Custom Rule Engine in WPF - MVVM

using System.Collections.Generic;
using System.Collections;

namespace CustomRulesMVVM.Models
{
    class CustomRulesGroups:CollectionBase,IRule 
    {   
        #region Members
        private const string AND = "AND";
        private const string OR = "OR";       
        #endregion

        #region Properties
        public bool IsSelected { get; set; }
        public string SelectedItem { get; set; }
        #endregion

        #region Public methods
        /// <summary>
        /// Evaluate rules groups in collection of rule groups
        /// </summary>
        /// <param name="collection"></param>
        /// <returns></returns>
        public bool Eval(Dictionary<string, object> collection)
        {
            return EvaluateCustomRulesGroups(collection);
        }
        #endregion

        #region Private Methods
        /// <summary>
        /// Evaluate two or more rules groups 
        /// </summary>
        /// <param name="collection"></param>
        /// <returns></returns>
        private bool EvaluateCustomRulesGroups(Dictionary<string, object> collection)
        {
            //In collection of RuleGroups evaluate between first two rules groups by calling Eval method of each rule group.
            if (null != this.Item(0).RuleGroupJoinOperator)
            {
                switch (this.Item(0).RuleGroupJoinOperator)
                {
                    case AND:
                        if (this.Item(0).Eval(collection) && this.Item(1).Eval(collection))
                            this.IsSelected = true;
                        break;
                    case OR:
                        if (this.Item(0).Eval(collection) || this.Item(1).Eval(collection))
                            this.IsSelected = true;
                        break;
                    default:
                        this.IsSelected = false;
                        break;
                }


                //Evaluate till last rule group if more than two rules groups present 
                if (this.Count > 2)
                {
                    for (int i = 1; i < this.Count - 2; i++) //Loop till count-2 as last rule will not have RuleJoinOperator
                    {
                        switch (this.Item(i).RuleGroupJoinOperator)
                        {
                            case AND:
                                if (this.IsSelected && this.Item(i).Eval(collection))
                                    this.IsSelected = true;
                                else
                                    this.IsSelected = false;
                                break;
                            case OR:
                                if (this.IsSelected || this.Item(i).Eval(collection))
                                    this.IsSelected = true;
                                else
                                    this.IsSelected = false;
                                break;
                            default:
                                this.IsSelected = false;
                                break;
                        }

                    }
                }
            }
            else
            {
                this.IsSelected = this.Item(0).Eval(collection);
            }

            return this.IsSelected;
        }
        #endregion

        #region CollectionBase methods
        public void Add(CustomRules item)
        {
            this.List.Add(item);
        }

        public void Remove(CustomRules item)
        {
            this.List.Remove(item);
        }
        public CustomRules Item(int index)
        {
            return this.List[index] as CustomRules;
        }
        #endregion
    }
}