public partial class Form1 : Form
{
    [DllImport("user32.dll")]
    public extern static bool ShutdownBlockReasonCreate(IntPtr hWnd, [MarshalAs(UnmanagedType.LPWStr)] string pwszReason);

    private bool blocked = false;

    protected override void WndProc(ref Message aMessage)
    {
        const int WM_QUERYENDSESSION = 0x0011;
        const int WM_ENDSESSION = 0x0016;

        if (blocked && (aMessage.Msg == WM_QUERYENDSESSION || aMessage.Msg == WM_ENDSESSION))
            return;

        base.WndProc(ref aMessage);
    }

    void Button1_Click(object sender, FormClosingEventArgs e)
    {
        if (ShutdownBlockReasonCreate(this.Handle, "DONT:"))
        {
            blocked = true;
            MessageBox.Show("Shutdown blocking succeeded");
        }
        else
            MessageBox.Show("Shutdown blocking failed");
    }
}