Arduino - PC communication

I wrote a small wpf programm to control some LED's. I built 4 different modi:

LEDON , LEDOFF, DIMMING, FLASH

It's very simple and works fine. I send a,b,c or d (4 different buttons) to the arduino and execute the corresponding loop.

Now I try to create a slider in my WPF program that can control the LED brightness. My slider creates values from 0 to 255 which I send to the arduino.

Now I ask if the incoming byte is a number, if yes I execute a method which should determine the brightness using analog.write(PIN,value);.

WPF:

 private void targetSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)        {            try            {                var val = Math.Round(targetSlider.Value).ToString();                Console.WriteLine("WPF:  " + val);                serialPort1.Write(val);                var ArduVal = serialPort1.ReadByte();                Console.WriteLine("Arduino: " + ArduVal);            }            catch (Exception ex)            {                MessageBox.Show(ex.Message);            }        }

Arduino:

void loop()  {    checkSerial();    if(lightON)LightON();    else if(lightOFF)LightOFF();    else if(lightDIM) LightDIM();    else if(lightBLINK) LightBLINK();  }  void checkSerial()  {    while(Serial.available()>0){      int incoming=Serial.read();      if(isdigit(incoming)){        executeSlider(incoming);        Serial.print(incoming);      }      else{          char c=incoming;          executeCommand(c);        }    }  }   void executeSlider(int val){        analogWrite(BLUE, val);        analogWrite(GREEN, val);        analogWrite(RED, val);    }

As you can see, I check the values of the slider (which are sent to the arduino) and the values which arrive at the arduino.

The arduino values always commute around 50. I don't understand why? Should I pass an array? Or is something wrong with arudino-PC communication?

Console Output:

WPF:  2 Arduino: 53 WPF:  4 Arduino: 48 WPF:  4 Arduino: 53 WPF:  5 Arduino: 50 WPF:  6 Arduino: 53 WPF:  8 Arduino: 50 WPF:  8 Arduino: 53 WPF:  10 Arduino: 51 WPF:  10 Arduino: 53 WPF:  12 Arduino: 52 WPF:  13 Arduino: 53 WPF:  14 Arduino: 54 WPF:  14 Arduino: 53 WPF:  15 Arduino: 54 WPF:  16 Arduino: 52 WPF:  16 Arduino: 57 WPF:  17 Arduino: 52 WPF:  18 Arduino: 56 WPF:  20 Arduino: 52 WPF:  20 Arduino: 57 WPF:  21 Arduino: 52 WPF:  22 Arduino: 56 WPF:  23 Arduino: 52 WPF:  25 Arduino: 57 WPF:  26 Arduino: 53 WPF:  27 Arduino: 48 WPF:  29 Arduino: 52 WPF:  30 Arduino: 57 WPF:  31 Arduino: 53 WPF:  32 Arduino: 49 WPF:  33 Arduino: 52 WPF:  34 Arduino: 57 WPF:  35 Arduino: 53 WPF:  37 Arduino: 50