Virtual member call in a constructor

Question

I m getting a warning from ReSharper about a call to a virtual member from my objects constructor    Why would this be something not to do

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In C   a base class  constructor runs before the derived class  constructor  so any instance fields that a derived class might use in the possibly-overridden virtual member are not initialized yet   Do note that this is just a warning to make you pay attention and make sure it s all-right   There are actual use-cases for this scenario  you just have to document the behavior of the virtual member that it can not use any instance fields declared in a derived class below where the constructor calling it is

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There are well-written answers above for why you wouldn t want to do that   Here s a counter-example where perhaps you would want to do that  translated into C  from Practical Object-Oriented Design in Ruby by Sandi Metz  p  126     Note that GetDependency   isn t touching any instance variables   It would be static if static methods could be virtual    To be fair  there are probably smarter ways of doing this via dependency injection containers or object initializers       public class MyClass       private IDependency  myDependency       public MyClass IDependency someValue   null                 myDependency   someValue    GetDependency                  If this were static  it could not be overridden        as static methods cannot be virtual in C       protected virtual IDependency GetDependency                  return new SomeDependency             public class MySubClass   MyClass       protected override IDependency GetDependency                 return new SomeOtherDependency             public interface IDependency      public class SomeDependency   IDependency     public class SomeOtherDependency   IDependency

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I think that ignoring the warning might be legitimate if you want to give the child class the ability to set or override a property that the parent constructor will use right away   internal class Parent       public Parent                 Console WriteLine  Parent ctor            Console WriteLine Something              protected virtual string Something   get       Parent      internal class Child   Parent       public Child                 Console WriteLine  Child ctor            Console WriteLine Something              protected override string Something   get       Child       The risk here would be for the child class to set the property from its constructor in which case the change in the value would occur after the base class constructor has been called   My use case is that I want the child class to provide a specific value or a utility class such as a converter and I don t want to have to call an initialization method on the base   The output of the above when instantiating the child class is   Parent ctor Child Child ctor Child

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One important aspect of this question which other answers have not yet addressed is that it is safe for a base-class to call virtual members from within its constructor if that is what the derived classes are expecting it to do   In such cases  the designer of the derived class is responsible for ensuring that any methods which are run before construction is complete will behave as sensibly as they can under the circumstances   For example  in C   CLI  constructors are wrapped in code which will call Dispose on the partially-constructed object if construction fails   Calling Dispose in such cases is often necessary to prevent resource leaks  but Dispose methods must be prepared for the possibility that the object upon which they are run may not have been fully constructed

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Reasons of the warning are already described  but how would you fix the warning  You have to seal either class or virtual member      class B         protected virtual void Foo              class A   B         public A               Foo       warning here             You can seal class A     sealed class A   B         public A               Foo       no warning             Or you can seal method Foo     class A   B         public A               Foo       no warning            protected sealed override void Foo               base Foo

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The warning is a reminder that virtual members are likely to be overridden on derived class  In that case whatever the parent class did to a virtual member will be undone or changed by overriding child class  Look at the small example blow for clarity  The parent class below attempts to set value to a virtual member on its constructor  And this will trigger Re-sharper warning  let see on code   public class Parent       public virtual object Obj get set       public Parent                    Re-sharper warning  this is open to change from             inheriting class overriding virtual member         this Obj   new Object              The child class here overrides the parent property  If this property was not marked virtual the compiler would warn that the property hides property on the parent class and suggest that you add  new  keyword if it is intentional   public class Child  Parent       public Child   base                 this Obj    Something             public override object Obj get set       Finally the impact on use  the output of the example below abandons the initial value set by parent class constructor  And this is what Re-sharper attempts to to warn you  values set on the Parent class constructor are open to be overwritten by the child class constructor which is called right after the parent class constructor   public class Program       public static void Main                 var child   new Child               anything that is done on parent virtual member is destroyed         Console WriteLine child Obj              Output   Something

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The rules of C  are very different from that of Java and C     When you are in the constructor for some object in C   that object exists in a fully initialized  just not  constructed   form  as its fully derived type   namespace Demo       class A              public A                   System Console WriteLine  This is a  0     this GetType                        class B   A                                      B b   new B       Output   This is a Demo B      This means that if you call a virtual function from the constructor of A  it will resolve to any override in B  if one is provided   Even if you intentionally set up A and B like this  fully understanding the behavior of the system  you could be in for a shock later  Say you called virtual functions in B s constructor   knowing  they would be handled by B or A as appropriate  Then time passes  and someone else decides they need to define C  and override some of the virtual functions there  All of a sudden B s constructor ends up calling code in C  which could lead to quite surprising behavior   It is probably a good idea to avoid virtual functions in constructors anyway  since the rules are so different between C   C    and Java  Your programmers may not know what to expect

