How to implement a read only property

Question

I need to implement a read only property on my type  Moreover the value of this property is going to be set in the constructor and it is not going to be changed  I am writing a class that exposes custom routed UI commands for WPF but it does not matter    I see two ways to do it    class MyClass       public readonly object MyProperty   new object       class MyClass       private readonly object my property   new object        public object MyProperty   get   return my property           With all these FxCop errors saying that I should not have public member variables  it seems that the second one is the right way to do it  Correct   Is there any difference between a get only property and a read only member in this case   I would appreciate any comments advice etc

User · Answer

The second way is the preferred option   private readonly int MyVal   5   public int MyProp   get   return MyVal        This will ensure that MyVal can only be assigned at initialization  it can also be set in a constructor    As you had noted - this way you are not exposing an internal member  allowing you to change the internal implementation in the future

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I agree that the second way is preferable   The only real reason for that preference is the general preference that  NET classes not have public fields   However  if that field is readonly  I can t see how there would be any real objections other than a lack of consistency with other properties   The real difference between a readonly field and get-only property is that the readonly field provides a guarantee that its value will not change over the life of the object and a get-only property does not

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The second method is preferred because of the encapsulation   You can certainly have the readonly field be public  but that goes against C  idioms in which you have data access occur through properties and not fields   The reasoning behind this is that the property defines a public interface and if the backing implementation to that property changes  you don t end up breaking the rest of the code because the implementation is hidden behind an interface

User · Answer

You can do this   public int Property   get         private set

User · Answer

With the introduction of C  6  in VS 2015   you can now have get-only automatic properties  in which the implicit backing field is readonly  i e  values can be assigned in the constructor but not elsewhere    public string Name   get     public Customer string name      Constructor       Name   name     private void SomeFunction         Name    Something Else       Compile-time error     And you can now also initialise properties  with or without a setter  inline   public string Name   get       Boris     Referring back to the question  this gives you the advantages of option 2  public member is a property  not a field  with the brevity of option 1    Unfortunately  it doesn t provide a guarantee of immutability at the level of the public interface  as in  CodesInChaos s  point about self-documentation   because to a consumer of the class  having no setter is indistinguishable from having a private setter

User · Answer

yet another way  my favorite   starting with C  6  private readonly int MyVal   5   public int MyProp   gt  MyVal    https   docs microsoft com en-us dotnet csharp programming-guide classes-and-structs properties expression-body-definitions

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In C  9 Microsoft will introduce a new way to have properties set only on initialization using the init  method like so  public class Person     public string firstName   get  init      public string lastName   get  init       This way  you can assign values when initializing a new object  var person   new Person     firstname    quot John quot     lastName    quot Doe quot     But later on  you cannot change it  person lastName    quot Denver quot      throws a compiler error

User · Answer

C  6 0 adds readonly auto properties public object MyProperty   get     So when you don t need to support older compilers you can have a truly readonly property with code that s just as concise as a readonly field   Versioning  I think it doesn t make much difference if you are only interested in source compatibility  Using a property is better for binary compatibility since you can replace it by a property which has a setter without breaking compiled code depending on your library  Convention  You are following the convention  In cases like this where the differences between the two possibilities are relatively minor following the convention is better  One case where it might come back to bite you is reflection based code  It might only accept properties and not fields  for example a property editor viewer  Serialization Changing from field to property will probably break a lot of serializers  And AFAIK XmlSerializer does only serialize public properties and not public fields  Using an Autoproperty Another common Variation is using an autoproperty with a private setter  While this is short and a property it doesn t enforce the readonlyness  So I prefer the other ones  Readonly field is selfdocumenting There is one advantage of the field though  It makes it clear at a glance at the public interface that it s actually immutable  barring reflection   Whereas in case of a property you can only see that you cannot change it  so you d have to refer to the documentation or implementation  But to be honest I use the first one quite often in application code since I m lazy  In libraries I m typically more thorough and follow the convention

[c#] How to implement a read only property

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