public delegate void AsyncLog(string str1, string str2);

private void Test()
{
    Console.WriteLine("Test Start...");

    for (int i = 0; i < 100; i++)
    {
        AsyncLog asyLog1 = new AsyncLog(WriteLog);
        asyLog1.BeginInvoke("EventActionA" + i.ToString(), "EventContentA" + i.ToString(), null, null);

        AsyncLog asyLog2 = new AsyncLog(WriteLog);
        asyLog2.BeginInvoke("EventActionB" + i.ToString(), "EventContentB" + i.ToString(), null, null);

        AsyncLog asyLog3 = new AsyncLog(WriteLog);
        asyLog3.BeginInvoke("EventActionC" + i.ToString(), "EventContentC" + i.ToString(), null, null);
    }

    Console.WriteLine("Test End...");
}

public static object lockObject = new object();

private void WriteLog(string strEventType, string strEventContent)
{
    lock (lockObject)
    {
        Console.WriteLine("Write Log start... ThreadID:{0}", Thread.CurrentThread.ManagedThreadId);

        string strLogPath = string.Format("D:\\Log\\Log{0}.log", DateTime.Now.ToString("yyyy-MM-dd"));

        if (!File.Exists(strLogPath))
        {
            File.Create(strLogPath).Close();
        }

        FileStream fs = new FileStream(strLogPath, FileMode.Append, FileAccess.Write);
        StreamWriter sw = new StreamWriter(fs, Encoding.UTF8);

        sw.WriteLine(string.Format("{0}  {1}", strEventType, strEventContent));

        sw.Close();
        fs.Close();

        Console.WriteLine("Write Log End   ThreadID:{0}", Thread.CurrentThread.ManagedThreadId);
    }
}