要禁用鼠标激活，请执行以下操作：

class NonFocusableForm : Form
{
    protected override void DefWndProc(ref Message m)
    {
        const int WM_MOUSEACTIVATE = 0x21;
        const int MA_NOACTIVATE = 0x0003;

        switch(m.Msg)
        {
            case WM_MOUSEACTIVATE:
                m.Result = (IntPtr)MA_NOACTIVATE;
                return;
        }
        base.DefWndProc(ref m);
    }
}

要在不激活的情况下显示表单（在无边界表单的情况下，这是唯一一种对我有效的方法）：

[DllImport("user32.dll")]
    public static extern bool ShowWindow(IntPtr handle, int flags);

    NativeMethods.ShowWindow(form.Handle, 8);

标准的方法来做到这一点（似乎它不适用于所有的表单样式）：

protected override bool ShowWithoutActivation
    {
        get { return true; }
    }

如果有其他激活表单的方法，则可以以类似的方式抑制它们。

这是我使用的“NoFocusForm”：

public class NoFocusForm : Form
{
    /// From MSDN <see cref="https://learn.microsoft.com/en-us/windows/win32/winmsg/extended-window-styles"/>
    /// A top-level window created with this style does not become the 
    /// foreground window when the user clicks it. The system does not 
    /// bring this window to the foreground when the user minimizes or 
    /// closes the foreground window. The window should not be activated 
    /// through programmatic access or via keyboard navigation by accessible 
    /// technology, such as Narrator. To activate the window, use the 
    /// SetActiveWindow or SetForegroundWindow function. The window does not 
    /// appear on the taskbar by default. To force the window to appear on 
    /// the taskbar, use the WS_EX_APPWINDOW style.
    private const int WS_EX_NOACTIVATE = 0x08000000;

    public NoFocusForm()
    { 
        // my other initiate stuff
    }

    /// <summary>
    /// Prevent form from getting focus
    /// </summary>
    protected override CreateParams CreateParams
    {
        get
        {
            var createParams = base.CreateParams;

            createParams.ExStyle |= WS_EX_NOACTIVATE;
            return createParams;
        }
    }
}