How to clear a combobox after research in WPF?

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C#

WPF

I had a research by criteria with Two combobox, it works fine after the research is finished, I have a button Display All : to reset the combobox to null..and the DataGrid display with all elements ,

The problem that the combobox must be empty when I click on the Button Dispaly All!

1) Without select an element in combobox(just dispaly the datagrid):I have 6 elements in the datagrid, it is correct..and the combobox are Empty.

2) After select the Search criteria, i have the result correct: (I have just 3 results, it is the correct action)

3) When I click on the button Display All:(I have all the elements in datagrid, 6 elements..It is correct) But the Combobox aren't empty!!

The view:

The ViewModel:

What I have tried:

<pre>    <Window x:Class="WPFAuthentification.Views.BusinesseventsView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" >   

     <Label Content="Entity Type" Width="128" Grid.Row="1" Grid.ColumnSpan="2"/>
     <ComboBox HorizontalAlignment="Center" VerticalAlignment="Center"  
         ItemsSource="{Binding EntityLevelEnum}" 
        SelectedItem="{Binding EntityType, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged, NotifyOnValidationError=True, TargetNullValue=''}"
        Grid.ColumnSpan="2" Grid.Column="1"  />


        <Button Content="Dislplay all" ToolTip="Display All Business Events" 
                VerticalAlignment="Top"  Command="{Binding Initialize}" 
                Visibility="{Binding Path=ShowDisplayAllButton, Converter={StaticResource BoolToVis}}"  />

         <DataGrid ..... />
    </Window>

<pre>    class BusinesseventsViewModel : ViewModelBase1
    {

        private ObservableCollection<BusinessEventClass> businessEventsList;    

        private RelayCommand<string> initialize;
        public RelayCommand<string> Initialize
        {
            get { return initialize; }
        }       
        public BusinesseventsViewModel()
        {
            //businessEventsList: to Get all the Business events
            businessEventsList = new ObservableCollection<BusinessEventClass>(WCFclient.getAllBusinessEvent());
            //Enumeration of Entity Type and Criticality
            levelCriticalityEnum = new ObservableCollection<Level_Criticality>(Enum.GetValues(typeof(Level_Criticality)).Cast<Level_Criticality>());
            entityLevelEnum = new ObservableCollection<BusinessEntityLevel>(Enum.GetValues(typeof(BusinessEntityLevel)).Cast<BusinessEntityLevel>());
            //the Button Display All :
            initialize = new RelayCommand<string>(initFunc);    
       }        

       //Function of the Button Display All
        private void initFunc(object obj)
        {
      entityLevelEnum.Clear();
          //  EntityLevelEnum = null;
            levelCriticalityEnum.Clear();
            //LevelCriticalityEnum = null;
            OnPropertyChanged("EntityLevelEnum");
            OnPropertyChanged("LevelCriticalityEnum");    
			}     

         private string  entityType;
        public string EntityType
        {
            get { return entityType; }
            set
            {
                entityType = value;
                businessEventsList = filterByCriteria(entityType, criticality);
                OnPropertyChanged("BusinessEventsList");
                OnPropertyChanged("EntityType");
            }
        } 

             //Function of the research :
                 public ObservableCollection<BusinessEventClass> filterByCriteria(string entityType, string criticality)
        {               
            BusinessEventsList = new ObservableCollection<BusinessEventClass>(WCFclient.getAllBusinessEvent());         

            ObservableCollection<BusinessEventClass> updatedList = new ObservableCollection<BusinessEventClass>();

            if ((entityType == null) && (Criticality == null))
            {
                updatedList = businessEventsList;
                levelCriticalityEnum = new ObservableCollection<Level_Criticality>(Enum.GetValues(typeof(Level_Criticality)).Cast<Level_Criticality>());     
                 entityLevelEnum = new ObservableCollection<BusinessEntityLevel>(Enum.GetValues(typeof(BusinessEntityLevel)).Cast<BusinessEntityLevel>());

            }          

            if ((entityType != null && entityType != "") && (Criticality != null))
                {
                    updatedList = new ObservableCollection<BusinessEventClass>(BusinessEventsList.Where(a => a.EntityType.ToString().ToLower().Equals(criticality.ToString())
                                                                             && a.Critciality.ToString().Equals(criticality.ToString())));
                } }