An interface can declare an event. The following example shows how to implement interface events in a class. Basically the rules are the same as when you implement any interface method or property.
To implement interface events in a class
Declare the event in your class and then invoke it in the appropriate areas.
namespace ImplementInterfaceEvents
{
public interface IDrawingObject
{
event EventHandler ShapeChanged;
}
public class MyEventArgs : EventArgs
{
// class members
}
public class Shape : IDrawingObject
{
public event EventHandler? ShapeChanged;
void ChangeShape()
{
// Do something here before the event…
OnShapeChanged(new MyEventArgs(/*arguments*/));
// or do something here after the event.
}
protected virtual void OnShapeChanged(MyEventArgs e)
{
ShapeChanged?.Invoke(this, e);
}
}
} Example
The following example shows how to handle the less-common situation in which your class inherits from two or more interfaces and each interface has an event with the same name. In this situation, you must provide an explicit interface implementation for at least one of the events. When you write an explicit interface implementation for an event, you must also write the add and remove event accessors. Normally these are provided by the compiler, but in this case the compiler cannot provide them.
By providing your own accessors, you can specify whether the two events are represented by the same event in your class, or by different events. For example, if the events should be raised at different times according to the interface specifications, you can associate each event with a separate implementation in your class. In the following example, subscribers determine which OnDraw event they will receive by casting the shape reference to either an IShape or an IDrawingObject.
sample · csharpsamples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideEvents/CS/Events.cs10
using System;
using System.Collections.Generic;
using System.Text;
//-----------------------------------------------------------------------------
//<Snippet1>
namespace BaseClassEvents
{
// Special EventArgs class to hold info about Shapes.
public class ShapeEventArgs : EventArgs
{
public ShapeEventArgs(double area)
{
NewArea = area;
}
public double NewArea { get; }
}
// Base class event publisher
public abstract class Shape
{
protected double _area;
public double Area
{
get => _area;
set => _area = value;
}
// The event. Note that by using the generic EventHandler<T> event type
// we do not need to declare a separate delegate type.
public event EventHandler<ShapeEventArgs> ShapeChanged;
public abstract void Draw();
//The event-invoking method that derived classes can override.
protected virtual void OnShapeChanged(ShapeEventArgs e)
{
// Safely raise the event for all subscribers
ShapeChanged?.Invoke(this, e);
}
}
public class Circle : Shape
{
private double _radius;
public Circle(double radius)
{
_radius = radius;
_area = 3.14 * _radius * _radius;
}
public void Update(double d)
{
_radius = d;
_area = 3.14 * _radius * _radius;
OnShapeChanged(new ShapeEventArgs(_area));
}
protected override void OnShapeChanged(ShapeEventArgs e)
{
// Do any circle-specific processing here.
// Call the base class event invocation method.
base.OnShapeChanged(e);
}
public override void Draw()
{
Console.WriteLine("Drawing a circle");
}
}
public class Rectangle : Shape
{
private double _length;
private double _width;
public Rectangle(double length, double width)
{
_length = length;
_width = width;
_area = _length * _width;
}
public void Update(double length, double width)
{
_length = length;
_width = width;
_area = _length * _width;
OnShapeChanged(new ShapeEventArgs(_area));
}
protected override void OnShapeChanged(ShapeEventArgs e)
{
// Do any rectangle-specific processing here.
// Call the base class event invocation method.
base.OnShapeChanged(e);
}
public override void Draw()
{
Console.WriteLine("Drawing a rectangle");
}
}
// Represents the surface on which the shapes are drawn
// Subscribes to shape events so that it knows
// when to redraw a shape.
public class ShapeContainer
{
private readonly List<Shape> _list;
public ShapeContainer()
{
_list = new List<Shape>();
}
public void AddShape(Shape shape)
{
_list.Add(shape);
// Subscribe to the base class event.
shape.ShapeChanged += HandleShapeChanged;
}
// ...Other methods to draw, resize, etc.
private void HandleShapeChanged(object sender, ShapeEventArgs e)
{
if (sender is Shape shape)
{
// Diagnostic message for demonstration purposes.
Console.WriteLine($"Received event. Shape area is now {e.NewArea}");
// Redraw the shape here.
shape.Draw();
}
}
}
class Test
{
static void Main()
{
//Create the event publishers and subscriber
var circle = new Circle(54);
var rectangle = new Rectangle(12, 9);
var container = new ShapeContainer();
// Add the shapes to the container.
container.AddShape(circle);
container.AddShape(rectangle);
// Cause some events to be raised.
circle.Update(57);
rectangle.Update(7, 7);
// Keep the console window open in debug mode.
