为什么会出现"集合已被修改，枚举操作可能无法执行在c# winform应用程序

我有两个winforms。frmMainMenu和frmLogin。在frmMainMenu中，我有一个菜单条。其中我保留了一些菜单，其中包括注销选项。所以当我登录时，frmMainMenu出现在前面。我只是不做任何事情，只是点击登出选项frmMainMenu关闭和frmLogin表单再次显示。因此，当点击frmLogin表单的红色十字按钮时，我得到如下异常错误:

应用程序中发生了未处理的异常。如果单击Continue，应用程序将忽略此错误并尝试继续。如果您点击退出，应用程序将立即关闭。

Collection被修改;不能执行枚举操作。

代码:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
namespace Restaurant_Management_System
{
   public partial class frmMainMenu : Form
  {
    public frmMainMenu()
    {
        InitializeComponent();
    }
    ConnectionString cs = new ConnectionString();
    private void productToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmProduct frm = new frmProduct();
        frm.Show();
    }
    private void categoryToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmCategory frm = new frmCategory();
        frm.Show();
    }
    private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmAbout frm = new frmAbout();
        frm.Show();
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        ToolStripStatusLabel4.Text = System.DateTime.Now.ToString();
    }
    private void registrationToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmRegistration frm = new frmRegistration();
        frm.Show();
    }
    private void loginDetailsToolStripMenuItem_Click(object sender, EventArgs e)
    {
           // frmLoginDetails frm = new frmLoginDetails();
        frmPassVerify frm = new frmPassVerify();
        frm.Show();
    }
    private void profileEntryToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmCustomers frm = new frmCustomers();
        frm.Show();
    }
    private void billingToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmSales frm = new frmSales();
        frm.Show();
    }
    private void customersToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmPassVerCus frm = new frmPassVerCus();
        frm.Show();
    }
    private void salesToolStripMenuItem_Click(object sender, EventArgs e)
    {
        //frmSalesRecord1 frm = new frmSalesRecord1();
        frmPassVerSale frm = new frmPassVerSale();
        frm.Show();
    }
    private void typeToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmTypeReg frm = new frmTypeReg();
        frm.Show();
    }
    private void logoutToolStripMenuItem_Click(object sender, EventArgs e)
    {
        frmInventory frm = new frmInventory();
        frm.Show();
    }
    //I think here is the problem arises. on clicking the logout
    private void logoutToolStripMenuItem1_Click(object sender, EventArgs e)
    {
        frmLogin frm = new frmLogin();
        this.Dispose();
        frm.Show();
    }
  }
}

代码:

using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Data; 
using System.Drawing; 
using System.Linq;  
using System.Text; 
using System.Windows.Forms; 
using System.Data.SqlClient; 
namespace Restaurant_Management_System 
{
    public partial class frmLogin : Form
    {
        ConnectionString cs = new ConnectionString();
        DataTable dt = new DataTable();
        public frmLogin()
        {
            InitializeComponent();
        }
        private void btnOK_Click(object sender, EventArgs e)
        {
            if (txtUserName.Text == "")
            {
                MessageBox.Show("Please enter user name", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtUserName.Focus();
                return;
            }
            if (txtPassword.Text == "")
            {
                MessageBox.Show("Please enter password", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                txtPassword.Focus();
                return;
            }
            try
            {
                SqlConnection myConnection = default(SqlConnection);
                myConnection = new SqlConnection(cs.DBConn);
                SqlCommand myCommand = default(SqlCommand);
                myCommand = new SqlCommand("SELECT Username,password FROM Registration WHERE Username = @username AND password = @UserPassword", myConnection);
                SqlParameter uName = new SqlParameter("@username", SqlDbType.VarChar);
                SqlParameter uPassword = new SqlParameter("@UserPassword", SqlDbType.VarChar);
                //if(txtUserName.Text != "admin")
                uName.Value = txtUserName.Text;
                uPassword.Value = txtPassword.Text;
                myCommand.Parameters.Add(uName);
                myCommand.Parameters.Add(uPassword);
                myCommand.Connection.Open();
                SqlDataReader myReader = myCommand.ExecuteReader(CommandBehavior.CloseConnection);
                if (myReader.Read() == true)
                {
                    int i;
                    ProgressBar1.Visible = true;
                    ProgressBar1.Maximum = 5000;
                    ProgressBar1.Minimum = 0;
                    ProgressBar1.Value = 4;
                    ProgressBar1.Step = 1;
                    for (i = 0; i <= 5000; i++)
                    {
                        ProgressBar1.PerformStep();
                    }
                        this.Hide();
                        if (txtUserName.Text == "admin")
                        {
                            frmMainMenu frm = new frmMainMenu();
                            frm.Show();
                            frm.lblUser.Text = txtUserName.Text;
                        }
                        if (txtUserName.Text != "admin")
                        {
                            frmMainMenu1 frm = new frmMainMenu1();
                            frm.Show();
                            frm.lblUser.Text = txtUserName.Text;
                        }

                    }

                else
                {
                    MessageBox.Show("Login is Failed...Try again !", "Login Denied", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    txtUserName.Clear();
                    txtPassword.Clear();
                    txtUserName.Focus();
                }
                if (myConnection.State == ConnectionState.Open)
                {
                    myConnection.Dispose();
                }

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            ProgressBar1.Visible = false;
            txtUserName.Focus();
        }
        private void btnCancel_Click(object sender, EventArgs e)
        {
            this.Dispose();
        }
        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            this.Dispose();
            Application.Exit();
        }
        private void linkLabel1_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            this.Hide();
            frmChangePassword frm = new frmChangePassword();
            frm.Show();
            frm.txtUserName.Text = "";
            frm.txtNewPassword.Text = "";
            frm.txtOldPassword.Text = "";
            frm.txtConfirmPassword.Text = "";
        }
        private void linkLabel3_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            this.Hide();
            frmRecoveryPassword frm = new frmRecoveryPassword();
            frm.txtEmail.Focus();
            frm.Show();
        }
     } 
 }

如何处理这个异常?我不想在关闭时看到这个错误。提前感谢:)

private void logoutToolStripMenuItem1_Click(object sender, EventArgs e)
    {
        frmLogin frm = new frmLogin();
        this.Dispose(); //Comment This Line
        frm.Show();
    }