Cập nhật ngày 10/11/2021
private void button1_Click(object sender, EventArgs e)
{
    int size = -1;
    OpenFileDialog openFileDialog1 = new OpenFileDialog();
    DialogResult result = openFileDialog1.ShowDialog(); // Show the dialog.

    if (result == DialogResult.OK) // Test result.
    {
       string file = openFileDialog1.FileName;
       
       try
       {
          string text = File.ReadAllText(file);
          size = text.Length;
       }
       catch (IOException)
       {
       }
    }

    Console.WriteLine(size); // <-- Shows file size in debugging mode.
    Console.WriteLine(result); // <-- For debugging use.
}