基类构造函数中Call abstract方法的替代方法

Question

my problem is this:

I Have a base class

public abstract class ViewModelBase 

wich contains an abstract method "RegisterCommands"

protected abstract void RegisterCommands();

All my Derived classes must, obviously, implements this method, for example my LoginViewModel has

protected override void RegisterCommands()
{
    LoginCommand?
        .Configure(
            execute: (msg) => { Login(User); },
            canExecute: (x) => { return CanLogin(); }
            );
}

and call it when class is instantiated, but i don't want call this method in every derived class constructor (if I have 100 derived classes i must call RegisterCommand 100 times).

Normal solution is call RegisterCommand in the base class constructor

protected abstract void RegisterCommands();

public ViewModelBase()
{
    RegisterCommands();
}

and this usually works (even if I do not know if it's a good practice) but...but...

in my scenario, in all the RegisterCommands methods I use my ICustomCommand objects, which are initialized in the derived class constructor with dependency injection

public class LoginViewModel : ViewModelBase
{

    private ICustomCommand _loginCommand;

    public ICustomCommand LoginCommand
    {
        get
        {
            return _loginCommand;
        }
        set
        {
            _loginCommand = value;
        }
    }

    public LoginViewModel(ICustomCommand loginCommand)
    {
        _loginCommand = loginCommand;
    }

    protected override void RegisterCommands()
    {
        LoginCommand?
            .Configure(
                execute: (msg) => { Login(User); },
                canExecute: (x) => { return CanLogin(); }
                );
    }

So, because base class constructor is called before derived class constructor, i can't call RegisterCommands() in base class constructor (because my ICustomCommands are not initialized yet in derived class so RegisterCommands() try to use my ICustomCommand which are still null).

I know that is not possible call derived class constuctor before base class constructor, so what could be a valid, simple and clean solution to call RegisterCommands in all derived class calling this command in only one point?

thanks for answer

UPDATE:

As I said, RegisterCommands() is plural because every derived class could have N ICustomCommand objects

So i can have

LoginCommand for my LoginViewModel

SaveCommand, DeleteCommand for another ViewModel

etc.

One solution I think now is to remove ICustomCommand initalization from constructor and resolve it "on the fly" in getter property trough a static Resolver class, something like this

public ICustomCommand LoginCommand
{
    get
    {
        if(_loginCommand == null)
            MyStaticResolver.Resolve<ICustomCommand>();
        return _loginCommand;

But I'm still not convinced

Answer 1

If you use interfaces to represent your commands, you could pass them to the base class and expose a method to retrieve the commands. Your Registration method can then use them as needed:

public abstract class ViewModelBase
{
    public ViewModelBase(params ICustomCommand[] commands)
    {
        _commands = commands;            
        RegisterCommands();
    }

    private IEnumerable<ICustomCommand> _commands;

    protected abstract void RegisterCommands();

    //This method gets you the commands
    protected T GetCommand<T>() where T : ICustomCommand
    {
        var command = _commands.FirstOrDefault(c => typeof(T).IsAssignableFrom(c.GetType()));
        return (T)command ;
    }
}

public class LoginViewModel : ViewModelBase
{
    public LoginViewModel(ILoginCommand command):base(command)
    {

    }

    protected override void RegisterCommands()
    {
        //Get the command from the base class
        var command = GetCommand<ILoginCommand>();
        command?
        .Configure(
            execute: (msg) => { Login(User); },
            canExecute: (x) => { return CanLogin(); }
            );
    }
}

public class LoginCommand : ILoginCommand
{
}

public interface ILoginCommand : ICustomCommand
{
}