title: c# SQL Listview selected item comparison to column
body: <p>what I'm trying to do is when I select an item on my <code>listview1</code>, then compare that value to the value in my datatable column, and then put that <code>osztalyzatok</code> column in the <code>listview3</code> where the <code>listview1</code> selected item equals to the <code>id_tanulo</code></p>
  
  <p>Here is my code:</p>
  
  <p><img src="https://i.stack.imgur.com/PI9U3.jpg" alt=""></p>
  
  <p>And my Datatable:</p>
  
  <p><img src="https://i.stack.imgur.com/G99h3.jpg" alt=""></p>