[ACCEPTED]-NullReferenceException when triggering event-event-handling

Accepted answer

Score: 34

You need a null check - in C# you can't 9 call events when there are no handlers registered 8 on that event.

The normal thing would be 7 to implement an OnConnectFailed method:

protected virtual void OnConnectFailed(e as EventArgs) {
    EventHandler tmp = connectFailed;
    if(tmp != null)
        tmp(this,e);
}

Also, the 6 first argument to the event handler should 5 be this, not the exception. If you need to pass 4 the exception to the event handler, create 3 an EventArgs class with an exception property.

Also, there's 2 no point in raising an event from the constructor... nothing 1 has a chance to add a handler to it.

Score: 11

Also in C# 6 you may do the null checking this way:

connectFailed?.Invoke(this, e);

Score: 9

Found it,

public delegate void OnRequestReceivedHandler(object sender);
public event OnRequestReceivedHandler ReqeustReceived = delegate { };

Score: 1

'connectFailed' is an event. If nobody susbcribes 13 to the event, it will be null, so you have 12 to check for the null case.

To make this 11 safe you would need a null check, i.e:

if (connectFailed != null)
    connectFailed(e, new EventArgs());

However, this 10 pattern is not enough, because of multithreading. The 9 recommended approach is this:

EventHandler temp = connectFailed;
if (temp != null)
    temp(e, new EventArgs());

This not only 8 checks for the null condition, but also 7 copies the event first to ensure it is thread-safe 6 (if the event queue is changed by one thread 5 while the event is being handled on another 4 thread, the behaviour could be undefined. By 3 copying it first you ensure that the subscriber 2 list remains the same for the duration of 1 the event-handling process)

Source: stackoverflow.com

[ACCEPTED]-NullReferenceException when triggering event-event-handling

Which methods can be used to make thread wait for an event and then continue its execution?

Passing arguments to an event handler

Do you need to remove an event handler in the destructor?

Shutdown event for console application C#

Best way to get the name of a button that called an event?

Form.Load event not firing, form showing

JTable: Detect cell data change

jQuery event on value change of input hidden

VBA: Using WithEvents on UserForms

JCheckbox change listener gets notified of mouse over events