in my csv reader How can I open a file from outside

***  my csv reader menu ***
       private void newToolStripMenuItem_Click(object sender, EventArgs e)
       {
           if (SaveIfModified())
               ClearFile();
       }

       private void openToolStripMenuItem_Click(object sender, EventArgs e)
       {
           if (SaveIfModified())
           {
               if (openFileDialog1.ShowDialog(this) == DialogResult.OK)
                   ReadFile(openFileDialog1.FileName);
           }
       }

       private void saveToolStripMenuItem_Click(object sender, EventArgs e)
       {
           if (FileName != null)
               WriteFile(FileName);
           else
               saveAsToolStripMenuItem_Click(sender, e);
       }

       private void saveAsToolStripMenuItem_Click(object sender, EventArgs e)
       {
           saveFileDialog1.FileName = FileName;
           if (saveFileDialog1.ShowDialog(this) == DialogResult.OK)
               WriteFile(saveFileDialog1.FileName);
       }