title: Find a match of certain columns from the first datatable in the second
body: <p>I have two <code>DataTables</code>: <em>dtChild</em> and <em>dtMaster</em>.
  <img src="https://i.stack.imgur.com/HM02P.png" alt="enter image description here"></p>
  
  <p>For every row in <em>dtChild</em>, I would like to see if there is a match in <em>dtMaster</em>. And this matching is based on specific columns.</p>
  
  <p>So in the image above, both <code>DataTables</code> have columns named <em>Col2</em> and <em>Col3</em>. And these are columns we are interested in. In the second row, we have a match. Because there is a row in <em>dtMaster</em> with the value of <em>Col2</em> equal to <em>Value22</em> and value of <em>Col3</em> equal to <em>Value23</em>. </p>
  
  <p>I want to write a LINQ query that returns <code>null</code> for the first row result (since there is no match for the first tow in <em>dtChild</em>) and for the second row returns the id of the found record in <em>dtMaster</em> (let's assume <em>dtMaster</em> also has a Primary Key column called Id in this case).</p>
  
  <p><strong>N.B. The column names can vary every time we run the program. So we would like our LINQ to be dynamic. Also the number of matching columns (2 in the example above) can vary. So there might be a case where our condition is based on the value of 5 columns.</strong></p>