Task List Throws "Collection was modified; enumeration operation may not execute."

Question

Creating a list of Tasks to run parallel like this:

if (call.Item.PictureDetails != null)
{
    List<Task> lstTasks = new List<Task>();
    foreach (var url in call.Item.PictureDetails.PictureURL.Cast<string>().ToList())
    {
        var newTile = AddNewPicTile(url);
        lstTasks.Add(Task.Run(() => {
            UploadPicture(newTile);
        }));
    }
    // Run all pic upload tasks in parallel...
    await Task.WhenAll(lstTasks);
}

Execution throws "Collection was modified..." exception but I can't figure out why. I use similar code to this elsewhere in my app where I perform a while() loop that reads data from a database and it works fine. Like this...

        SqlDataReader rdr = cmd.ExecuteReader();
        if (rdr.HasRows)
        {
            while (rdr.Read())
            {
                RadTileElement newTile = AddNewPicTile(rdr["PictureUrl"].ToString());   //...must create this var here otherwise we won't be able to access rdr from a background thread (it will be closed by then)
                
                lstTasks.Add(Task.Run(() => {
                    UploadPicture(newTile);
                }));
            }
        }
        rdr.Close();
    }
    conn.Close();
}

// Run all pic upload tasks in parallel...
await Task.WhenAll(lstTasks);

What is it about the string list that could cause it to yell at me like that?

Answer 1

Actually - problem was not related to List. Not even in the code posted. Thanks to Caius Jard who got me looking elsewhere for it.