#region 无焦点窗体

[System.Runtime.InteropServices.DllImport("user32.dll")]
private extern static IntPtr SetActiveWindow(IntPtr handle);
private const int WM_ACTIVATE = 0x006;
private const int WM_ACTIVATEAPP = 0x01C;
private const int WM_NCACTIVATE = 0x086;
private const int WA_INACTIVE = 0;
private const int WM_MOUSEACTIVATE = 0x21;
private const int MA_NOACTIVATE = 3;
protected override void WndProc(ref Message m)
{
if (m.Msg == WM_MOUSEACTIVATE)
{
m.Result = new IntPtr(MA_NOACTIVATE);
return;
}
else if (m.Msg == WM_NCACTIVATE)
{
if (((int)m.WParam & 0xFFFF) != WA_INACTIVE)
{
if (m.LParam != IntPtr.Zero)
{
SetActiveWindow(m.LParam);
}
else
{
SetActiveWindow(IntPtr.Zero);
}
}
}
base.WndProc(ref m);
}

#endregion

C# 实现无焦点窗体（转载）