C# 只读列表线程同步

问：

我在这里所做的是在多个线程中通过 for-each 和 index 方法导航只读列表。结果看起来线程安全，但我不相信。

谁能告诉下面的代码（从只读列表中读取）是线程安全的？如果是，那为什么？

public class ThreadTest
{
   readonly List<string> port;

    public  ThreadTest()
    {
        port = new List<string>() { "1", "2", "3", "4", "5", "6" };            
    }

    private void Print()
    {
        foreach (var itm in port)
        {
            Thread.Sleep(50);
            Console.WriteLine(itm+"----"+Thread.CurrentThread.ManagedThreadId);
        }
    }

    private void Printi()
    {
        for(int i=0;i<5;i++)
        {
            Thread.Sleep(100);
            Console.WriteLine(port[i] + "--iiiii--" + Thread.CurrentThread.ManagedThreadId);
        }
    }

    public void StartThread()
    {
        Task[] tsks = new Task[10];
        tsks[0] = new Task(Print);
        tsks[1] = new Task(Print);
        tsks[2] = new Task(Print);
        tsks[3] = new Task(Print);
        tsks[4] = new Task(Print);
        tsks[5] = new Task(Printi);
        tsks[6] = new Task(Printi);
        tsks[7] = new Task(Printi);
        tsks[8] = new Task(Printi);
        tsks[9] = new Task(Printi);

        foreach (var tsk in tsks)
        {
            tsk.Start();
        }

        Task.WaitAll(tsks);
    }
}

class Program
{
    static void Main(string[] args)
    {

        new ThreadTest().StartThread();

        Console.ReadLine();
    }
}