在下面的代码中，即使仅在lock语句中修改mCounter变量，有时也会显示CheckCounter方法。如果我注解DoAnythingElseWithUI调用，则永远不会出现问题。DoAnythingElseWithUI似乎中断了lock语句，并允许timer1事件在timer2事件释放锁之前继续。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Timers;
using System.Windows.Forms;

namespace LockThreadTest
{
    public partial class Form1 : Form
    {
        private SynchronizationContext mUIContext;
        private System.Timers.Timer mTimer1 = new System.Timers.Timer();
        private System.Timers.Timer mTimer2 = new System.Timers.Timer();

        public delegate void CompletedEventHandler(object sender);
        public event CompletedEventHandler CompletedEvent1;
        public event CompletedEventHandler CompletedEvent2;

        private object lockObject = new object();

        private static int mCounter = 0;

        public Form1()
        {
            mUIContext = WindowsFormsSynchronizationContext.Current;

            InitializeComponent();

            mTimer1.Interval = 1000;
            mTimer2.Interval = 1000;
            mTimer1.Elapsed += new System.Timers.ElapsedEventHandler(Timer1_Elapsed);
            mTimer2.Elapsed += new System.Timers.ElapsedEventHandler(Timer2_Elapsed);

            this.CompletedEvent1 += Form1_CompletedEvent1;
            this.CompletedEvent2 += Form1_CompletedEvent2;
        }

        public virtual void OnCompletedEvent1()
        {
            if (CompletedEvent1 != null)
                CompletedEvent1(this);
        }

        public virtual void OnCompletedEvent2()
        {
            if (CompletedEvent2 != null)
                CompletedEvent2(this);
        }

        private void Timer1_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            OnCompletedEvent1();
        }

        private void Timer2_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            OnCompletedEvent2();
        }

        private void ProcessTimer1()
        {
            lock (lockObject)
            {
                CheckCounter();

                mCounter++;
                WriteLog($"ProcessTimer1 started {mCounter}");

                Random random = new Random();
                Thread.Sleep(random.Next(50));

                mCounter--;
                WriteLog($"ProcessTimer1 finished {mCounter}");
            }
        }

        private void ProcessTimer2()
        {
            lock (lockObject)
            {
                CheckCounter();

                mCounter++;
                WriteLog($"ProcessTimer2 started {mCounter}");

                Random random = new Random();
                Thread.Sleep(random.Next(300));
                DoAnythingElseWithUI(); //after comment this line, the problem is gone

                mCounter--;
                WriteLog($"ProcessTimer2 finished {mCounter}");
            }
        }

        private void DoAnythingElseWithUI()
        {
            ExecuteUIContextAction(() =>
            {
                WriteLog("Anything else");
            });
        }

        private void CheckCounter()
        {
            if (mCounter != 0)
            {
                MessageBox.Show($"Alert! {mCounter}");
            }
        }

        private void WriteLog(string message)
        {
            richTextBox1.AppendText($"{DateTime.Now.ToString("HH:mm:ss:fffff")} {message}{Environment.NewLine}");
        }

        private void Form1_CompletedEvent1(object sender)
        {
            ExecuteUIContextAction(() =>
            {
                ProcessTimer1();
            });
        }

        private void Form1_CompletedEvent2(object sender)
        {
            ExecuteUIContextAction(() =>
            {
                ProcessTimer2();
            });
        }

        private void btnStart_Click(object sender, EventArgs e)
        {
            ThreadPool.QueueUserWorkItem(state =>
            {
                mTimer1.Start();
                mTimer2.Start();
            });
        }

        public void ExecuteUIContextAction(Action action)
        {
            if (mUIContext == null)
            {
                if (WindowsFormsSynchronizationContext.Current == null)
                {
                    WindowsFormsSynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
                    mUIContext = WindowsFormsSynchronizationContext.Current;
                }
            }
            mUIContext.Send(new SendOrPostCallback(delegate (object state)
            {
                action();
            }), null);
        }

        public SynchronizationContext UIContext
        {
            get { return mUIContext; }
        }

        private void btnStop_Click(object sender, EventArgs e)
        {
            mTimer1.Stop();
            mTimer2.Stop();
        }
    }
}

有人能告诉我为什么吗？
正确的解决方案是像这样将锁移动到Form1_CompletedEventX中吗？

lock (lockObjectForRevisible)
{
   ExecuteUIContextAction(() =>
   {
      ProcessTimer1();
   });
}