C#线程和ShowDialog问题

ProgressForm类：

public partial class ProgressForm : Form
    {
        public int prc = 0, sz;
        MainForm mf;
        public ProgressForm(MainForm MF)
        {
            InitializeComponent();
            mf = MF;
            sz = 0;
        }
        public ProgressForm(int mx)
        {
            InitializeComponent();
            sz = mx;
        }
        public void SetMax(int mx)
        {
            sz = mx;
        }
        public void StartProgress()
        {
            timer1.Enabled = true;
        }
        public void IncProgress(int prg)
        {
            prc += prg;
        }
        private void timer1_Tick(object sender, EventArgs e)
        {
            double pos = (double)prc / (double)sz * 100;
            progressBar.Value = (int)pos;
        }
        private void ProgressForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            timer1.Enabled = false;
        }
        private void cancelBtn_Click(object sender, EventArgs e)
        {
            mf.isCanceled = true;
            this.Close();
        }
        private void ProgressForm_Shown(object sender, EventArgs e)
        {
            progressBar.Value = 0;
            StartProgress();
        }
    }

主窗体类别：

void DeleteFiles()
            {
                int x = 0;
                int cnt = resultList.Count;
                isCanceled = false;
                DeleteThreadHandler("beginprogress");
                try
                {
                    DeleteThreadHandler("begindelete");
                    for (int j = 0; j < cnt; j++)
                    {
                        if (resultList[x].isChecked)
                        {
                            DeleteThreadHandler("progress");
                            DeleteFile(resultList[x].name, deleteForm.isDeletePermanently);
                            if (File.Exists(resultList[x].name))
                            {
                                DeleteErrorHandler(resultList[x].name);
                                isError = true;
                            }
                            else
                                resultList.RemoveAt(x);
                        }
                        else
                            ++x;
                        if (isCanceled)
                            break;
                    }
                }
                finally
                {
                    validity(true);
                    DeleteThreadHandler("enddelete");
                }
            }
            void DeleteErrorHandler(string val)
            {
                Action action = null;
                action = () =>
                {
                    errorReportForm.AddError(val);
                };
                this.BeginInvoke(action);
            }
            void DeleteThreadHandler(String title)
            {
                Action action = null;
                if (title == "beginprogress")
                {
                    action = () =>
                    {
                    };
                }
                else
                if (title == "begindelete")
                {
                    action = () =>
                    {
                        olvVirtual.BeginUpdate();
                    };
                }
                else
                    if (title == "enddelete")
                    {
                        action = () =>
                        {
                            olvVirtual.VirtualListSize = resultList.Count;
                            olvVirtual.EndUpdate();
                            RefreshStatus();
                            progressForm.Close();
                            if (isError)
                                errorReportForm.ShowDialog();
                        };
                    }
                if (title == "progress")
                {
                    action = () =>
                    {
                        progressForm.IncProgress(1);
                    };
                }
                this.BeginInvoke(action);
            }    

    private void DeleteBtn_Click(object sender, EventArgs e)
        {
            int checkedcount = GetCheckedCount();
            if (checkedcount == 0)
            {
                MessageBox.Show("Please mark at least a file first");
                return;
            }
            DialogResult dr = new DialogResult();
            if (deleteForm == null)
                deleteForm = new DeleteForm();
            dr = deleteForm.ShowDialog();
            if (dr == DialogResult.OK)
            {
                //if (progressForm == null)
                progressForm = new ProgressForm(this);
                progressForm.Text = "Deleting...";
                progressForm.SetMax(checkedcount);
                if (errorReportForm == null)
                    errorReportForm = new ErrorReportForm();
                errorReportForm.ClearMemo();
                isError = false;
                Thread t = new Thread(DeleteFiles);
                t.Start();
                progressForm.ShowDialog();
            }
        }

在progressForm中，有一个进度条&定时器，每500ms更新一次进度。问题是我仍然可以访问主窗体，我也尝试了BeginInvoke，但两者都不起作用有人知道怎么了吗？

感谢

编辑：我找到了这个烂摊子的来源，它是使用Microsoft.VisualBasic.FileIO.FileSystem.DeleteFile的DeleteFile。用非托管代码替换它后，它工作得很好。