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Code Analyzer for C#

There is a newer version: 10.2.0.105762

Why is this an issue?
Field-like events are events that do
not have explicit add and remove accessors.
public event EventHandler MyEvent; // No add and remove accessors

The compiler generates a private delegate field to back the event, as well as generating the implicit add
and remove accessors.
When a virtual field-like event is overridden by another field-like event, the behavior of the C# compiler
is to generate a new private delegate field in the derived class, separate from the parent’s field. This results in multiple
and separate events being created, which is rarely what’s actually intended.
Noncompliant code example
abstract class Car
{
  public virtual event EventHandler OnRefuel; // Noncompliant

  public void Refuel()
  {
    // This OnRefuel will always be null
     if (OnRefuel != null)
     {
       OnRefuel(this, EventArgs.Empty);
     }
  }
}

class R2 : Car
{
  public override event EventHandler OnRefuel;
}

class Program
{
  static void Main(string[] args)
  {
    var r2 = new R2();
    r2.OnRefuel += (o, a) =>
    {
        Console.WriteLine("This event will be called");
    };
    r2.Refuel();
  }
}

Compliant solution
To prevent this, remove the virtual designation from the parent class event.
abstract class Car
{
  public event EventHandler OnRefuel; // Compliant

  public void Refuel()
  {
    if (OnRefuel != null)
    {
      OnRefuel(this, EventArgs.Empty);
    }
  }
}

class R2 : Car
{

}

class Program
{
  static void Main(string[] args)
  {
    var r2 = new R2();
    r2.OnRefuel += (o, a) =>
    {
        Console.WriteLine("This event will be called");
    };
    r2.Refuel();
  }
}

Resources
Documentation

   Add keyword 
   Remove keyword 
   Delegates 
   Introduction to events 
   Field-like events