I implemented a sample application to see the behavior of the retries inside a parallel foreach loop. As per my observations following application not showing all the values inside the array ,in the textbox (Showing character number changes randomly).

I want to know what cause this behavior. Thanks

    public string DisplayText = "Numbers =";

    private void button1_Click(object sender, EventArgs e)
    {
        int[] numbers = { 1, 2, 3,4,5,6,7,8,9};
        Start(numbers);
        textBox1.Text = DisplayText;
    }
    
    private void Start(int[] numbers)
    {
        Parallel.ForEach(numbers, k =>
        {
            Write(k.ToString());
        });
    }

    private void Write(string text)
    {
        int maxRetryCount = 3;
        int retryCount = 0;
        int retryInterval = 1000;

        while (retryCount < maxRetryCount)
        {
            if(retryCount == maxRetryCount-1)
            {
                DisplayText = DisplayText + "," + text;
                break;
            }

            Thread.Sleep(retryInterval);
            retryCount++;
        }
    }