title: Use List.ToLookup()
body: <p>I have a List like the following:</p>
  
  <pre><code>var products = new List&lt;Product&gt;
  {
      new Product { Id = 1, Category = "Electronics", Value = 15.0 },
      new Product { Id = 2, Category = "Groceries", Value = 40.0 },
      new Product { Id = 3, Category = "Garden", Value = 210.3 },
      new Product { Id = 4, Category = "Pets", Value = 2.1 },
      new Product { Id = 5, Category = "Electronics", Value = 19.95 },
      new Product { Id = 6, Category = "Pets", Value = 5.50 },
      new Product { Id = 7, Category = "Electronics", Value = 250.0 },
  };
  </code></pre>
  
  <p>I want to group by category and get the <strong>sum</strong> of 'Values' belonging to that category..
  Example: Electronics: 284.95</p>
  
  <p>While I can do this in some other way, I want to learn usage of Look-Up.</p>
  
  <p>Is it possible to get these 2 values (category and Value) in a Look-Up? If yes, How can I do that?</p>