Presentation Model (MVVM) Good Practices

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using PresentationModelBase;
using System.Windows.Input;
using App.Configuration;
using System.Collections;

namespace App.Configuration
{
    class CommandPropertyDescriptor : PropertyDescriptor
    {
        public CommandPropertyDescriptor()
            : base(
            TypeDescriptor.CreateProperty
            (typeof(PresentationModel), "Call", typeof(ICommand), null))
        {
        }

        Dictionary<PresentationModel, ICommand> _propertyValues = new Dictionary<PresentationModel, ICommand>();

        public override bool CanResetValue(object component)
        {
            return false;
        }

        public override Type ComponentType
        {
            get { return typeof(PresentationModel); }
        }

        public override object GetValue(object component)
        {
            PresentationModel pm = (PresentationModel)component;
            if (!_propertyValues.ContainsKey(pm)) 
            {
                MethodCallCommand cmd = new MethodCallCommand(pm);
                _propertyValues.Add(pm, cmd);
            }
            return _propertyValues[pm];
        }

        public override bool IsReadOnly
        {
            get { return true; }
        }

        public override Type PropertyType
        {
            get { return typeof(ICommand); }
        }

        public override void ResetValue(object component)
        {
            throw new NotImplementedException();
        }

        public override void SetValue(object component, object value)
        {
            throw new NotImplementedException();
        }

        public override bool ShouldSerializeValue(object component)
        {
            return false;
        }
    }
}