Question

This is the call:

    bool isValid = true;
    if (this.ExternalConstraint != null)
    {
      isValid &= this.ExternalConstraint(this, EventArgs.Empty);
    }
    if (isValid)
    {
        //...
    }

The event look like:

public delegate bool externalConstraint(object sender, EventArgs args);
event externalConstraint ExternalConstraint;

When debugging I notice that all method attached to the event is called but only the last return seem to return to this line : isValid &= this.ExternalConstraint(this, EventArgs.Empty);. How can I get the event to return every method or to handle the all return value to take a decision?

Answer 1

It’s considered bad practice to depend on return values when using events. Rather, you should use a user-defined EventArgs to get back results.

public class ConstraintEventArgs : EventArgs
{
    public bool IsValid { get; set; }
}

It’s also important to implement some mechanism in which multiple event listeners won’t override each other’s values. Most of the time (with a boolean flag), we will allow (or encourage) users to only explicitly turn it on (never off).

For example, with CancelEventArgs it’s never a good idea to set it to false explicitly; Users should always set it to true or do nothing. The same idea should apply here with IsValid.