removed the additional vliadtion rule, as it wasn't really necessary. In fact, for the demo it is a bit better to binf an editable combo box's Text preoprty...as it's editable...it could be any value

Author: ButchersBoy

MainDemo.Wpf/Domain/NotNullValidationRule.cs

@@ -13,15 +13,15 @@ namespace MaterialDesignColors.WpfExample.Domain
     public class TextFieldsViewModel : INotifyPropertyChanged
     {
         private string _name;
-        private int _selectedValueOne;
-        private int? _selectedValueTwo;
+        private int? _selectedValueOne;
+        private string _selectedTextTwo;
 
         public TextFieldsViewModel()
         {
             LongListToTestComboVirtualization = new List<int>(Enumerable.Range(0, 1000));
 
             SelectedValueOne = LongListToTestComboVirtualization.Skip(2).First();
-            SelectedValueTwo = null;
+            SelectedTextTwo = null;
         }
 
         public string Name
@@ -33,21 +33,21 @@ public string Name
             }
         }
 
-        public int SelectedValueOne
+        public int? SelectedValueOne
         {
             get { return _selectedValueOne; }
             set
             {
                 this.MutateVerbose(ref _selectedValueOne, value, RaisePropertyChanged());
             }
-        }
+        }        
 
-        public int? SelectedValueTwo
+        public string SelectedTextTwo
         {
-            get { return _selectedValueTwo; }
+            get { return _selectedTextTwo; }
             set
             {
-                this.MutateVerbose(ref _selectedValueTwo, value, RaisePropertyChanged());
+                this.MutateVerbose(ref _selectedTextTwo, value, RaisePropertyChanged());
             }
         }
 

MainDemo.Wpf/Domain/TextFieldsViewModel.cs

@@ -93,7 +93,6 @@
     <Compile Include="Domain\DemoItem.cs" />
     <Compile Include="Domain\DialogsViewModel.cs" />
     <Compile Include="Domain\ListFieldsViewModel.cs" />
-    <Compile Include="Domain\NotNullValidationRule.cs" />
     <Compile Include="Domain\Sample4Dialog.xaml.cs">
       <DependentUpon>Sample4Dialog.xaml</DependentUpon>
     </Compile>

MainDemo.Wpf/MaterialDesignDemo.csproj

@@ -133,7 +133,7 @@
                     <ComboBox.SelectedItem>
                         <Binding Path="SelectedValueOne" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
                             <Binding.ValidationRules>
-                                <domain1:NotNullValidationRule ValidatesOnTargetUpdated="True"/>
+                                <domain1:NotEmptyValidationRule ValidatesOnTargetUpdated="True"/>
                             </Binding.ValidationRules>
                         </Binding>
                     </ComboBox.SelectedItem>
@@ -149,13 +149,13 @@
                           VerticalAlignment="Bottom" 
                           IsEditable="True"                          
                           ItemsSource="{Binding LongListToTestComboVirtualization}">
-                    <ComboBox.SelectedItem>
-                        <Binding Path="SelectedValueTwo" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
+                    <ComboBox.Text>
+                        <Binding Path="SelectedTextTwo" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
                             <Binding.ValidationRules>
-                                <domain1:NotNullValidationRule ValidatesOnTargetUpdated="True"/>
+                                <domain1:NotEmptyValidationRule ValidatesOnTargetUpdated="True"/>
                             </Binding.ValidationRules>
                         </Binding>
-                    </ComboBox.SelectedItem>
+                    </ComboBox.Text>
                     <ComboBox.ItemsPanel>
                         <ItemsPanelTemplate>
                             <VirtualizingStackPanel />