Is this use of Parallel.ForEach() thread safe?

c#, multithreading, parallel-processing, parallel.foreach, thread-safety

Solution

Yes; `List<T>` is not thread safe, so adding to it ad-hoc from arbitrary threads (quite possibly at the same time) is doomed. You should use a thread-safe list instead, or add locking manually. Or maybe there is a `Parallel.ToList`.

Also, if it matters: insertion order will not be guaranteed.

This version is safe, though:

var output = new string[data.Count];

Parallel.ForEach<String>(data, (line,state,index) =>
{
    String outputLine = index.ToString();
    // ** Do something with "line" and store result in "outputLine" **

    // Additionally, there are some this.Invoke statements for updating UI
    output[index] = outputLine;
});

here we are using `index` to update a different array index per parallel call.

Problem

Essentially, I am working with this: ``` var data = input.AsParallel(); List<String> output = new List<String>(); Parallel.ForEach<String>(data, line => { String outputLine = ""; // ** Do something with "line" and store result in "outputLine" ** // Additionally, there are some this.Invoke statements for updating UI output.Add(outputLine); }); ``` Input is a `List<String>` object. The `ForEach()` statement does some processing on each value, updates the UI, and adds the result to the `output` `List`. Is there anything inherently wrong with this? Notes: - Output order is unimportant Update: Based on feedback I've gotten, I've added a manual `lock` to the `output.Add` statement, as well as to the UI updating code.

Original source