Is it safe to use List with Parallel.Foreach?

Question

I’m making one by making a style method Crawler which receives a string list, makes a call on a website, inserts the return into another list and returns.

My method is working, but it’s too slow, so I want to parallelize the calls.
So I researched the Parallel.ForEach easily solves this problem, but it is safe to use it with List?

My current code:

private List<Veiculo> ObtemTodosVeiculos(List<string> modelos)
{
    List<Veiculo> veiculos = new List<Veiculo>();
    foreach (var modelo in modelos)
    {
        string retorno = GET(string.Format("https://192.168.0.1/modelo/{0}", modelo));
        var veiculo = Deserializa<Veiculo>(retorno);
        if (veiculo != null)
            veiculos.Add(veiculo);
    }
    return veiculos;
}

Answer 1

My method is working, but it is very slow, so I want to parallelize the calls. So I searched the Parallel.ForEach easily solves this problem, but it is safe to use it with List?

Not

List and any class outside of System.Collections.Concurrent are not thread-safe. Prefer one of the collections listed here.

Another way is protect the list in a critical region, but this will bring other performance penalties because List is not optimized for competition.

private Object obj = new Object();
...

var veiculo = Deserializa<Veiculo>(retorno);
lock (obj) 
{
    if (veiculo != null)
        veiculos.Add(veiculo);
}

Answer 2

It depends. Whether you can ensure that the list will not be modified while the Parallel.ForEach is running so you will have no problem. Otherwise, follow @Ciganomorrisonmendez’s suggestion.

Of documentation of List<T> in the MSDN, thread-safe access section: "It is safe to perform multiple read operations on a List, but problems can occur if the collection is modified while being read".

As Bruno Costa mentioned in the comment - in the above case there is no problem in using the list modelos in parallel. However, the list veiculos is being modified into multiple threads, so the type of this variable should be changed to one of the threadsafe collections.