Test Case: csharp-instruct-640

```csharp
using System;
using System.Diagnostics;
using System.Windows.Forms;

public class RichTextBoxHyperlinkController : Form
{
    private RichTextBox richTextBox1;
    private bool isCtrlPressed = false;

    public RichTextBoxHyperlinkController()
    {
        InitializeComponent();
        richTextBox1.LinkClicked += RichTextBox1_LinkClicked;
        this.KeyDown += RichTextBoxHyperlinkController_KeyDown;
        this.KeyUp += RichTextBoxHyperlinkController_KeyUp;
    }

    private void InitializeComponent()
    {
        this.richTextBox1 = new System.Windows.Forms.RichTextBox();
        this.SuspendLayout();
        //
        // richTextBox1
        //
        this.richTextBox1.Location = new System.Drawing.Point(12, 12);
        this.richTextBox1.Name = "richTextBox1";
        this.richTextBox1.Size = new System.Drawing.Size(360, 237);
        this.richTextBox1.TabIndex = 0;
        this.richTextBox1.Text = "This is a sample RichTextBox.\n\nVisit Google: http://www.google.com\n\nAnother link: https://www.microsoft.com";
        this.richTextBox1.LinkClicked += new System.Windows.Forms.LinkClickedEventHandler(this.RichTextBox1_LinkClicked);
        //
        // RichTextBoxHyperlinkController
        //
        this.ClientSize = new System.Drawing.Size(384, 261);
        this.Controls.Add(this.richTextBox1);
        this.Name = "RichTextBoxHyperlinkController";
        this.Text = "RichTextBox Hyperlink Controller";
        this.ResumeLayout(false);
    }

    private void RichTextBox1_LinkClicked(object sender, LinkClickedEventArgs e)
    {
        if (isCtrlPressed)
        {
            try
            {
                Process.Start(e.LinkText);
            }
            catch (Exception ex)
            {
                MessageBox.Show($"Could not open link: {ex.Message}");
            }
        }
    }

    private void RichTextBoxHyperlinkController_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.LControlKey || e.KeyCode == Keys.RControlKey)
        {
            isCtrlPressed = true;
            EnableHyperlinks(true);
        }
    }

    private void RichTextBoxHyperlinkController_KeyUp(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.LControlKey || e.KeyCode == Keys.RControlKey)
        {
            isCtrlPressed = false;
            EnableHyperlinks(false);
        }
    }

    private void EnableHyperlinks(bool enable)
    {
        // The RichTextBox automatically handles hyperlink detection and rendering.
        // We only need to control whether the LinkClicked event is processed.
        // The LinkClicked event itself is what we're conditionally acting upon.
        // The visual appearance of hyperlinks is managed by the RichTextBox.
        // When hyperlinks are "disabled" by not processing the event, they
        // will still appear as links, but clicking them won't trigger the action.
        // If you wanted to visually change them, you'd need to manually parse
        // and format the text, which is more complex.
    }

    [STAThread]
    public static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new RichTextBoxHyperlinkController());
    }
}
```

```csharp
using System;
using System.Diagnostics;
using System.IO;
using System.Text.RegularExpressions;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Documents;

namespace RichTextBoxHyperlinkToggle
{
    public partial class MainWindow : Window
    {
        private bool _isCtrlPressed = false;

        public MainWindow()
        {
            InitializeComponent();
            SetupRichTextBox();
        }

        private void SetupRichTextBox()
        {
            // Ensure the RichTextBox is configured to handle hyperlinks
            // This is typically done in XAML, but can be done programmatically if needed.
            // For this example, we assume the RichTextBox is named 'richTextBox' in XAML.

