How to filter lines in textbox

Question

I have three radio buttons. When i press every button the text in textbox should get filtered according to that.

for example: if lastlog radio button is clicked only last 5 lines should appear

                        if lastfiveradio button  is clicked only last 25 lines should appear

 

How can i do that? 

 

if (lastlogradio.Checked == true)

{

StreamReader objstream = new StreamReader(listBox1.Items[listBox1.SelectedIndex].ToString());

resulttxtbox.Text = objstream.ReadToEnd();

}

else if (lastfiveradio.Checked == true)

{

StreamReader objstream = new StreamReader(listBox1.Items[listBox1.SelectedIndex].ToString());

resulttxtbox.Text = objstream.ReadToEnd();

}

else if (alllogsradio.Checked == true)

{

StreamReader objstream = new StreamReader(listBox1.Items[listBox1.SelectedIndex].ToString());

resulttxtbox.Text = objstream.ReadToEnd();

}