title: How to initialize IEnumerable<Object> that be empty and allow to Concat to it?
body: <p>I tried this code for adding <code>b</code> to <code>books</code>:</p>
  
  <pre><code>IEnumerable&lt;Book&gt; books =null;
  foreach (Book b in context.Books.AsEnumerable())
      if (someConditions)
         books = books.Concat(new[] {b});
  </code></pre>
  
  <p>but gives me this error on last line of code:</p>
  
  <blockquote>
    <p>System.ArgumentNullException: Value cannot be null.
    Parameter name: first</p>
  </blockquote>
  
  <p>it seems that <code>null</code> <code>Collection</code> could not concatenated. I use EF,so how should I initialize my <code>Collection</code> that have no thing in it and I could concatenate to it?</p>