（有时）词典中不存在给定的键

Question

I am using the code below to start threads with a list parameter but sometimes it throws an exception:

The given key was not present in the dictionary

From this line:

Thread MoveThread = new Thread(() => MoveTask(ControllerDictionary[i]));

How can I fix that error?

Full code:

var ControllerDictionary = ConfigFile.ControllerList.Select((c, i) => new { Controller = c, Index = i })
    .GroupBy(x => x.Index % AppSettings.SimultaneousProcessNumber)
    .Select((g, i) => new { GroupIndex = i, Group = g })
    .ToDictionary(x => x.GroupIndex, x => x.Group.Select(xx => xx.Controller).ToList());

for (int i = 0; i < ControllerDictionary.Count; i++)
{
     Thread MoveThread = new Thread(() => MoveTask(ControllerDictionary[i]));
     MoveThread.Start();

     foreach (var Controller in ControllerDictionary[i])
         Logger.Write(string.Format("{0} is in move thread {1}.", Controller.Ip, (i + 1)),EventLogEntryType.Information, AppSettings.LogInfoMessages);
}

Answer 1

You're capturing the variable i , rather than its value. So currently you could have several threads calling MoveTask using the same index... and sometimes the value of i could be equal to ControllerDictionary.Count .

If you take a copy of i into a variable within the loop, that fixes the problem as you'll get a separate variable on each iteration of the loop:

for (int i = 0; i < ControllerDictionary.Count; i++)
{
    int index = i;
    Thread MoveThread = new Thread(() => MoveTask(ControllerDictionary[index]));
    ... 
}

Or even better, extract the ControllerDictionary fetch from the thread entirely:

for (int i = 0; i < ControllerDictionary.Count; i++)
{
    var value = ControllerDictionary[i];
    Thread MoveThread = new Thread(() => MoveTask(value));
    ... 
}

Additionally, it's not really clear why you're using a dictionary at all. Given that you know the keys will all be in the range [0, count) why don't you just use an array? You'd change your query to:

var controllerLists = ConfigFile.ControllerList
    .Select((c, i) => new { Controller = c, Index = i })
    .GroupBy(x => x.Index % AppSettings.SimultaneousProcessNumber)
    .Select(g => g.Select(xx => xx.Controller).ToList())
    .ToArray();

Answer 2

The problem is here:

Thread MoveThread = new Thread(() => MoveTask(ControllerDictionary[i]));

That anonymous function is going to be executed on a different thread and there is no guarantee that the local variables are going to be valid.