :
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/interface-properties.
While I intended to refer to the following link:
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/auto-implemented-properties. I have updated my question with the correct link now.

MSDN never said anything like "all properties without bodies are auto-implemented properties". They might say "auto-implemented properties don't have bodies", but the latter doesn't imply the former. MSDN is not contradicting itself.
Properties without bodies in an interface are abstract, whereas auto-implemented properties are those that are non-abstract, without bodies, and in a class/struct.
Therefore, MyProperty1 in public MyInterface { int MyProperty1 {get;set;} } is not an auto-implemented property, but an abstract one.
I fail to understand what is declaring a property without body
It's just like declaring two methods without bodies in an interface:
public MyInterfaceWithTwoMethods {
int GetMyProperty1();
void SetMyProperty1(int value);
}
Except it's more idiomatic to use properties in C#.
You could implement MyInterface with an auto-implemented property:
public class MyImpl : MyInterface {
public int MyProperty1 { get; set; }
}
Even though you seem to be just repeating what is written in MyInterface, this is analogous to implementing MyInterfaceWithTwoMethods like this:
public class MyImpl : MyInterfaceWithTwoMethods {
private int myProperty1;
int GetMyProperty1() => myProperty1;
void SetMyProperty1(int value) { myProperty1 = value; }
}
You could also implement MyInterface not with an auto-implemented property:
public class MyImpl : MyInterface {
public int MyProperty1 {
get => 1;
set { Console.WriteLine("foo"); }
}
}

How to create auto implemented properties when implementing an interface into a class?

When I implement an interface for the first time into a class I want either resharper 6 or visual studio 2010 to implement my properties as auto implemented properties and not put in the default value of throw new NonImplementedException();. How can I do this? For example:
public interface IEmployee
{
// want this to stay just like this when implemented into class
ID { get; set; }
}
public class Employee : IEmployee
{
// I do not want the throw new NonImplemented exception
// I want it to just appear as an auto implemented property
// as I defined it in the interface
public int ID
{
get
{
throw new NotImplementedException();
}
set
{
throw new NotImplementedException();
}
}
}
Because this happens all the time, I am finding myself having to constantly refactor and manually remove those throw new UnImplimented() exceptions and manually make the properties be auto implemented... a pain! After all, I defined it as an auto implemented property in my interface.
Any help or advice much appreciated! Thanks.

Note: your R# keyboard shortcuts may differ, I am using the Resharper 2.x keyboard schema.
If you declare the interface on the class and then use Alt+Enter and select “Implement members”:
Then you will get the default implementation, which happens to be throwing NotImplementedException, unless you change that.
But if you ignore the suggested course of action and instead use Alt+Insert to open the Generate menu, you can select “Missing members”:
This will open Generate window, where you can select to implement the property (or properties) as auto-implemented:
That will do exactly what you want:
class Employee : IEmployee
{
public int Id { get; set; }
}

After all, I defined it as an auto implemented property in my interface.
No, you didn't. You declared it as a property without an implementation. That's all you can do in an interface: you're just saying that classes implementing the interface must provide the concrete implementations of such properties.
Personally I would be wary of having too many writable properties within interfaces - if this is something you find "happens all the time" I wonder whether you're using interfaces where possibly abstract classes would be more appropriate.
In terms of your exact question: I don't know whether it's possible to change the default implementation either VS or R# provides for interfaces - but I would resist making those changes anyway, to be honest.
EDIT: Under R# options, "Code Generation", you can choose between throwing an exception, returning a default value, or giving uncompilable code. It's possible that this will do what you want. Give it a go, but I'd still strongly urge you to think carefully before going down this path.

An interface is not meant to specify how the methods will be implemented so there is no way around it using the interface. One solution would be to make an abstract base class with the auto-implemented properties and inherit that class instead of directly implementing the interface.

Here's a quick workaround that I found in VS2015. Mark your class as abstract then implement the interface abstractly. This adds the auto properties for you then you just replace the "abstract " with "" in your file. Then you can remove the abstract keyword from your class.

In the case of VS2015 I'm using a find and replace macro as a workaround:
Find:
(\r|\n| |\t)+\{(\r|\n| |\t)+get(\r|\n| |\t)+\{(\r|\n| |\t)+throw(\r|\n| |\t)+new(\r|\n| |\t)+NotImplementedException\(\);(\r|\n| |\t)+\}(\r|\n| |\t)+set(\r|\n| |\t)+\{(\r|\n| |\t)+throw(\r|\n| |\t)+new(\r|\n| |\t)+NotImplementedException\(\);(\r|\n| |\t)+\}(\r|\n| |\t)+\}
replace:
{get;set;}

In addition to Jon's answer... if you really want to change this (out of box) behavior of Visual Studio and creating auto properties when implement interface, you can try one of following...
Use T4 for code generation - http://msdn.microsoft.com/en-us/library/bb126445.aspx
Write a custom plugin for Visual Studio using VS SDK achieve this

It's not related to this question completely, however, a different approach when you have multiple of such properties in an Interface, instead of interface you can have a class and refer to that class as a return type in your main class. You can create a class and refer to that class in your main class. Example
public class Asset
{
public int AssetTrackingID { get; set; }
public Category AssetCategoryInfo { get; set; }
public Manufacturer AssetManufacturerInfo { get; set; }
public ManufacturerModel AssetModelInfo { get; set; }
public Status AssetStatusInfo { get; set; }
public EmployeeDetails AssetEmployeeInfo { get; set; }
public string AssetNumber { get; set; }
public string SerialNumber { get; set; }
public DateTime? AcquiredDate { get; set; }
}

How to hide set method of an implemented property from an interface in C#?

