集合被修改，枚举操作可能无法执行

Question

好的，所以我想打开一个新表单，如果它还没有打开。所以我根据表格的标题或文本检查表格。现在，到目前为止它可以工作，就像在表单打开中一样，如果它已经打开，它只是把它带到前面。但我的问题是，如果它没有打开，并且我尝试创建它的新实例，它会抛出"集合已修改;枚举操作可能无法执行"。我一辈子都想不通为什么。任何帮助，不胜感激。

foreach (DataRow iRow in chatcheck.Rows)
{
   FormCollection fc = Application.OpenForms;
   foreach (Form f in fc)
   {
      if (f.Text != ChatReader["Sender"].ToString())
      {
         ChatBox chat = new ChatBox();
         Connection.ConnectionStrings.chatopen = ChatReader["Sender"].ToString();
         chat.Text = Connection.ConnectionStrings.chatopen;
         chat.Show();
         chat.BringToFront();
      }
      else if (f.Text == ChatReader["Sender"].ToString())
      {
              f.BringToFront();
      }
   }
}

Answer 1

不要使用 foreach 而是使用 for 循环：

for (int i = 0; i < Application.OpenForms.Count; i++ )
{
    Form f = Application.OpenForms[i];
    if (f.Text != ChatReader["Sender"].ToString())
    {
        //...
        chat.Show();
        chat.BringToFront();
    }
    // ...
}

在枚举期间，不能更改foreach的基础集合。但是，如果您创建一个新表单并在那里显示它，就会发生这种情况。将另一个窗体添加到打开集合。

Answer 2

您可以将信息保存在 foreach 循环中（例如，在 List <> 中），然后使用此信息打开表单。

                var myList = new List<something>();
                foreach (DataRow iRow in chatcheck.Rows)
                {
                    FormCollection fc = Application.OpenForms;
                    foreach (Form f in fc)
                    {
                        if (f.Text != ChatReader["Sender"].ToString())
                        {
                           myList.Add(...)
                        }
                        else if (f.Text == ChatReader["Sender"].ToString())
                        {
                            f.BringToFront();
                        }
                    }
                }

 

foreach (var val in myList)
{
   ChatBox chat = new ChatBox();
   ...
}

<</div> div class="one_answers">

为什么在循环和修改Dictionary时不使用foraeach？我们可以使用 foreach 及其键遍历字典，如下所示：

            //get key collection from dictionary into a list to loop through    
            List<int> keys = new List<int>(Dictionary.Keys);
            // iterating key collection using simple for-each loop
            foreach (int key in keys)
            { 
                // Now we can perform any modification with values of dictionary.    
                Dictionary[key] = Dictionary[key] - 1; 
            }

这是一篇关于它的帖子：如何在 C# 中使用键迭代字典

Answer 3

现实生活中的代码位于WinForms应用程序的MainForm内。

    /// <summary>
    /// Creates and connects the hub connection and hub proxy. 
    /// </summary>
    private async void ConnectWithRetryAsync()
    {
        Connection = new HubConnection(Properties.Settings.Default.ServerBaseUrl);
        Connection.Closed += Connection_Closed;
        Connection.Error += Connection_Error;
        HubProxy = Connection.CreateHubProxy("signalcalendar");
        //Handle incoming event from server: use Invoke to write to log from SignalR's thread
        HubProxy.On<CalendarUpdateRequest>("UpdateCalendarEvent", (calendarUpdateRequest) =>
            this.Invoke((Action)(() =>
            {
                try
                {
                    if (calendarUpdateRequest == null) return;
                    // Reject my own calendar's changes
                    if (calendarUpdateRequest.UserInfo.UserId == Program.UserInfo.UserId) return;
                    //Notify all opened Form about Calendar changes
                    for (int i = 0; i < Application.OpenForms.Count; i++)
                    {
                        var openForm = Application.OpenForms[i];
                        try
                        {
                            var currentFormType = openForm.GetType();
                            if (currentFormType == typeof(CommonForm))
                            {
                                if ((openForm as CommonForm).AppWindowType == AppWindowTypes.FactTruckForm ||
                                    (openForm as CommonForm).AppWindowType == AppWindowTypes.PlanTruckForm ||
                                    (openForm as CommonForm).AppWindowType == AppWindowTypes.FactExcForm ||
                                    (openForm as CommonForm).AppWindowType == AppWindowTypes.PlanExcForm)
                                {
                                    (openForm as CommonForm).CalendarHasBeenChanged(calendarUpdateRequest);
                                }
                            }
                        }
                        catch (Exception ex)
                        {
                            logger.Error(ex);
                        }
                    }
                }
                catch (Exception ex)
                {
                    logger.Error(ex);
                }
            }
            ))
        );
        #region  Connect to the Server
        try
        {
            await Connection.Start();
        }
        catch (HttpRequestException ex)
        {
            var errorMessage = "There is no connection with Server. Check your netwrok and Server App state";
            logger.Error(errorMessage);
            logger.Error(ex);
            MetroMessageBox.Show(this, errorMessage, this.Text, MessageBoxButtons.OK, MessageBoxIcon.Error);
            Close();
        }
        #endregion
        //Activate UI          
        logger.Info("COnnection has been established OK");
    }