Example of Event in C# - Delegate with the custom EventArgs

Step-by-Step

1. Define a custom EventArgs class:

   In this context, NewIssueEventArgs is a derived class of EventArgs. This means that NewIssueEventArgs inherits from EventArgs, making it a specialized version of EventArgs with additional properties specific to the new issue event (e.g., IssueNumber and IssueTitle).

   Here's a breakdown:

   • Base Class: EventArgs is a predefined class in the .NET framework used as the base class for classes containing event data.
   • Derived Class: NewIssueEventArgs inherits from EventArgs and adds additional properties (IssueNumber and IssueTitle).

public class NewIssueEventArgs : EventArgs
{
    public int IssueNumber { get; }
    public string IssueTitle { get; }

    public NewIssueEventArgs(int issueNumber, string issueTitle)
    {
        IssueNumber = issueNumber;
        IssueTitle = issueTitle;
    }
}

2. The event to use the custom EventArgs class:

public class MagazineCompany
{
    // Define a delegate with the custom EventArgs
    public delegate void NewIssueEventHandler(object sender, NewIssueEventArgs e);

    // Define an event based on the delegate
    public event NewIssueEventHandler NewIssue;

    // Method to raise the event with custom event data
    public void PublishNewIssue(int issueNumber, string issueTitle)
    {
        Console.WriteLine("Magazine Company: Publishing new issue...");
        OnNewIssue(new NewIssueEventArgs(issueNumber, issueTitle));
    }

    // Protected virtual method to allow derived classes to override the event invocation behavior
    protected virtual void OnNewIssue(NewIssueEventArgs e)
    {
        // Invoke the event handler, if there are any subscribers
        NewIssue?.Invoke(this, e);
    }
}

3. The event handler to use the custom EventArgs class:

public class Reader
{
    // Method that will be called when the event is raised
    public void OnNewIssueReceived(object sender, NewIssueEventArgs e)
    {
        Console.WriteLine($"Reader: Received new issue {e.IssueNumber} titled '{e.IssueTitle}'.");
    }
}

4. Example usage:

public class Program
{
    public static void Main(string[] args)
    {
        // Create a magazine company (Publisher)
        MagazineCompany magazineCompany = new MagazineCompany();
        
        // Create a reader (Subscriber)
        Reader reader = new Reader();
        
        // Subscribe the reader to the magazine company's new issue event
        magazineCompany.NewIssue += reader.OnNewIssueReceived;
        
        // Publish a new issue with additional event data
        magazineCompany.PublishNewIssue(42, "The Future of Technology");
    }
}

Explanation

• Custom EventArgs Class: NewIssueEventArgs contains properties for the issue number and title.
• Delegate and Event: NewIssueEventHandler uses the custom EventArgs.
• Raising the Event: OnNewIssue method raises the event, passing an instance of NewIssueEventArgs with the issue number and title.
• Event Handler: OnNewIssueReceived method of Reader class handles the event and can access the additional data (issue number and title).

When you run the program, the output will be:

Magazine Company: Publishing new issue...
Reader: Received new issue 42 titled 'The Future of Technology'.