Safely Calling Virtual Members From Constructors...

• Download VirtualMemberCallFromConstructor.zip - 19.9 KB

Introduction

The idea is simple - you should make calls to virtual members only from the top of the inheritance hierarchy. This tip shows it can be done in a simple manner.

Background

You can read more about the solving problem here or try to find on the internet - 'virtual member call from constructor'.

Using the Code

Let's suppose we have three simple classes A, B, C inherited in such sequence A -> B -> C.

A has virtual member, B and C override it and use it in their constructors.

Code A: Will cause runtime error, because there are not initialized members in B and C.

class A
{
    public A()
    {
        VirtualMethod();
    }

    protected virtual void VirtualMethod()
    {
        Console.WriteLine("A: Do nothing");
    }
}

class B : A
{
    private readonly String _someString;

    public B(String someString)
        :base () //Just to show that base constructor called, may be skipped
    {
        if(someString == null) throw new ArgumentNullException("someString");
        _someString = someString;

        VirtualMethod();
    }

    protected override void VirtualMethod()
    {
        Console.WriteLine("B : Length of some string - {0}", _someString.Length);
    }
}

class C : B
{
    private readonly String _someString;

    public C(String someStringC, String someStringB)
        : base(someStringB) //Just to show that base constructor called, may be skipped
    {
        if (someStringC == null) throw new ArgumentNullException("someStringC");
        _someString = someStringC;

        VirtualMethod();
    }

    protected override void VirtualMethod()
    {
        Console.WriteLine("C : Length of some string - {0}", _someString.Length);
    }
}

To solve a problem, we should make some changes to base and inherited classes, these changes will be visible in the following code:

Code B: Will not cause any errors.

class A
{
    private List<Action> _virtualMembersCallActions;

    /// <summary>
    /// This function should be called from every inherited  constructor,
    /// if you forget to call it you will have some not initialized
    /// members through all inheritance hierarchy be careful!
    /// </summary>
    /// <typeparam name="TType">Type of currently calling class.</typeparam>
    /// <param name="action">Action which contains virtual members calls.</param>
    protected void CallVirtualMembers<TType>(Action action = null)
    {
        if (action != null)
        {
            if (_virtualMembersCallActions == null)
            {
                _virtualMembersCallActions = new List<Action>();
            }

            if (!_virtualMembersCallActions.Contains(action))
            {
                _virtualMembersCallActions.Add(action);
            }
        }

        if (GetType() == typeof (TType))
        {
            _virtualMembersCallActions.ForEach(i=>i());
            _virtualMembersCallActions = null;
        }
    }

    public A()
    {
        CallVirtualMembers<A>(VirtualMethod);
    }

    protected virtual void VirtualMethod()
    {
        Console.WriteLine("A: Do nothing");
    }
}

class B : A
{
    private readonly String _someString;

    public B(String someString)
        :base () //Just to show that base constructor called, may be skipped
    {
        if(someString == null) throw new ArgumentNullException("someString");
        _someString = someString;

        CallVirtualMembers<B>(VirtualMethod);
    }

    protected override void VirtualMethod()
    {
        Console.WriteLine("B : Length of some string - {0}", _someString.Length);
    }
}

class C : B
{
    private readonly String _someString;

    public C(String someStringC, String someStringB)
        : base(someStringB)
    {
        if (someStringC == null) throw new ArgumentNullException("someStringC");
        _someString = someStringC;

        CallVirtualMembers<C>(VirtualMethod);
    }

    protected override void VirtualMethod()
    {
        Console.WriteLine("C : Length of some string - {0}", _someString.Length);
    }
}

Of course, this just a sample, you can add any business logic to CallVirtualMembers function, you can have several such functions to change sequence if virtual members are calling. In my current example, I have added:

if (!_virtualMembersCallActions.Contains(action))
{
      _virtualMembersCallActions.Add(action);
}

to suppress repeat calling of same virtual members twice or more times, if you remove this line you will see repeats in console window.

Code C: No errors, but use singleton extension

public static class VirtualsCaller
{
    private static readonly Dictionary<Object, List<Action>> _actionsStore = new Dictionary<object, List<Action>>();

    /// <summary>
    /// This function should be called from every inherited  constructor,
    /// if you forget to call it you will have some not initialized
    /// members through all inheritance hierarchy be careful!
    /// </summary>
    /// <typeparam name="TType">Type of currently calling class.</typeparam>
    /// <param name="source">Source object</param>
    /// <param name="action">Action which contains wirtual members calls.</param>
    public static void CallVirtualMembers<TType>(Object source, Action action)
    {
        List<Action> actions;
        if (!_actionsStore.TryGetValue(source, out actions))
        {
                    actions = new List<Action>();
                    _actionsStore[source] = actions;
        }

        if (action != null)
        {
            if (!actions.Contains(action))
            {
                actions.Add(action);
            }
        }

        if (source.GetType() == typeof(TType))
        {
            actions.ForEach(i => i());
            _actionsStore.Remove(source);
        }
    }
}

class A
{
    public A()
    {
        VirtualsCaller.CallVirtualMembers<A>(this, VirtualMethod);
    }

    protected virtual void VirtualMethod()
    {
        Console.WriteLine("A: Do nothing");
    }
}

class B : A
{
    private readonly String _someString;

    public B(String someString)
        :base () //Just to show that base constructor called, may be skipped
    {
        if(someString == null) throw new ArgumentNullException("someString");
        _someString = someString;

        VirtualsCaller.CallVirtualMembers<B>(this, VirtualMethod);
    }

    protected override void VirtualMethod()
    {
        Console.WriteLine("B : Length of some string - {0}", _someString.Length);
    }
}

class C : B
{
    private readonly String _someString;

    public C(String someStringC, String someStringB)
        : base(someStringB) //Just to show that base constructor called, may be skipped
    {
        if (someStringC == null) throw new ArgumentNullException("someStringC");
        _someString = someStringC;

        VirtualsCaller.CallVirtualMembers<C>(this, VirtualMethod);
    }

    protected override void VirtualMethod()
    {
        Console.WriteLine("C : Length of some string - {0}", _someString.Length);
    }
}

History

• 23/03/2015 - Just created