Why does trackbar work as only ON and OFF mode?

Question

When i scrolled this trackbar right or left, The value should has changed from zero to its max. value (255) and the LED at the output should has been brighter over trackbar values while scrolling right.

But this trackbar works like switch. Before trackbar is 127 the led is off and after that, led suddenly turns on. Where do you think problem is?

c# code

{
public partial class Form1 : Form
{
    SerialPort serialport;
    public Form1()
    {
        InitializeComponent();
        serialport = new SerialPort();
        serialport.BaudRate = 9600;

    }

    private void Form1_Load(object sender, EventArgs e)
    {
        textBox1.Text = "COM3";
    }

    private void button1_Click(object sender, EventArgs e)
    {
        timer1.Start();
        try
        {
            serialport.PortName = textBox1.Text;
            if (!serialport.IsOpen)
                serialport.Open();
            MessageBox.Show("connected");
        }
        catch
        {
            MessageBox.Show("error!");
        }
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        try
        {
            string ledval = trackBar1.Value.ToString();
            textBox2.Text = ledval;
            serialport.WriteLine(ledval);
            serialport.WriteLine(",");
        }
        catch (Exception ex) { }
    }

    private void button2_Click(object sender, EventArgs e)
    {
        timer1.Stop();
        serialport.Close();
    }
}}

arduino code

int led = 13;
String x;
int val;

void setup(){
Serial.begin(9600);
pinMode(led, OUTPUT);
}

void loop(){
if(Serial.available()>0)
{
 x=Serial.readStringUntil(',');
 val=x.toInt();
 analogWrite(led, val);
}
}