为什么Form_FormClosing事件在c＃.net中执行2次

Question

I have a windows form application in C#.net and I want to backUp while closing the form by X button So I Wroted below code to do this action .but when I run the program I noticed that this event execute 2 times Please Help me to solve this problem

private void MenuFrm_FormClosing(object sender, FormClosingEventArgs e)
        {
            //  DialogResult dialogResult = MessageBox.Show("ایا مایل به گرفتن نسخه پشتیبان می باشید", "هشدار", MessageBoxButtons.YesNo);
            //  if (dialogResult == DialogResult.Yes && !closefrm)
            try
            {
                DialogResult dialogResult = MessageBox.Show("آیا مایل به خروج از نرم افزار میباشید؟", "خروج", MessageBoxButtons.YesNo);
                if (dialogResult == DialogResult.Yes)
                {
                    SaveFileDialog f = new SaveFileDialog();
                    f.InitialDirectory = "D:\\";
                    f.Title = "HoghooghDastmozdBackup";
                    if (Directory.Exists("E:\\MobtakeranSoftBackup\\"))
                    {
                        f.FileName = "E:\\MobtakeranSoftBackup\\" + getPersianDate() + ".BAK";
                        f.FilterIndex = 1;
                        f.OverwritePrompt = true;
                        f.Filter = @"SQL Backup files (*.BAK) |*.BAK|All files(*.*) |*.*";
                        SqlConnection sqlconn = new SqlConnection(DBsetting.Connstring);
                        SqlCommand sqlcmd = new SqlCommand("BACKUP DATABASE HoghooghDastmozd TO  DISK =@n", sqlconn);
                        sqlcmd.Parameters.AddWithValue("@n", f.FileName);
                        sqlconn.Open();
                        sqlcmd.ExecuteNonQuery();
                        sqlconn.Close();
                        Application.Exit();
                    }
                    else
                    {
                        Directory.CreateDirectory("E:\\MobtakeranSoftBackup\\");
                        f.FileName = "E:\\MobtakeranSoftBackup\\" + getPersianDate() + ".BAK";
                        f.FilterIndex = 1;
                        f.OverwritePrompt = true;
                        f.Filter = @"SQL Backup files (*.BAK) |*.BAK|All files(*.*) |*.*";
                        SqlConnection sqlconn = new SqlConnection(DBsetting.Connstring);
                        SqlCommand sqlcmd = new SqlCommand("BACKUP DATABASE HoghooghDastmozd TO  DISK =@n", sqlconn);
                        sqlcmd.Parameters.AddWithValue("@n", f.FileName);
                        sqlconn.Open();
                        sqlcmd.ExecuteNonQuery();
                        sqlconn.Close();
                        Application.Exit();
                    }
                }
                else
                {
                    e.Cancel = true;
                }

            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                Application.Exit();
            }


        }

Answer 1

You're calling Application.Exit() . This is done before the form has fully closed... it's still open. Thus, a new Close operation is triggered, which involves a second Form_Closing event. By the time you are able to process the second event, the first has finished and form is fully closed, and so it stops there (you end up with the event firing exactly twice, and not more).

You can adjust for this by adding a boolean to the class the defaults to false, check that it's false at the top of the method, and set it true at the end of the method. But that's just patching over the real problem... you're doing too much in the form_closing event. The Application.Exit() call belongs in Form_Closed , which would typically only fire once.

---

While I'm here, I want to point out this code:

sqlconn.Open();
sqlcmd.ExecuteNonQuery();
sqlconn.Close(); 

This is poor practice. The .Close() call for an SqlConnection object should always be in a finally block (the easiest way to accomplish that is via a using block). In this case you're probably okay, because the application is about to quit anyway, but it's just not a good habit to execute queries this way, and it make one wonder if the same mistake is made elsewhere in the program. I also wonder why you're repeating so much code here. That whole section can be reduced to this:

Directory.CreateDirectory("E:\\MobtakeranSoftBackup\\"); //it's good to just call this, even if the directory already exists.
f.FileName = System.IO.Path.Combine(@"E:\MobtakeranSoftBackup", getPersianDate() + ".BAK");
f.FilterIndex = 1;
f.OverwritePrompt = true;
f.Filter = @"SQL Backup files (*.BAK) |*.BAK|All files(*.*) |*.*";
using (var sqlconn = new SqlConnection(DBsetting.Connstring))
using (var sqlcmd = new SqlCommand("BACKUP DATABASE HoghooghDastmozd TO  DISK =@n", sqlconn))
{
     sqlcmd.Parameters.AddWithValue("@n", f.FileName);
      sqlconn.Open();
      sqlcmd.ExecuteNonQuery();
}
Application.Exit();