Implementeer ICommand patroon met RelayCommand voor MVVM

- Voeg RelayCommand class toe voor herbruikbare ICommand implementatie
- Vervang button click event handler door AddProductCommand in MainViewModel
- Update XAML om Command binding te gebruiken in plaats van Click event
- Verwijder business logic uit code-behind (MainWindow.xaml.cs)
- Los DataContext duplicatie op (was twee keer MainViewModel instantie)

Dit maakt de applicatie beter testbaar en volgt proper MVVM principes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Views/MainWindow.xaml

@@ -26,7 +26,7 @@
                     Content="Add Product"
                     Width="100px"
                     Height="25px"
-                    Click="btnAddProduct_Click" />
+                    Command="{Binding AddProductCommand}" />
             <Label Content="Selected Product Details"
                    FontWeight="Bold"
                    FontSize="16"

Views/MainWindow.xaml.cs

@@ -18,12 +18,10 @@ namespace MVVM_DEMO
     /// </summary>
     public partial class MainWindow : Window
     {
-        MainViewModel viewModel = new MainViewModel();
-
         public MainWindow()
         {
             InitializeComponent();
-            this.DataContext = viewModel;
+            // ViewModel is already set in XAML via Window.DataContext
             comboBox.SelectionChanged += ComboBox_SelectionChanged;
             // initial selection
             if (comboBox.Items.Count > 0)
@@ -35,25 +33,15 @@ namespace MVVM_DEMO
         private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
         {
             // display selected product details
-            if (comboBox.SelectedItem != null)
+            if (comboBox.SelectedItem != null && DataContext is MainViewModel viewModel)
             {
-                viewModel.productName= ((Product)comboBox.SelectedItem).ProductName;
+                viewModel.productName = ((Product)comboBox.SelectedItem).ProductName;
                 viewModel.productPrice = (int)((Product)comboBox.SelectedItem).Price;
                 viewModel.OnPropertyChanged("productName");
                 viewModel.OnPropertyChanged("productPrice");
             }
-
         }
 
-        private void btnAddProduct_Click(object sender, RoutedEventArgs e)
-        {
-            // toevoegen van een product in het viewmodel
-            Product p = new Product();
-            p.ProductName = $"Product {viewModel.Products.Count + 1}";
-            // generate a random price between 10 and 100
-            Random rand = new Random();
-            p.Price = rand.Next(10, 100);
-            viewModel.Products.Add(p);
-        }
+        // btnAddProduct_Click removed - now using Command binding in XAML
     }
 }