C# List.AddRange in Parallel.For occur ArgumentException

Question

My code like that.

List<MyPanel> list_panel = new List<MyPanel>();
.......
List<string> list_sql = new List<string>();
Parallel.For(0, list_panel.Count, i =>
{
    if (list_panel[i].R == 0)
    {
        list_sql.AddRange(list_panel[i].MakeSqlForSave()); // it returns two string
    }
});

But AddRange occur System.ArgumentException sometimes.

I found 'list isn't for multi write'. So I fix it using lock.

string[] listLock = new string[2];
Parallel.For(0, list_panel.Count, i =>
{
    if (list_panel[i].R == 0)
    {
        listLock = list_panel[i].MakeSqlForSave();    
        lock(listLock)
            list_sql.AddRange(listLock);
    }
});

But it still occur System.ArgumentException that 'Source array was not long enough. Check srcIndex and length, and the array's lower bounds.' sometimes.

An error occurred in list_sql. If the count is 34, but the IndexOfRangeException occurs when you call list_sql[32] and list_sql[33].

How can I handle it?

List<T> (and especially List<T>.AddRange) is not thread-safe. — Dai
– Dai, Commented Jan 2, 2020 at 2:24
Your lock() statement is incorrect because it locks on a thread-local object rather than a shared object. — Dai
– Dai, Commented Jan 2, 2020 at 2:25
Is MyPanel a WinForms control? If so, this code is unsafe because WinForms controls should only be accessed from the GUI thread. — Dai
– Dai, Commented Jan 2, 2020 at 2:29
You reassign listlock, then lock on it. This makes the lock meaningless as you will have many locks. You don't lock on the variable, you lock on the (new) object it refers to. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Jan 2, 2020 at 13:17

Wai Ha Lee · Accepted Answer · 2020-01-02 10:15:48Z

3

Use ConcurrentBag<T> as a thread-safe collection that you can safely append to from multiple threads:

ConcurrentBag<String> result = new ConcurrentBag<String>();

Parallel.For(0, list_panel.Count, i =>
{
    if (list_panel[i].R == 0)
    {
        foreach( String s in list_panel[i].MakeSqlForSave() )
        {
            result.Add( s );
        }
    }
});

List<String> list_sql = result.Select( s => s ).ToList(); // Serialize to a single List<T> after the concurrent operations are complete.

edited Jan 2, 2020 at 10:15

Wai Ha Lee

8,856102 gold badges61 silver badges100 bronze badges

answered Jan 2, 2020 at 2:28

Dai

158k31 gold badges314 silver badges436 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Theodor Zoulias · Accepted Answer · 2020-01-02 13:04:19Z

1

You must use a dedicated lock for the specific List, and use it every time you access this list (for both read and write).

List<MyPanel> list_panel = new List<MyPanel>();
List<string> list_sql = new List<string>();
object listSqlLock = new object();
Parallel.For(0, list_panel.Count, i =>
{
    if (list_panel[i].R == 0)
    {
        var sqlCommands = list_panel[i].MakeSqlForSave();
        lock (listSqlLock)
            list_sql.AddRange(sqlCommands);
    }
});

answered Jan 2, 2020 at 13:04

Theodor Zoulias

46.1k8 gold badges112 silver badges155 bronze badges

Collectives™ on Stack Overflow

C# List.AddRange in Parallel.For occur ArgumentException

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related