The issue you're experiencing is likely due to mismatches or conflicts between the event handler declaration and how the event is referenced in your XAML. Here’s a breakdown of the possible causes and solutions:

1. Check the Namespace Ensure that the namespace in your MainWindow.xaml and MainWindow.xaml.cs files matches the namespace of your project. In your example: namespace C__experiments_WPF_

If there’s a typo or discrepancy, the event handler might not be found.

2. Event Handler Definition The error suggests that the ToggleB1_Click event handler is not being found during compilation. Verify that:

The method ToggleB1_Click is exactly spelled the same in both the XAML and the code-behind. The method signature in MainWindow.xaml.cs matches the required signature:

private void ToggleB1_Click(object sender, RoutedEventArgs e)

3. XAML Definition The Click attribute in your XAML references the method by its name. Ensure it's correctly written:

4. Avoid Redundant Attachments Since you're specifying Click="ToggleB1_Click" in XAML, the line in your constructor:

ToggleB1.Click += ToggleB1_Click;

5. Rebuild and Clean the Project