拖放和拖动输入触发两次

我有以下代码，我正在尝试在其中实现拖放功能：

public Location_Alert()
    {
        InitializeComponent();
        #region Form Initialization
        Recur_Txt.Text = "1";
        End_Date.Value = Start_Date.Value.Add(new TimeSpan(1,0,0,0));
        Recur_Time_Txt.Text = DateTime.Now.Add(new TimeSpan(0,15,0)).ToString("HH:mm");
        Location_Alert_Timer.Tick += new EventHandler(Location_Alert_Timer_Tick);//allow for timed recurrences in code
        this.DragEnter += new DragEventHandler(Location_Alert_DragEnter);//set up monitoring for a Drag event, changing the cursor as users drags file(s) onto the form.
        this.DragDrop +=new DragEventHandler(Location_Alert_DragDrop);//set up monitoring for a Drop event, allowing user to drag file onto the form
        #endregion
    }
    private void Location_Alert_DragDrop(object sender, DragEventArgs e)
    {
        string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
        if (files.Length == 1)
        {
            int i = 0;
            int ii = -1;
            foreach(string file in files)
            {
                if (file.Contains("enot"))
                {
                    if (ii == -1)
                    {
                        ii = i;
                    }
                }
                i++;
            }
            ImportFile(files[ii]);
        }
        else
        {
            MessageBox.Show("This application only supports a single drag and drop file, only the first *.enot file will be imported.");
            int i = 0;
            int ii = -1;
            foreach (string file in files)
            {
                if (file.Contains("enot"))
                {
                    if (ii == -1)
                    {
                        ii = i;
                    }
                }
                i++;
            }
            ImportFile(files[ii]);
        }
    }
    private void Location_Alert_DragEnter(object sender, DragEventArgs e)
    {
        if (e.Data.GetDataPresent(DataFormats.FileDrop))
        {
            int i = 0;
            int ii = -1;
            string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
            foreach (string file in files)
            {
                if (file.Contains("enot"))
                {
                    if (ii == -1)
                    {
                        ii = i;
                    }
                }
                i++;
            }
            if (ii != -1)
            {
                e.Effect = DragDropEffects.Copy;
            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
        }
    }

并且拖放和拖动输入事件总是触发两次。 我的滴答事件没有，我不确定这背后的原因。 额外的触发不会导致问题，除非用户遇到异常。 这将处理两次异常，在这种情况下，这意味着将 2 个消息框返回给用户。

private void Location_Alert_DragDrop(object sender, DragEventArgs e)
{
   ToDo();
}
private void Location_Alert_DragEnter(object sender, DragEventArgs e)
{
   e.Effect = DragDropEffects.Copy;
}