Why is this an issue?
Field-like events are events that do not have explicit add and remove methods. The compiler generates a
private delegate field to back the event, as well as generating the implicit add and remove
methods.
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.
To prevent this, remove the virtual designation from the parent class event.
Noncompliant code example
abstract class Car
{
public virtual event EventHandler OnRefueled; // Noncompliant
public void Refuel()
{
// This OnRefueled will always be null
if (OnRefueled != null)
{
OnRefueled(this, null);
}
}
}
class R2 : Car
{
public override event EventHandler OnRefueled;
}
class Program
{
static void Main(string[] args)
{
var r2 = new R2();
r2.OnRefueled += new EventHandler((o, a) =>
{
Console.WriteLine("This event will never be called");
});
r2.Refuel();
}
}
Compliant solution
abstract class Car
{
public event EventHandler OnRefueled; // Compliant
public void Refuel()
{
if (OnRefueled != null)
{
OnRefueled(this, null);
}
}
}
class R2 : Car {}
class Program
{
static void Main(string[] args)
{
var r2 = new R2();
r2.OnRefueled += new EventHandler((o, a) =>
{
Console.WriteLine("This event will be called");
});
r2.Refuel();
}
}
