borderless-window.cs

public MainPage()
{
InitializeComponent();

#if WINDOWS

// WindowLongFlags
const int GWL_STYLE = -16;
const int GWL_EXSTYLE = -20;

// WindowStyles
const int WS_CAPTION = 0xc00000;
const int WS_THICKFRAME = 0x00040000;
const int WS_MINIMIZE = 0x20000000;
const int WS_MAXIMIZE = 0x01000000;
const int WS_SYSMENU = 0x00080000;
const int WS_EX_DLGMODALFRAME = 0x00000001;
const int WS_EX_CLIENTEDGE = 0x00000200;
const int WS_EX_STATICEDGE = 0x00020000;

// SetWindowPosFlags
const uint SWP_FRAMECHANGED = 0x0020;
const uint SWP_NOMOVE = 0x0002;
const uint SWP_NOSIZE = 0x0001;
const uint SWP_NOZORDER = 0x0004;
const uint SWP_NOOWNERZORDER = 0x0200;

Loaded += MainPage_Loaded;

void MainPage_Loaded(object sender, EventArgs e)
{
    var wnd = Window.Handler.PlatformView as Microsoft.UI.Xaml.Window;
    var hwnd = WinRT.Interop.WindowNative.GetWindowHandle(wnd);

    var lStyle = GetWindowLongPtr(hwnd, GWL_STYLE);
    lStyle &= ~(WS_CAPTION | WS_THICKFRAME | WS_MINIMIZE | WS_MAXIMIZE | WS_SYSMENU);
    SetWindowLongPtr(hwnd, GWL_STYLE, lStyle);

    var lExStyle = GetWindowLongPtr(hwnd, GWL_EXSTYLE);
    lExStyle &= ~(WS_EX_DLGMODALFRAME | WS_EX_CLIENTEDGE | WS_EX_STATICEDGE);
    SetWindowLongPtr(hwnd, GWL_EXSTYLE, lExStyle);

    SetWindowPos(hwnd, IntPtr.Zero, 0, 0, 0, 0, SWP_FRAMECHANGED | SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER | SWP_NOOWNERZORDER);
}

static IntPtr GetWindowLongPtr(IntPtr hWnd, int nIndex) =>
    IntPtr.Size == 8
        ? GetWindowLongPtr64(hWnd, nIndex)
        : GetWindowLongPtr32(hWnd, nIndex);

static IntPtr SetWindowLongPtr(IntPtr hWnd, int nIndex, IntPtr dwNewLong) =>
    IntPtr.Size == 8
        ? SetWindowLongPtr64(hWnd, nIndex, dwNewLong)
        : new IntPtr(SetWindowLong32(hWnd, nIndex, dwNewLong.ToInt32()));

[DllImport("user32.dll", SetLastError = true)]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

[DllImport("user32.dll", EntryPoint = "GetWindowLong")]
static extern IntPtr GetWindowLongPtr32(IntPtr hWnd, int nIndex);

[DllImport("user32.dll", EntryPoint = "GetWindowLongPtr")]
static extern IntPtr GetWindowLongPtr64(IntPtr hWnd, int nIndex);

[DllImport("user32.dll", EntryPoint = "SetWindowLong")]
static extern int SetWindowLong32(IntPtr hWnd, int nIndex, int dwNewLong);

[DllImport("user32.dll", EntryPoint = "SetWindowLongPtr")]
static extern IntPtr SetWindowLongPtr64(IntPtr hWnd, int nIndex, IntPtr dwNewLong);
#endif
}