title: The event can only appear in the left hand side of += or -+
body: <p>I have an event that is defined in a base class as:</p>
  
  <pre><code>public event EventHandler&lt;HighestSeverityLevelChangedEventArgs&gt; HighestSeverityLevelChanged;
  </code></pre>
  
  <p>The event argument is defined as:</p>
  
  <pre><code>   public sealed class HighestSeverityLevelChangedEventArgs : EventArgs
  {
      /// &lt;summary&gt;
      /// Hold the highest severity level.
      /// &lt;/summary&gt;
      public int HighestSeverityLevel { get; set; }
  
      /// &lt;summary&gt;
      /// Holds the previously high severity level.
      /// &lt;/summary&gt;
      public int PreviousHighestSeverityLevel { get; set; }
  
      /// &lt;summary&gt;
      /// Initializes a new instance of the &lt;see cref="HighestSeverityLevelChangedEventArgs" /&gt; class.
      /// &lt;/summary&gt;
      /// &lt;param name="highestSeverityLevel"&gt;Input current high severity level&lt;/param&gt;
      /// &lt;param name="previousHighestSeverityLevel"&gt;&lt;/param&gt;
      public HighestSeverityLevelChangedEventArgs(int highestSeverityLevel, int previousHighestSeverityLevel)
      {
          #region Preconditions
  
          if (highestSeverityLevel &lt; 0)
          {
              throw new InvalidOperationException(nameof(highestSeverityLevel));
          }
  
          if (previousHighestSeverityLevel &lt; 0)
          {
              throw new InvalidOperationException(nameof(previousHighestSeverityLevel));
          }
  
          #endregion
  
          HighestSeverityLevel = highestSeverityLevel;
          PreviousHighestSeverityLevel = previousHighestSeverityLevel;
      }
  }
  </code></pre>
  
  <p>Now in a subclass, I have to raise the event, so I use the following line:</p>
  
  <pre><code>HighestSeverityLevelChanged?.Invoke(this, new HighestSeverityLevelChangedEventArgs(m_highestLevel, previousHighestLevel));
  </code></pre>
  
  <p>But this is not compiling, it is telling me that:
  The event can only appear in the left hand side of += or -+ excep when used within the base class.</p>
  
  <p>I have this code in an inherited class, so is the syntax different here since I am trying to raise it in the inherited class. What am I doing wrong?</p>