How to pass a parameter to Singleton in MVVM Xamarin Forms

Question

I want to add an element to an ObservableCollection from another ViewModel ... the problem is that when generating the instance of that ViewModel through a singleton I get the error that the constructor receives a parameter

There is no argument given that corresponds to the required formal parameter 'centroLegado' of 'GenerarRetiroCentroViewModel.GenerarRetiroCentroViewModel(CentroGeneracion)'

How can I implement the singleton so that I can call it from another ViewModel?

I attach the code of the ViewModel that has the Observable Collection and the constructor..

GenerarRetiroCentroViewModel.CS:

 #region Constructor
  public GenerarRetiroCentroViewModel(CentroGeneracion centroLegado)
  {
      instance = this;   

      ListaResiduosTemporales = new ObservableCollection<ResiduosTemporal>();

      centroSeleccionado = centroLegado;

 }    
 #endregion

 #region Singleton
  static GenerarRetiroCentroViewModel instance;

  public static  GenerarRetiroCentroViewModel GetInstance()
  {
      if (instance == null)
      {
          return new GenerarRetiroCentroViewModel();
      }
      return instance;
 }
 #endregion

I attach the code of how I "wish" to call my ObservableCollection from the other ViewModel (SelectResiduoViewModel.CS)

SeleccionarResiduoViewModel.CS:

           var objeto = new ResiduosTemporal
            {
                IdResiduo = IdResiduo,
                NombreResiduo = NombreResiduo,
                IdEstimado = IdUnidad,
                NombreEstimado = NombreUnidad,
                IdContenedor = IdContenedor,
                NombreContenedor = NombreContenedor,

            };

            var generarRetiroCentroViewModel = GenerarRetiroCentroViewModel.GetInstance();

            generarRetiroViewModel.ListaResiduosTemporales.Add(objecto);

How can I add the Mode object to fill a control that is in another ViewModel? Is this possible with a SINGLETON? how can I do it? any help for me?

Answer 1

(I actually forgot to make an answer. Sorry ¯\\_(ツ)_/¯)

As explained in my comment, you simply have to fill the parameter correct. You're expecting a CentroGeneracion but you're not passing any arguments. If you don't need it, create a second, empty constructor.

Also, you're never assigning the instance property, that means you'll always return a new singleton instance if you call that function. It should be

public GenerarRetiroCentroViewModel GetInstance()
{
    if(instance == null){
        instance = new GenerarRetiroCentroViewModel();
    }

    return instance;
}

Actually, you don't even need a function.

public GenerarRetiroCentroViewModel Instance
{
    get
    {
        if(instance == null){
            instance = new GenerarRetiroCentroViewModel();
        }

        return instance;
    }
}

---

Note: That is not thread-safe. If you want to make it thread-safe, you can use Lazy (.NET 4+).

public sealed class SingletonClass
{
    private static readonly Lazy<SingletonClass> _lazy =  new Lazy<SingletonClass>(() => new SingletonClass());

    public static SingletonClass Instance 
    {
        get
        {
            return _lazy.Value;
        }
    }

}

It basically creates a Singleton the first time someone tries to access that property.