MonitorBrightness.cs

public static ushort GetMonitorBrightness() {
    using (var mclass = new ManagementClass("WmiMonitorBrightness")) {
        mclass.Scope = new ManagementScope(@"\\.\root\wmi");
        using (var instances = mclass.GetInstances()) {
            foreach (ManagementObject instance in instances) {
                return (byte) instance.GetPropertyValue("CurrentBrightness");
            }
        }
    }
    return 0;
}

public static void SetMonitorBrightness(ushort brightness) {
    using (var mclass = new ManagementClass("WmiMonitorBrightnessMethods")) {
        mclass.Scope = new ManagementScope(@"\\.\root\wmi");
        using (var instances = mclass.GetInstances()) {
            foreach (ManagementObject instance in instances) {
                object[] args = new object[] { 1, brightness };
                instance.InvokeMethod("WmiSetBrightness", args);
            }
        }
    }
}