"is this use of parallel.foreach() thread safe?" Code Answer

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.

By MSL on January 26 2022