Greetings everyone...
If I have the following interface:
interface IMyInterface
{
int property { get; set; }
}
And the following implementation:
class MyClass : IMyInterface
{
// anything
}
How can I hide the set method of property from the instances of MyClass... In other words, I don't want the set method of property to be public, is that possible?
It would be easy to do with abstract class:
abstract class IMyInterface
{
int property { get; protected set; }
}
Then I could only set the property within the class that implements the abstract class above...

Don't have the set in the interface to begin with. You can still implement it as private.
You can't "hide" it, it's part of the contract. If you don't want it to be part of the contract, don't define it.

If you use the following interface the set method will be unavailable when classes are manipulated via the interface:
interface IMyInterface
{
int property { get; }
}
You could then implement the class like this:
class MyClass : IMyInterface
{
int property { get; protected set; }
}

If some implementations will only implement some parts of an interface, it may be a good idea to subdivide the interface into the parts which each implementation will either implement completely or not at all, and then define interfaces which inherit all the common combinations of them. Adapting your example:
interface IMyReadableInterface
{
int property { get; }
}
interface IMyFullInterface : IMyReadableInterface
{
new int property { get; set; }
}
Classes which want to support read-write access should implement IMyFullInterface; those which want to only support read access should only implement IMyReadableInterface. This segregation will not require any extra work for implementations of either interface which are written in C# and implement property implicitly. Code which implements property in VB, or explicitly implements property in C#, will have to define two implementations of property--a read-only one and a read-write one, but such is life. Note that while one could define an IMyWritableInterface which just had a setter, and have IMyFullInterface inherit both IMyReadableInterface and IMyWritableInterface, IMyFullInterface would still have to define a read-write property of its own, and when using explicit implementation one would then have to define three properties (I really don't understand why C# can't use a read-only and write-only property together as thought they were a read-write property, but it can't).

Assuming you need the setter to be part of the interface but for some reason it does not make sense for it to be used on a particular implementer (in this case MyClass) you can always throw an exception in the setter (such as an InvalidOperationException). This will not protect you at compile time, only at run time. It is a bit strange though, as code that operates on the interface has no idea whether calling the setter is allowed.

There are certainly cases where you want the interface to have a set and then hide it in some concrete class.
I believe the code below shows what we want to accomplish. I.e. the implementation hides the setter, but any IMyInterface aware component will have access to it.
public static void Main()
{
var myClass = new MyClass();
myClass.Property = 123; // Error
((IMyInterface)myClass).Property = 123; // OK
}
It's basically the same pattern you often see for IDisposable.Dispose() where you have an Explicit Interface Implementation. Here's an example for completeness.
public interface IMyInterface
{
int Property { get; set; }
}
public class MyClass : IMyInterface, IDisposable
{
public int Property { get; private set; }
int IMyInterface.Property
{
get => Property;
set => Property = value;
}
void IDisposable.Dispose() {}
}
Too much typing :(
C# doesn't help us much here. Ideally, it would be possible to have an explicit interface implementation for the setter:
// In C# 10 maybe we can do this instead:
public class MyFutureClass : IMyInterface
{
public int Property { get; IMyInterface.set; }
}
See C# feature proposal here.

There is no protected or private in interface, everything is public. Either you don't define any set or use it as public.

Why can't C# interfaces contain fields?

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.

Accessibility of abstract properties

I have an abstract class with an abstract property that is set to have both Get and Set. I know I'll always want to be able to get this property from derived classes but there are some cases where it doesn't make sense to set this property in certain types of derived classes.
I can't just omit the Set accessor in a derived class (see code example below). I could override the set accessor in a derived classes to do nothing with the values passed by the user. But is there another way that actually make the property in a specific derived class read only? Ultimately I'm displaying these properties in a property grid and I don't want the user to be entering values into a field that is going to do nothing. Maybe I just attribute the property as read only in specific derived classes?
Also I'd really really rather not mess with any of the type descriptor stuff to get properties to display correctly in a property grid, such as overriding ICustomTypeDescriptor.
public abstract class MyClass
{
public abstract string MyProperty
{
get;
set;
}
}
public abstract class MyDerivedClass
{
public override string MyProperty
{
//VS complains that the Set accessor is missing
get;
}
}

You should not do this. What you are saying by defining your getter and setter in the abstract class is "you must implement this if you want to inherit from me." Then you are asking, "how can I made a derived class ignore this rule."
The answer is that if you have a situation that every derived class needs a getter, put that in the abstract class and let the derived class decide if they will implement a setter or not by leaving it out of the abstract class.
Or alternatively, you can create two more classes that derive from the initial abstract class, one that implement the setter and one that does not and then have your derived class generalize the one of those that makes sense, but that is overkill I think.

look like you searching for
[ReadOnly(true)] attribute
this will show to property grid your property, as readonly.
but in your class you can use it as usual property (with read and write possibilities)

You should use abstract not override:
public abstract class MyClass
{
public abstract string MyProperty
{
get;
set;
}
}
public abstract class MyDerivedClass
{
public abstract string MyProperty
{
get;
}
}
but like #JP wrote, you shouldn't do this.