C# ToolStrip System.StackOverflowException was unhandled

Question

I use toolstrip to control show PDF document. But I has key down or anything about keyboard. My application show error message :

[System.StackOverflowException was unhandled]
[An handled exception of type 'System.stackOverflowException' occurred in 
System.Windows.Forms.dll]

I use pdf tools for .net

using MigraDoc;
using PdfSharp.Pdf;
using PDFLibNet;

Help ! Please and Thank you.

My Code public partial class frm_showpdf : Form {

    int pageindex = 0;
    int pagecount = 0;
    int def_pagewidth, def_pageheight = 0;
    public string filename = "";
    int workingnow = 0;

    public frm_showpdf()
    {
        InitializeComponent();        
    }
    private void updateUI()
    {
        ts_pageindex.Text = Convert.ToString(pageindex);
        ts_pagetotal.Text = Convert.ToString(pagecount);
    }

    private void show_pagepdf(int indexpage)
    {

      PDFLibNet.PDFWrapper pdfview = new PDFLibNet.PDFWrapper();

      workingnow = 1;

      pdfview.LoadPDF(filename.ToString());
      pdfview.CurrentPage = indexpage;
      pageindex = pdfview.CurrentPage;
      pagecount = pdfview.PageCount;

      pdfview.UseMuPDF = true;

      pdfview.FitToWidth(pictureBox1.Handle);
      pdfview.RenderPage(pictureBox1.Handle);
      pdfview.CurrentX = 0;
      pdfview.CurrentY = 0;

      if (def_pagewidth == 0)
      {
         def_pagewidth = pdfview.PageWidth;
         def_pageheight = pdfview.PageHeight;
      }

      pdfview.ClientBounds = new Rectangle(0, 0, def_pagewidth, def_pageheight);
      pagecount = pdfview.PageCount;
      updateUI();
      Bitmap _backbuffer = new Bitmap(def_pagewidth, def_pageheight);
      using (Graphics g = Graphics.FromImage(_backbuffer))
      {
         //pdfview.RenderHDC(g.GetHdc);
         pdfview.DrawPageHDC(g.GetHdc());
         g.ReleaseHdc();
      }

      pictureBox1.SizeMode = PictureBoxSizeMode.AutoSize;
      pictureBox1.Image = _backbuffer;
      workingnow = 0;
      pdfview.Dispose();

    }

    private void showfile(object sender, EventArgs e)
    {
        Boolean hasfilename = false;

        if (pictureBox1.Image != null)
        {
            pictureBox1.Image.Dispose();
        }

        if (filename.ToString().Trim() != "") hasfilename = true;

        if (hasfilename == false)
        {
            openFileDialog1.Filter = "PDF files|*.pdf";
            DialogResult result = openFileDialog1.ShowDialog();
            if (result == DialogResult.OK) // Test result.
            {
                hasfilename = true;
                pgpdf1.Text = openFileDialog1.FileName;
                filename = pgpdf1.Text.ToString();
            }
            else
            {
                //pgpdf1.Text = "";
                filename = pgpdf1.Text.ToString();
            }
        }

        if (hasfilename == true)
        {

            show_pagepdf(1);
        }
    }

    private void frm_showpdf_Load(object sender, EventArgs e)
    {
        toolStrip1.Cursor = System.Windows.Forms.Cursors.Hand;
    }

    private void panel1_MouseEnter(object sender, EventArgs e)
    {
        panel1.Focus();
    }

    private void pictureBox1_MouseEnter(object sender, EventArgs e)
    {
        panel1.Focus();
    }

    private void ts_first_Click(object sender, EventArgs e)
    {
        show_pagepdf(1);
    }

    private void ts_back_Click(object sender, EventArgs e)
    {
        if (pageindex > 1) { show_pagepdf(pageindex - 1); };
    }

    private void ts_next_Click(object sender, EventArgs e)
    {
        if (pageindex < pagecount) { show_pagepdf(pageindex + 1); };
    }

    private void ts_last_Click(object sender, EventArgs e)
    {
        show_pagepdf( pagecount) ;
    }

    private void ts_pageindex_TextChanged(object sender, EventArgs e)
    {

    }

    private void frm_showpdf_KeyDown(object sender, KeyEventArgs e)
    {
        switch (e.KeyCode)
        {
            case Keys.Up :
                ts_back.PerformClick();
                e.Handled = true;
                break;
            case Keys.Left:
                ts_back.PerformClick();
                e.Handled = true;
                break;
            case Keys.Right:
                ts_next.PerformClick();
                e.Handled = true;
                break;
            case Keys.Down:
                ts_next.PerformClick();
                e.Handled = true;
                break;
        }

        if (!e.Handled)
           base.OnKeyDown(e);
    }

    private void frm_showpdf_Shown(object sender, EventArgs e)
    {
        pgpdf1.Text = filename.ToString().Trim();
        pgpdf1.Text = filename.Replace(@"\\", @"\");
        showfile(sender,e) ;
    }

    private void toolStrip1_TextChanged(object sender, EventArgs e)
    {
       // Hello Man
    }

}

Answer 1

I found error and change my code to :

    private void frm_showpdf_KeyDown(object sender, KeyEventArgs e)
    {
       switch (e.KeyCode)
       {
          case Keys.Up:
             ts_back.PerformClick();
             e.Handled = true;
             break;
          case Keys.Left:
             ts_back.PerformClick();
             e.Handled = true;
             break;
          case Keys.Right:
             ts_next.PerformClick();
             e.Handled = true;
             break;
          case Keys.Down:
             ts_next.PerformClick();
             e.Handled = true;
             break;
       }
    }

Why C# Error At Line :

    if (!e.Handled)
       base.OnKeyDown(e);

Advice Me Please.