Show validation in Demo App

samples/ControlCatalog/Pages/CalendarDatePickerPage.xaml

@@ -1,5 +1,7 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:vm="clr-namespace:ControlCatalog.ViewModels"
+             x:DataType="vm:MainWindowViewModel"
              x:Class="ControlCatalog.Pages.CalendarDatePickerPage">
   <StackPanel Orientation="Vertical" Spacing="4">
     <TextBlock Classes="h2">A control for selecting dates with a calendar drop-down</TextBlock>
@@ -39,6 +41,9 @@
                 
         <TextBlock Text="Disabled"/>
         <CalendarDatePicker IsEnabled="False"/>
+        
+        <TextBlock Text="Validation Example"/>
+        <CalendarDatePicker SelectedDate="{CompiledBinding ValidatedDateExample, Mode=TwoWay}"/>
       </StackPanel>
 
     </StackPanel> 

samples/ControlCatalog/ViewModels/MainWindowViewModel.cs

@@ -5,6 +5,7 @@ using Avalonia.Controls.Notifications;
 using Avalonia.Dialogs;
 using Avalonia.Platform;
 using System;
+using System.ComponentModel.DataAnnotations;
 using MiniMvvm;
 
 namespace ControlCatalog.ViewModels
@@ -164,5 +165,17 @@ namespace ControlCatalog.ViewModels
         public MiniCommand ExitCommand { get; }
 
         public MiniCommand ToggleMenuItemCheckedCommand { get; }
+
+        private DateTime? _validatedDateExample;
+
+        /// <summary>
+        ///    A required DateTime which should demonstrate validation for the DateTimePicker
+        /// </summary>
+        [Required]
+        public DateTime? ValidatedDateExample
+        {
+            get => _validatedDateExample;
+            set => this.RaiseAndSetIfChanged(ref _validatedDateExample, value);
+        }
     }
 }