Disable Close Button In Title Bar of a WPF Window (C#)

You have to override and in OnCLosing event set e.cancel=true

public MyWindow()
{
    InitializeComponent();
    this.Closing += new System.ComponentModel.CancelEventHandler(MyWindow_Closing);
}

void MyWindow_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
    e.Cancel = true;
}

This post which an answer using Behavior, GetWindowLong and SetWindowLong:

public class HideCloseButtonOnWindow : System.Windows.Interactivity.Behavior<Window>
{
    #region bunch of native methods

    private const int GWL_STYLE = -16;
    private const int WS_SYSMENU = 0x80000;

    [DllImport("user32.dll", SetLastError = true)]
    private static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32.dll")]
    private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

    #endregion

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.Loaded += OnLoaded;
    }

    protected override void OnDetaching()
    {
        AssociatedObject.Loaded -= OnLoaded;
        base.OnDetaching();
    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        var hwnd = new System.Windows.Interop.WindowInteropHelper(AssociatedObject).Handle;
        SetWindowLong(hwnd, GWL_STYLE, GetWindowLong(hwnd, GWL_STYLE) & ~WS_SYSMENU);
    }
}

How to use it:

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
    xmlns:w="clr-namespace:WpfApplication2">

<i:Interaction.Behaviors>
    <w:HideCloseButtonOnWindow />
</i:Interaction.Behaviors>

</Window>

Try this:

public partial class MainWindow : Window
{

    [DllImport("user32.dll")]
    static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

    [DllImport("user32.dll")]
    static extern bool EnableMenuItem(IntPtr hMenu, uint uIDEnableItem, uint uEnable);


    const uint MF_BYCOMMAND = 0x00000000;
    const uint MF_GRAYED = 0x00000001;

    const uint SC_CLOSE = 0xF060;

    public MainWindow()
    {
        InitializeComponent();
    }

    protected override void OnSourceInitialized(EventArgs e)
    {
        base.OnSourceInitialized(e);

        // Disable close button
        IntPtr hwnd = new WindowInteropHelper(this).Handle;
        IntPtr hMenu = GetSystemMenu(hwnd, false);
        if (hMenu != IntPtr.Zero)
        {
            EnableMenuItem(hMenu, SC_CLOSE, MF_BYCOMMAND | MF_GRAYED);
        }
    }
}

Taken from here.

Make sure you set the ResizeMode to NoResize.