有人可以通过调用openfile showdialog来解决此代码中的错误吗？

Question

有人可以协助解决此代码中涉及openfile showdialog的错误吗？

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;

namespace North_America
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        //The GetFileName method gets a filename from the
        //user and assignes it to the variable passed as
        //an argument.
        private void GetFileName(out string selectedFile)
        {
            if (openFile.ShowDialog() == DialogResult.OK)
            {
                selectedFile = openFile.FileName;
            }
            else
            {
                selectedFile = "";
            }
        }

        //The GetCountries method accpets a filename as an
        //argument.  It opens the specified file and displays
        //its contents in the countriesListBox control.
        private void GetCountries (string filename)
        {
            try
            {
                //Delcare a variable to hold a country name.
                string countryName;

                //Declare a StreamReader variable
                StreamReader inputFile;

                //Open the file to get a streamreader object.
                inputFile = File.OpenText(filename);

                //Clear anything cueerntly in the listbox.
                countriesListBox.Items.Clear();

                //Read the file's contents.
                while (!inputFile.EndOfStream)
                {
                    //Get a country name.
                    countryName = inputFile.ReadLine();

                    //Add the counry name to the ListBox.
                    countriesListBox.Items.Add(countryName);
                }

                //Close the file.
                inputFile.Close();
            }
            catch (Exception ex)
            {
                //Display an error message.
                MessageBox.Show(ex.Message);
            }

       }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void countriesListBox_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void getCountriesButton_Click(object sender, EventArgs e)
        {
            string filename;    //To hold the filename

            //Get the filename from the user.
            GetFileName(out filename);

            //Get the countries from the file.
            GetCountries(filename);
        }

        private void exitButton_Click(object sender, EventArgs e)
        {
            // Close the form.
            this.Close();
        }
    }
}

如何解决此问题？