title: Merge two (or more) lists into one, in C# .NET
body: <p>Is it possible to convert two or more lists into one single list, in .NET using C#?</p>
  
  <p>For example, </p>
  
  <pre><code>public static List&lt;Product&gt; GetAllProducts(int categoryId){ .... }
  .
  .
  .
  var productCollection1 = GetAllProducts(CategoryId1);
  var productCollection2 = GetAllProducts(CategoryId2);
  var productCollection3 = GetAllProducts(CategoryId3);
  </code></pre>