r_ahlskog r_ahlskog - 1 month ago 5
C# Question

Using EventArgs to pass information back to invoking class

It is frowned upon to modify EventArgs in event handlers for the purpose of passing information back to the class invoking the event?

For instance if I have a low-level communication class needing to validate a certificate for SSL but it has no way of knowing what a valid certificate looks like since that is the knowledge of the different users of the class.

class ValidationEventArgs : System.EventArgs
{
public X509Certificate Certificate { get; set; }
public bool Valid { get; set; }
}


Then in the using objects they hook up to the event, and check it somehow changing the
Valid
flag to indicate if the certificate is acceptable or not.

comms.ValidationEvent += CertValidationHandler;

void CertValidationHandler(ValidationEventArgs args)
{
if (args.Certificate.Issuer.Contains(COMPANY_NAME)
args.Valid = true;
}


I have found references of EventArgs being used like this but I have also seen people saying it is not recommended.

Edit: Maybe I should clarify that this is not about inheriting EventArgs, but using them as a bi-directional channel of communication. As others commented this is acceptable and whatever noise Google picks up to the opposite is probably just people having misunderstood/misused the concept and now has the same personal crusade against as
goto
.

Answer

Ask yourself the following question, "when I publish an event, do I want any subscriber to alter any of the EventArgs values"? If the answer is no, e.g. you are broadcasting readonly information then make the class immutable, however if you need some feedback from a subscriber then make the properties which need to be altered mutable.

To clarify, in a broadcast example, we want to tell any subscriber something but not let them alter the value.

public class ProgressEventArgs : EventArgs
{
    public ProgressEventArgs(int current)
    {
        this.Current = current;
    }

    public int Current { get; private set; }
}

Equally though, we could also raise an event to ask for information the class itself does not know.

public class FeedbackEventArgs : EventArgs
{
    public bool ShouldContinue { get; set; }
    public string Reason { get; set; }
}