如何正确使用并行计算对集合进行写操作？

using System;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Diagnostics;

namespace ParallelTest1
{
public class CalcTask
{
public event EventHandler<TaskItemEventArgs> OnTaskFinish;

public void ExecuteTasks()
{
int icount = 100;
List<Task> items = new List<Task>(icount);
PInitializeTask(items, icount);
Console.WriteLine(items.Count);

List<Task> items2 = new List<Task>();
SInitializeTask(items2, icount);

Console.WriteLine(items2.Count);

Console.ReadKey();
}

private void PInitializeTask(List<Task> items,int iCount)
{
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();

Parallel.For(0, iCount, new ParallelOptions()
{

}, i =>
{
lock (items)                //如果不加锁，观察最终集合的个数？
{
items.Add(new Task()
{
Source = i,
Dest = i + i,
});

//if (items.Count == iCount)
OnTaskFinish(this, new TaskItemEventArgs()
{
ThreadId=Thread.CurrentThread.ManagedThreadId,
Data=items,
});
}
});

stopwatch.Stop();
Console.WriteLine("Parallel loop time in milliseconds: {0}", stopwatch.ElapsedMilliseconds);

}

private void SInitializeTask(List<Task> items, int iCount)
{
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for(int i=0;i<iCount;i++)
{
items.Add(new Task()
{
Source = i,
Dest = i + i,
});

OnTaskFinish(this, new TaskItemEventArgs()
{
ThreadId = Thread.CurrentThread.ManagedThreadId,
Data = items,
});
}
stopwatch.Stop();
Console.WriteLine("Sequential loop time in milliseconds: {0}", stopwatch.ElapsedMilliseconds);
}
}

public class Task
{
public int Source { get; set; }
public int Dest { get; set; }

public override string ToString()
{
return string.Format("{0}-{1}", this.Source, this.Dest);
}
}

public class TaskItemEventArgs:EventArgs
{
public int ThreadId{get;set;}
public object Data{get;set;}
}
}

static void Main(string[] args)
{
Console.WriteLine("Processor Count:" + Environment.ProcessorCount);
CalcTask task1 = new CalcTask();
task1.OnTaskFinish += new EventHandler<TaskItemEventArgs>((sender, e) =>
{
Console.WriteLine(string.Format("Finish:{0}-{1}", e.ThreadId, (e.Data as List<Task>).Count));
});
task1.ExecuteTasks();

}

在上述代码中，我测试的结果规律如下：
如果并行中，不加lock，如果是向集合填充100万个对象 ，那么并行代码 执行完毕后，那个集合的数目永远都是小于100万，我的结果显示是80多万。。。。估计中间发生冲突导致部分的项添加不成功，但是.net并没有报错，这样很容易让我们中招。所以用一般集合时需要小心，这里也证实了一般的集合List<T>并不是线程安全的。。。

后来我换成System.Collections.Concurrent.ConcurrentDictionary<int, Task>试了下，发现不用加lock，结果也是正确的，所以大家在并行运算中记得多使用这个哈希结构。。。但是线程安全的。