Console.WriteLine("Press any key to continue...");
Console.ReadKey();
}
}
}
/* Output:
Received event. Shape area is now 10201.86
Drawing a circle
Received event. Shape area is now 49
Drawing a rectangle
*/
//</Snippet1>
//--------------------------------------------------------------------------------------
//<Snippet2>
namespace DotNetEvents
{
// Define a class to hold custom event info
public class CustomEventArgs : EventArgs
{
public CustomEventArgs(string message)
{
Message = message;
}
public string Message { get; set; }
}
// Class that publishes an event
class Publisher
{
// Declare the event using EventHandler<T>
public event EventHandler<CustomEventArgs>? RaiseCustomEvent;
public void DoSomething()
{
// Write some code that does something useful here
// then raise the event. You can also raise an event
// before you execute a block of code.
OnRaiseCustomEvent(new CustomEventArgs("Event triggered"));
}
// Wrap event invocations inside a protected virtual method
// to allow derived classes to override the event invocation behavior
protected virtual void OnRaiseCustomEvent(CustomEventArgs e)
{
// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
EventHandler<CustomEventArgs>? raiseEvent = RaiseCustomEvent;
// Event will be null if there are no subscribers
if (raiseEvent != null)
{
// Format the string to send inside the CustomEventArgs parameter
e.Message += $" at {DateTime.Now}";
// Call to raise the event.
raiseEvent(this, e);
}
}
}
//Class that subscribes to an event
class Subscriber
{
private readonly string _id;
public Subscriber(string id, Publisher pub)
{
_id = id;
// Subscribe to the event
pub.RaiseCustomEvent += HandleCustomEvent;
}
// Define what actions to take when the event is raised.
void HandleCustomEvent(object? sender, CustomEventArgs e)
{
Console.WriteLine($"{_id} received this message: {e.Message}");
}
}
class Program
{
static void Main()
{
var pub = new Publisher();
var sub1 = new Subscriber("sub1", pub);
var sub2 = new Subscriber("sub2", pub);
// Call the method that raises the event.
pub.DoSomething();
// Keep the console window open
Console.WriteLine("Press any key to continue...");
Console.ReadLine();
}
}
}
//</Snippet2>
namespace Winforms
{
using System.Windows.Forms;
class Form1 : System.Windows.Forms.Form
{
//<Snippet11>
private void Form1_Load(object sender, System.EventArgs e)
{
// Add your form load event handling code here.
}
//</Snippet11>
}
}
namespace ExplicitEvents
{
using System;
using System.Collections.Generic;
//<Snippet20>
public class EventSource2
{
private EventHandler TestEventHandler;
public event EventHandler TestEvent
{
add
{
lock (TestEventHandler)
{
TestEventHandler += value;
}
}
remove
{
lock (TestEventHandler)
{
TestEventHandler -= value;
}
}
}
private void RaiseTestEvent()
{
// Safely invoke an event.
EventHandler temp = TestEventHandler;
if (temp != null)
{
temp(this, new System.EventArgs());
}
}
}
//</Snippet20>
}
namespace WrapEvent
{
using System;
using System.Collections.Generic;
//<Snippet7>
namespace TestCollections
{
// A class that works just like ArrayList, but sends event
// notifications whenever the list changes:
public class ListWithChangedEvent : System.Collections.ArrayList
{
// An event that clients can use to be notified whenever the
// elements of the list change:
public event System.EventHandler Changed;
// Invoke the Changed event; called whenever list changes:
protected virtual void OnChanged(System.EventArgs e)
{
if (Changed != null)
{
Changed(this, e);
}
}
// Override some of the methods that can change the list;
// invoke event after each:
public override int Add(object value)
{
int i = base.Add(value);
OnChanged(System.EventArgs.Empty);
return i;
}
public override void Clear()
{
base.Clear();
OnChanged(System.EventArgs.Empty);
}
public override object this[int index]
{
set
{
base[index] = value;
OnChanged(System.EventArgs.Empty);
}
}
}
}
namespace TestEvents
{
using TestCollections;
class EventListener
{
private ListWithChangedEvent list;
public EventListener(ListWithChangedEvent list)
{
this.list = list;
// Add "ListChanged" to the Changed event on list:
this.list.Changed += new System.EventHandler(ListChanged);
}
// This will be called whenever the list changes:
private void ListChanged(object sender, System.EventArgs e)
{
