C#

[StructLayout(LayoutKind.Sequential)]
private struct RECT
{
	public int left;
	public int top;
	public int right;
	public int bottom;
}
[DllImport("user32.dll")]
private static extern int GetClientRect(IntPtr hWnd, ref RECT rect);

if (IntPtr.Zero != hWnd)
{
	Application.Run(new MainForm(hWnd));
}

private IntPtr parentWindow;
public MainForm(IntPtr parent)
{
	parentWindow = parent;
	if (IntPtr.Zero != parent)
		this.TopLevel = false;
.
.
.
}

protected override CreateParams CreateParams
{
	get
	{
		CreateParams cp = base.CreateParams;
		cp.Parent = parentWindow;
		return cp;
	}
}

const int WS_SYSMENU = 0x00080000;
protected override System.Windows.Forms.CreateParams CreateParams
{
	get
	{
		CreateParams cp = base.CreateParams;
		cp.Style |= WS_SYSMENU;
		return cp;
	}
}

const uint MF_BYCOMMAND = 0x00000000;
const uint MF_GRAYED    = 0x00000001;
const uint MF_DISABLED  = 0x00000002;
const uint SC_CLOSE     = 0xF060;


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


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

	IntPtr hMenu = GetSystemMenu(Handle, false);
	if (IntPtr.Zero != hMenu)
	{
		EnableMenuItem(
			hMenu,
			SC_CLOSE,
			MF_BYCOMMAND | MF_DISABLED | MF_GRAYED
			);
	}
}

const int CS_NOCLOSE = 0x0200;
protected override System.Windows.Forms.CreateParams CreateParams
{
	get
	{
		CreateParams cp = base.CreateParams;
		cp.ClassStyle |= CS_NOCLOSE;
		return cp;
	}
}