title: IEnumerable Concat Missing, does not contain a definition for 'Concat'
body: <p>I have the following Class which inherits IEnumerable</p>
  
  <pre><code>public class LinesEnumerable : IEnumerable&lt;Point&gt;
  {
      protected readonly IPointSeries _pointSeries;
      protected readonly ICoordinateCalculator&lt;double&gt; _xCoordinateCalculator;
      protected readonly ICoordinateCalculator&lt;double&gt; _yCoordinateCalculator;
      protected readonly bool _isDigitalLine;
  
      /// &lt;summary&gt;
      /// Initializes a new instance of the &lt;see cref="LinesEnumerable" /&gt; class.
      /// &lt;/summary&gt;
      /// &lt;param name="pointSeries"&gt;The point series.&lt;/param&gt;
      /// &lt;param name="xCoordinateCalculator"&gt;The x coordinate calculator.&lt;/param&gt;
      /// &lt;param name="yCoordinateCalculator"&gt;The y coordinate calculator.&lt;/param&gt;
      /// &lt;param name="isDigitalLine"&gt;if set to &lt;c&gt;true&lt;/c&gt; return a digital line .&lt;/param&gt;
      public LinesEnumerable(IPointSeries pointSeries, ICoordinateCalculator&lt;double&gt; xCoordinateCalculator, ICoordinateCalculator&lt;double&gt; yCoordinateCalculator, bool isDigitalLine)
      {
          _pointSeries = pointSeries;
          _xCoordinateCalculator = xCoordinateCalculator;
          _yCoordinateCalculator = yCoordinateCalculator;
          _isDigitalLine = isDigitalLine;
      }
  
      /// &lt;summary&gt;
      /// Returns an enumerator that iterates through a collection.
      /// &lt;/summary&gt;
      /// &lt;returns&gt;
      /// An &lt;see cref="T:System.Collections.IEnumerator" /&gt; object that can be used to iterate through the collection.
      /// &lt;/returns&gt;
      public virtual IEnumerator&lt;Point&gt; GetEnumerator()
      {
          return _isDigitalLine ?
              (IEnumerator&lt;Point&gt;)new DigitalLinesIterator(_pointSeries, _xCoordinateCalculator, _yCoordinateCalculator) :
              new LinesIterator(_pointSeries, _xCoordinateCalculator, _yCoordinateCalculator);
      }
  
      /// &lt;summary&gt;
      /// Returns an enumerator that iterates through a collection.
      /// &lt;/summary&gt;
      /// &lt;returns&gt;
      /// An &lt;see cref="T:System.Collections.IEnumerator" /&gt; object that can be used to iterate through the collection.
      /// &lt;/returns&gt;
      IEnumerator IEnumerable.GetEnumerator()
      {
          return GetEnumerator();
      }
  }
  </code></pre>
  
  <p>However, when I try to do the following:</p>
  
  <pre><code>linesEnumerable = linesEnumerable.Concat(new[] { new Point(viewportWidth, lastYCoordinate) });
  </code></pre>
  
  <p>it says 'System.Collections.IEnumerable' does not contain a definition for 'Concat' and the best extension method overload 'System.Linq.Queryable.Concat(System.Linq.IQueryable, System.Collections.Generic.IEnumerable)' has some invalid arguments</p>
  
  <p>I already have System.Linq namespace added</p>
  
  <p>Anybody know why this is happening?</p>