Clearing cache

Now I am trying to do clear the cache and the collection in the GroupedItemsPage this is part of the code:

if (DefaultViewModel.Keys.Contains("Groups"))
                {
                    base.ClearNavigationCache();

                    // TODO: Assign a collection of bindable groups to this.DefaultViewModel["Groups"]
                    var sampleDataGroups = VMDataSource.GetGroups((String)navigationParameter);
                    this.DefaultViewModel["Groups"] = sampleDataGroups;
                }
                else
                {
                    // TODO: Assign a collection of bindable groups to this.DefaultViewModel["Groups"]
                    var sampleDataGroups = VMDataSource.GetGroups((String)navigationParameter);
                    this.DefaultViewModel["Groups"] = sampleDataGroups;
                }

Than after the base.ClearNavigationCache I want to set the allGroups to null from the DataSource.

This is the DataSource:

private static VMDataSource _sampleDataSource = new VMDataSource();

        private ObservableCollection<VMDataGroup> _allGroups = new ObservableCollection<VMDataGroup>();
        public ObservableCollection<VMDataGroup> AllGroups
        {
            get { return this._allGroups; }
            set { this._allGroups = value; }
        }

        public static IEnumerable<VMDataGroup> GetGroups(string uniqueId)
        {
            if (!uniqueId.Equals("AllGroups")) throw new ArgumentException("Only 'AllGroups' is supported as a collection of groups");
            return _sampleDataSource.AllGroups;
        }

I implement the set for allGroups to set it to null cause the first time I load the app and choose a Category the code goes into the else on the code, after I navigate back to the mainPage I select another category but it still shows me the old data.

First loading the app, when it enters the GetGroups(string uniqueId) the return _sampleDataSource.AllGroups; is count "0" but after I choose another category and navigate to that same page the return _sampleDataSource.AllGroups; is count 1 so I think maybe set the allGroups to null on the GroupedItemsPage. Thanks.

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Thanks, Smiths