Question

How can I use a value from the reading function in the button1_Click function?

public void reading(object sender, EventArgs e)
{
   DialogResult reading_from_folder = new DialogResult();
   reading_from_folder = folderBrowserDialog1.ShowDialog();

   if (reading_from_folder == DialogResult.OK)
   {
      string[] files_in_folder = Directory.GetFiles(folderBrowserDialog1.SelectedPath);
      ...
   }
}

private void button1_Click(object sender, EventArgs e)
{ 
   foreach (string file in files_in_folder) // How do I access files_in_folder?
   {
      ListViewItem li = new ListViewItem(file);
   }
}                

Answer 1

You need to store it somehow, for example as a private member:

string some_value = null;

public void reading(object sender, EventArgs e)
{
    some_value = "Foobar";
}

private void button1_Click(object sender, EventArgs e)
{
    if (some_value != null)
    {
        // ...
    }
}