Standard pattern EventHandler and EventArgs

Context: The .NET standard event pattern uses EventHandler delegate and EventArgs (or a derived class) to pass event data.

EventHandler delegate

public delegate void EventHandler(object sender, EventArgs e);
public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);

Custom EventArgs

public class OrderEventArgs : EventArgs
{
    public int OrderId { get; set; }
    public decimal Amount { get; set; }
}

Implementing the pattern

public class OrderProcessor
{
    public event EventHandler<OrderEventArgs> OrderProcessed;

    protected virtual void OnOrderProcessed(OrderEventArgs e)
    {
        OrderProcessed?.Invoke(this, e);
    }

    public void Process(int orderId, decimal amount)
    {
        // processing logic
        OnOrderProcessed(new OrderEventArgs { OrderId = orderId, Amount = amount });
    }
}

Real-world usage example

All .NET UI frameworks: Button.Click uses EventHandler. Form.Load uses EventHandler. DataGridView.CellClick uses DataGridViewCellEventArgs derived from EventArgs.

Example: System.EventArgs documentation shows many derived types.

Related sections

• 7.2 Events
• 7.5 Project Button Click Simulator Event