title: C# copying distinct data from 1 datatable to another
body: <p>I have 2 datatables, Tab1 and Tab2.
  Tab1 is full of data, Tab2 is empty.</p>
  
  <p>Tab1 looks like that:<br>
  Col1 | Col2 | Col3 | Key | Country | Col.... <br>
  abc | xyz | 103 | AK1 | POL| .....<br>
  pol | man | 1212 | AK2 | POL| ....<br>
  moro | kom | 11 | AK1 | POL | ....<br>
  bca | oni | 10a| AK1 | GER|  ....<br>
  bca | oni | 10a| AK3 | GER|  ....<br>
  al | 1n | zxc | AK2 | POL| ....<br></p>
  
  <p>I need to select distinct data by Key and Country(Key+Country combination must be unique) and put it to Tab2. </p>
  
  <p>How to do it in c#?
  I have a lot of rows, so I need quiet fast method to do it.</p>
  
  <p>For this example a result should be:<br>
  Col1 | Col2 | Col3 | Key | Country | Col.... <br>
  abc | xyz | 103 | AK1 | POL| .....<br>
  pol | man | 1212 | AK2 | POL| ....<br>
  bca | oni | 10a| AK1 | GER|  ....<br>
  bca | oni | 10a| AK3 | GER|  ....<br></p>