title: 'System.Collections.Generic.List<float>' does not contain a definition for 'Sum'
body: <p>I am trying to Sum a list of floats with built in <code>Sum()</code> function but I keep getting this error :</p>
  
  <blockquote>
    <p>Error CS1061: 'System.Collections.Generic.List' does not
    contain a definition for 'Sum' and no extension method 'Sum' accepting
    a first argument of type 'System.Collections.Generic.List'
    could be found (are you missing a using directive or an assembly
    reference?) (CS1061)</p>
  </blockquote>
  
  <p>and I have </p>
  
  <pre><code>using System.Collections;
  using System.Collections.Generic;
  </code></pre>
  
  <p>in the beginning of the file:</p>
  
  <p>code :</p>
  
  <pre><code>List&lt;float&gt; x = new List&lt;float&gt;();
  x.add(5.0f);
  //..
  float f = x.Sum();
  </code></pre>