C#: Winforms Selected text and Highliight in RichtextBox

How do you ensure a line of text has been selected?

In the mock up below on pressing the button a Line of text is written to a text box

However if no line has been actually selected IE user has not clicked in the RichTextBox the fist line is written

How do you ensure a user  has actually chosen a line?

Also a line of text could be longer then the RichTextBox width is it possible, on hover, to show the whole line in some kind of pop up label?  

BTW: In the Real App the lines in the RichTextBox will be file paths so using word wrap not really an option

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 TestForms
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            richTextBox1.DragDrop += new DragEventHandler(richTextBox1_DragDrop);
            richTextBox1.AllowDrop = true;
            richTextBox1.AutoWordSelection = false;

        }

        private void richTextBox1_DragEnter(object sender, System.Windows.Forms.DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.Rtf))
                e.Effect = DragDropEffects.Move;
            else
                e.Effect = DragDropEffects.None;
        }
        private void richTextBox1_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
        {
 
            var  Files = (Array)e.Data.GetData(DataFormats.FileDrop);
            foreach (string Path in Files)
            //tring Path = Files[0].ToString();
            {
                if (File.Exists(Path))
                {
                    richTextBox1.AppendText(Environment.NewLine + Path  );
                    textBox1.Text = Path;
                }
            }

        }
        private void RichTextBox1_MouseClick(object sender, MouseEventArgs e)
        {
            int firstcharindex = richTextBox1.GetFirstCharIndexOfCurrentLine();

            int currentline = richTextBox1.GetLineFromCharIndex(firstcharindex);

            string currentlinetext = richTextBox1.Lines[currentline];

            richTextBox1.Select(firstcharindex, currentlinetext.Length);

        }

        private void Form1_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar== (char)Keys.Escape)
            {
                 this.Close();            
            }
        }

        private void Button1_Click(object sender, EventArgs e)
        {
            int firstcharindex = richTextBox1.GetFirstCharIndexOfCurrentLine();
                int currentline = richTextBox1.GetLineFromCharIndex(firstcharindex);

                string currentlinetext = richTextBox1.Lines[currentline];
            if (richTextBox1.SelectedText == null )
            {
                MessageBox.Show("You need to select a file first!");

            }
            else
            {

                textBox1.Text = currentlinetext;
            }
        }

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Escape)
            {
               //  This fires After KeyPres!;
                //this.Close();
            }
        }
    }
}

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