null values in List<string> when adding and removing in multiple threads

Question

I know the C# System.Collections.Generic.List object is not thread safe. But I am wondering why this piece of code generates null values.

Task.Run(() =>
{
    for (var i = 0; i < 10; i++)
    {
        var str = $"Test {i}";
        list.Add(str);

        if (i == 9)
        {
            i = 0;
        }
    }
});

Task.Run(() =>
{
    while (true)
    {
        list.Remove("Test 1");
        list.Remove("Test 2");
        list.Remove("Test 3");
        list.Remove("Test 4");
        list.Remove("Test 5");
        list.Remove("Test 6");
        list.Remove("Test 7");
        list.Remove("Test 8");
        list.Remove("Test 9");
    }
});

This is a part of the list after some seconds:

The thread which is responsible to remove the entries from the list can crash, if the entry is not present in the list. Therefore and for other multithreading reasons I understand why some objects are not removed from the list, but I do not understand how these null values are generated. Has anyone an explanation how these values are generated?

Answer 1

in addition to @Marc Gravell answer.

but I do not understand how these null values are generated

The first thread continuously adds "Test {i}". However, the next thread removes "Test {i}". Therefore, Those null values are as a result of removal action of the next thread.

of worth to say that, next "Test {i}" won't be replaced to removed one but appended at the end of the generic collection.

The final result would behave like below:

Test 1 null Test 3 null null Test 6 .... Test 1 Test 2 null null Test 6 ...  

Answer 2

List<T> is not thread-safe except for N reads and zero writes; any non-zero number of writes alongside anything else is not supported, and such behavior is completely undefined. If you need concurrency: either add synchronization, or use a concurrent collection type.