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When an object written in C  is constructed  what happens is that the initializers run in order from the most derived class to the base class  and then constructors run in order from the base class to the most derived class  see Eric Lippert s blog for details as to why this is    Also in  NET objects do not change type as they are constructed  but start out as the most derived type  with the method table being for the most derived type  This means that virtual method calls always run on the most derived type   When you combine these two facts you are left with the problem that if you make a virtual method call in a constructor  and it is not the most derived type in its inheritance hierarchy  that it will be called on a class whose constructor has not been run  and therefore may not be in a suitable state to have that method called    This problem is  of course  mitigated if you mark your class as sealed to ensure that it is the most derived type in the inheritance hierarchy - in which case it is perfectly safe to call the virtual method

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Another interesting thing I found is that the ReSharper error can be  satisfied  by doing something like below which is dumb to me  However  as mentioned by many earlier  it still is not a good idea to call virtual properties methods in constructor  public class ConfigManager      public virtual int MyPropOne   get  private set       public virtual string MyPropTwo   get  private set        public ConfigManager            Setup             private void Setup            MyPropOne   1      MyPropTwo    quot test quot

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Your constructor may  later  in an extension of your software  be called from the constructor of a subclass that overrides the virtual method   Now not the subclass s implementation of the function  but the implementation of the base class will be called   So it doesn t really make sense to call a virtual function here   However  if your design satisfies the Liskov Substitution principle  no harm will be done  Probably that s why it s tolerated - a warning  not an error

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Just to add my thoughts  If you always initialize the private field when define it  this problem should be avoid  At least below code works like a charm   class Parent       public Parent                 DoSomething              protected virtual void DoSomething                  class Child   Parent       private string foo    HELLO       public Child       Originally foo initialized here  Removed          protected override void DoSomething                 Console WriteLine foo ToLower

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I would just add an Initialize   method to the base class and then call that from derived constructors  That method will call any virtual abstract methods properties AFTER all of the constructors have been executed

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Yes  it s generally bad to call virtual method in the constructor   At this point  the objet may not be fully constructed yet  and the invariants expected by methods may not hold yet

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Beware of blindly following Resharper s advice and making the class sealed   If it s a model in EF Code First it will remove the virtual keyword and that would disable lazy loading of it s relationships        public   virtual   User User  get  set

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Because until the constructor has completed executing  the object is not fully instantiated  Any members referenced by the virtual function may not be initialised  In C    when you are in a constructor  this only refers to the static type of the constructor you are in  and not the actual dynamic type of the object that is being created  This means that the virtual function call might not even go where you expect it to

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One important missing bit is  what is the correct way to resolve this issue   As Greg explained  the root problem here is that a base class constructor would invoke the virtual member before the derived class has been constructed   The following code  taken from MSDN s constructor design guidelines  demonstrates this issue    public class BadBaseClass       protected string state       public BadBaseClass                 this state    BadBaseClass           this DisplayState               public virtual void DisplayState                  public class DerivedFromBad   BadBaseClass       public DerivedFromBad                 this state    DerivedFromBad              public override void DisplayState                    Console WriteLine this state             When a new instance of DerivedFromBad is created  the base class constructor calls to DisplayState and shows BadBaseClass because the field has not yet been update by the derived constructor   public class Tester       public static void Main                 var bad   new DerivedFromBad              An improved implementation removes the virtual method from the base class constructor  and uses an Initialize method  Creating a new instance of DerivedFromBetter displays the expected  DerivedFromBetter    public class BetterBaseClass       protected string state       public BetterBaseClass                 this state    BetterBaseClass           this Initialize               public void Initialize                 this DisplayState               public virtual void DisplayState                  public class DerivedFromBetter   BetterBaseClass       public DerivedFromBetter                 this state    DerivedFromBetter              public override void DisplayState                 Console WriteLine this state

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In order to answer your question  consider this question  what will the below code print out when the Child object is instantiated   class Parent       public Parent                 DoSomething               protected virtual void DoSomething                   class Child   Parent       private string foo       public Child                   foo    HELLO               protected override void DoSomething                 Console WriteLine foo ToLower       NullReferenceException              The answer is that in fact a NullReferenceException will be thrown  because foo is null  An object s base constructor is called before its own constructor  By having a virtual call in an object s constructor you are introducing the possibility that inheriting objects will execute code before they have been fully initialized

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There s a difference between C   and C  in this specific case  In C   the object is not initialized and therefore it is unsafe to call a virutal function inside a constructor  In C  when a class object is created all its members are zero initialized  It is possible to call a virtual function in the constructor but if you ll might access members that are still zero  If you don t need to access members it is quite safe to call a virtual function in C

[c#] Virtual member call in a constructor

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