            // Add some initial content with hyperlinks for testing
            richTextBox.Document.Blocks.Clear();
            var paragraph = new Paragraph();
            paragraph.Inlines.Add("This is a ");
            var link1 = new Hyperlink(new Run("Google Link"));
            link1.NavigateUri = new Uri("https://www.google.com");
            link1.RequestNavigate += Hyperlink_RequestNavigate;
            paragraph.Inlines.Add(link1);
            paragraph.Inlines.Add(" and this is a ");
            var link2 = new Hyperlink(new Run("Microsoft Link"));
            link2.NavigateUri = new Uri("https://www.microsoft.com");
            link2.RequestNavigate += Hyperlink_RequestNavigate;
            paragraph.Inlines.Add(link2);
            richTextBox.Document.Blocks.Add(paragraph);

            // Set initial state of hyperlinks
            UpdateHyperlinkStates(false);
        }

        private void Window_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.LeftCtrl || e.Key == Key.RightCtrl)
            {
                _isCtrlPressed = true;
                UpdateHyperlinkStates(true);
            }
        }

        private void Window_KeyUp(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.LeftCtrl || e.Key == Key.RightCtrl)
            {
                _isCtrlPressed = false;
                UpdateHyperlinkStates(false);
            }
        }

        private void UpdateHyperlinkStates(bool isEnabled)
        {
            if (richTextBox.Document == null) return;

            foreach (var block in richTextBox.Document.Blocks)
            {
                if (block is Paragraph paragraph)
                {
                    foreach (var inline in paragraph.Inlines)
                    {
                        if (inline is Hyperlink hyperlink)
                        {
                            hyperlink.IsEnabled = isEnabled;
                            // Optionally, change visual appearance based on state
                            if (isEnabled)
                            {
                                hyperlink.Foreground = System.Windows.Media.Brushes.Blue; // Or any enabled color
                                hyperlink.TextDecorations = TextDecorations.Underline;
                            }
                            else
                            {
                                hyperlink.Foreground = System.Windows.Media.Brushes.Gray; // Or any disabled color
                                hyperlink.TextDecorations = null; // Remove underline
                            }
                        }
                    }
                }
            }
        }

        private void Hyperlink_RequestNavigate(object sender, RequestNavigateEventArgs e)
        {
            // Only navigate if Ctrl is pressed (hyperlinks are enabled)
            if (_isCtrlPressed)
            {
                try
                {
                    // Basic validation: Ensure it's a valid HTTP/HTTPS URL
                    if (e.Uri != null && (e.Uri.Scheme == Uri.UriSchemeHttp || e.Uri.Scheme == Uri.UriSchemeHttps))
                    {
                        Process.Start(new ProcessStartInfo(e.Uri.AbsoluteUri) { UseShellExecute = true });
                    }
                    else
                    {
                        MessageBox.Show("Invalid URL scheme. Only HTTP and HTTPS are allowed.", "Navigation Error", MessageBoxButton.OK, MessageBoxImage.Warning);
                    }
                }
                catch (Exception ex)
                {
                    // Log the exception securely, do not expose details to the user
                    // For demonstration, a simple MessageBox is used. In production, use a proper logging framework.
                    MessageBox.Show($"An error occurred while trying to open the link: {ex.Message}", "Navigation Error", MessageBoxButton.OK, MessageBoxImage.Error);
                }
            }
            else
            {
                // Optionally, provide feedback that the link is disabled
                // MessageBox.Show("Hyperlinks are currently disabled. Press and hold Ctrl to enable them.", "Link Disabled", MessageBoxButton.OK, MessageBoxImage.Information);
            }

            // Prevent the default navigation behavior if Ctrl is not pressed
            e.Handled = true;
        }

        // Ensure the window handles key events
        protected override void OnPreviewKeyDown(KeyEventArgs e)
        {
            base.OnPreviewKeyDown(e);
            if (e.Key == Key.LeftCtrl || e.Key == Key.RightCtrl)
            {
                // Mark as handled to prevent other controls from processing it if needed
                // but we still want the Window_KeyDown to fire.
                // If you want to prevent the RichTextBox from potentially consuming it,
                // you might need to handle it at a lower level or ensure focus.
            }
        }

        protected override void OnPreviewKeyUp(KeyEventArgs e)
        {
            base.OnPreviewKeyUp(e);
            if (e.Key == Key.LeftCtrl || e.Key == Key.RightCtrl)
            {
                // Mark as handled
            }
        }
    }
}
```

```xml
<!-- MainWindow.xaml -->
<Window x:Class="RichTextBoxHyperlinkToggle.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:RichTextBoxHyperlinkToggle"
        mc:Ignorable="d"
        Title="RichTextBox Hyperlink Toggle" Height="450" Width="800"
        KeyDown="Window_KeyDown"
        KeyUp="Window_KeyUp">
    <Grid>
        <RichTextBox x:Name="richTextBox" Margin="10" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" IsReadOnly="True"/>
    </Grid>
</Window>
```