Set the display brightness from a Win32 application

BrightnessHelper.cs

public static class BrightnessHelper
{
	public static void SetBrightness(uint brightness)
    	{
    		if (brightness > 100)
        	{
        		throw new ArgumentOutOfRangeException("brightness");
        	}

        	// get handle to primary display
        	IntPtr hmon = NativeMethods.MonitorFromWindow(IntPtr.Zero, NativeMethods.MONITOR_DEFAULTTO.PRIMARY);

        	// get number of physical displays (assume only one for simplicity)
        	int num;
        	bool success = NativeMethods.GetNumberOfPhysicalMonitorsFromHMONITOR(hmon, out num);
        	NativeMethods.PHYSICAL_MONITOR pmon = new NativeMethods.PHYSICAL_MONITOR();

        	success = NativeMethods.GetPhysicalMonitorsFromHMONITOR(hmon, num, ref pmon);

        	uint min, max, current;
        	// commonly min and max are 0-100 which represents a percentage brightness
        	success = NativeMethods.GetMonitorBrightness(pmon.hPhysicalMonitor, out min, out current, out max);

        	// set to full brightness
        	success = NativeMethods.SetMonitorBrightness(pmon.hPhysicalMonitor, brightness);
        
        	success = NativeMethods.DestroyPhysicalMonitors(num, ref pmon);
	}

   	private static class NativeMethods
   	{
                [DllImport("Dxva2")]
                [return: MarshalAs(UnmanagedType.Bool)]
                internal static extern bool GetMonitorBrightness(IntPtr hMonitor,
                    out uint pdwMinimumBrightness,
                    out uint pdwCurrentBrightness,
                    out uint pdwMaximumBrightness);

                [DllImport("Dxva2")]
                [return: MarshalAs(UnmanagedType.Bool)]
                internal static extern bool SetMonitorBrightness(IntPtr hMonitor, uint newBrightness);

                [DllImport("Dxva2")]
                [return: MarshalAs(UnmanagedType.Bool)]
                internal static extern bool GetNumberOfPhysicalMonitorsFromHMONITOR(IntPtr hMonitor, out int numberOfPhysicalMonitors);

                [DllImport("Dxva2", CharSet = CharSet.Unicode)]
                [return: MarshalAs(UnmanagedType.Bool)]
                internal static extern bool GetPhysicalMonitorsFromHMONITOR(IntPtr hMonitor, int physicalMonitorArraySize, ref PHYSICAL_MONITOR physicalMonitorArray);

                [DllImport("Dxva2", CharSet = CharSet.Unicode)]
                [return: MarshalAs(UnmanagedType.Bool)]
                internal static extern bool DestroyPhysicalMonitors(int physicalMonitorArraySize, ref PHYSICAL_MONITOR physicalMonitorArray);

                [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
                internal struct PHYSICAL_MONITOR
                {
                    internal IntPtr hPhysicalMonitor;
                    //[PHYSICAL_MONITOR_DESCRIPTION_SIZE]
                    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
                    internal string szPhysicalMonitorDescription;
                }

                [DllImport("User32")]
                internal static extern IntPtr MonitorFromWindow(IntPtr hwnd, MONITOR_DEFAULTTO dwFlags);

                internal enum MONITOR_DEFAULTTO
                {
                    NULL = 0x00000000,
                    PRIMARY = 0x00000001,
                    NEAREST = 0x00000002,
                }
	}
}

For laptops you can use:
https://github.com/mbahrami/LaptopBrightnessControl/blob/master/SetBrightness.cs