Microsoft's own control the brightness of the screen the API, is indeed a call SetDeviceGammaRamp () the function, there is a very detailed graphic tutorials you can look at: http://bit.ly/2EkTArr
Create a new application form, drag a slider and a button, the code sent to you, you can try:

 using System; 
using System.Collections.Generic; 
Using System.Com ponentModel; 
Using System. The Data; 
Using System. Drawing; 
Using System. Linq; 
using System.Text; 
Using System. The Threading. The Tasks; 
Using System. Windows. Forms; 
Using System. The Runtime. InteropServices; 
The namespace brightnesscontrol 
{
Public partial class Form1: Form 
{
[DllImport (" gdi32. DLL)] 
Private unsafe static extern bool SetDeviceGammaRamp (Int32 HDC, void * ramp). 
Private static bool initialized=false; 
Private static Int32 HDC. 
Private static int a; 
Public _click () 
{
InitializeComponent(); 
} 
Private static void InitializeClass () 
{
If (the initialized) 
return; 
HDC=Graphics. FromHwnd (IntPtr. Zero.) GetHdc () ToInt32 (); 
An initialized=true; 
} 
Public static unsafe bool SetBrightness (int brightness) 
{
InitializeClass (); 
If (brightness & gt; 255) 
Brightness=255; 
If (brightness & lt; 0) 
Brightness=0; 
Short * gArray=stackalloc short [3 * 256]; 
Short * independence idx=gArray. 
For (int j=0; J & lt; 3; J++) 
{
For (int I=0; I & lt; 256; I++) 
{
Int arrayVal=I * (brightness + 128); 
If (arrayVal & gt; 65535) 
ArrayVal=65535; 
* independence idx=(short) arrayVal; 
Idx++; 
} 
} 
Bool retVal=SetDeviceGammaRamp (HDC, gArray); 
Return retVal; 
} 
Private void trackBar1_Scroll (object sender, EventArgs e) 
{
} 
Private void button1_Click (object sender, EventArgs e) 
{
A=trackBar1. Value; 
SetBrightness (a); 
} 
} 
} 

You put the value of the slider changed try