System.Console.WriteLine("This is called when the event fires.");
}
public void Detach()
{
// Detach the event and delete the list:
list.Changed -= new System.EventHandler(ListChanged);
list = null;
}
}
class Test
{
// Test the ListWithChangedEvent class:
static void Main()
{
// Create a new list:
ListWithChangedEvent list = new ListWithChangedEvent();
// Create a class that listens to the list's change event:
EventListener listener = new EventListener(list);
// Add and remove items from the list:
list.Add("item 1");
list.Clear();
listener.Detach();
}
}
}
//</Snippet7>
}
//-----------------------------------------------------------------------------
namespace WrapHash
{
//<Snippet9>
using System;
using System.Collections.Generic;
public delegate void EventHandler1(int i);
public delegate void EventHandler2(string s);
public class PropertyEventsSample
{
private const string Event1Key = nameof(Event1);
private const string Event2Key = nameof(Event2);
private readonly Dictionary<string, Delegate> handlers;
public PropertyEventsSample()
{
handlers = new Dictionary<string, Delegate>
{
[Event1Key] = null,
[Event2Key] = null
};
}
public event EventHandler1 Event1
{
add
{
lock (handlers)
{
handlers[Event1Key] = (EventHandler1)handlers[Event1Key] + value;
}
}
remove
{
lock (handlers)
{
handlers[Event1Key] = (EventHandler1)handlers[Event1Key] - value;
}
}
}
public event EventHandler2 Event2
{
add
{
lock (handlers)
{
handlers[Event2Key] = (EventHandler2)handlers[Event2Key] + value;
}
}
remove
{
lock (handlers)
{
handlers[Event2Key] = (EventHandler2)handlers[Event2Key] - value;
}
}
}
internal void RaiseEvent1(int i)
{
EventHandler1 handler;
lock (handlers)
{
handler = (EventHandler1)handlers[Event1Key];
}
handler?.Invoke(i);
}
internal void RaiseEvent2(string s)
{
EventHandler2 handler;
lock (handlers)
{
handler = (EventHandler2)handlers[Event2Key];
}
handler?.Invoke(s);
}
}
public static class TestClass
{
private static void Delegate1Method(int i) => Console.WriteLine(i);
private static void Delegate2Method(string s) => Console.WriteLine(s);
private static void Main()
{
var p = new PropertyEventsSample();
p.Event1 += Delegate1Method;
p.Event1 += Delegate1Method;
p.Event1 -= Delegate1Method;
p.RaiseEvent1(2);
p.Event2 += Delegate2Method;
p.Event2 += Delegate2Method;
p.Event2 -= Delegate2Method;
p.RaiseEvent2("TestString");
// Keep the console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
/* Output:
2
TestString
*/
//</Snippet9>
}
//-----------------------------------------------------------------------------
//<Snippet10>
namespace WrapTwoInterfaceEvents
{
using System;
public interface IDrawingObject
{
// Raise this event before drawing
// the object.
event EventHandler OnDraw;
}
public interface IShape
{
// Raise this event after drawing
// the shape.
event EventHandler OnDraw;
}
// Base class event publisher inherits two
// interfaces, each with an OnDraw event
public class Shape : IDrawingObject, IShape
{
// Create an event for each interface event
event EventHandler PreDrawEvent;
event EventHandler PostDrawEvent;
object objectLock = new Object();
// Explicit interface implementation required.
// Associate IDrawingObject's event with
// PreDrawEvent
#region IDrawingObjectOnDraw
event EventHandler IDrawingObject.OnDraw
{
add
{
lock (objectLock)
{
PreDrawEvent += value;
}
}
remove
{
lock (objectLock)
{
PreDrawEvent -= value;
}
}
}
#endregion
// Explicit interface implementation required.
// Associate IShape's event with
// PostDrawEvent
event EventHandler IShape.OnDraw
{
add
{
lock (objectLock)
{
PostDrawEvent += value;
}
}
remove
{
lock (objectLock)
{
PostDrawEvent -= value;
}
}
}
// For the sake of simplicity this one method
// implements both interfaces.
public void Draw()
{
// Raise IDrawingObject's event before the object is drawn.
PreDrawEvent?.Invoke(this, EventArgs.Empty);
Console.WriteLine("Drawing a shape.");
// Raise IShape's event after the object is drawn.
PostDrawEvent?.Invoke(this, EventArgs.Empty);
}
}
public class Subscriber1
{
// References the shape object as an IDrawingObject
public Subscriber1(Shape shape)
{
IDrawingObject d = (IDrawingObject)shape;
d.OnDraw += d_OnDraw;
}
void d_OnDraw(object sender, EventArgs e)
{
Console.WriteLine("Sub1 receives the IDrawingObject event.");
}
}
// References the shape object as an IShape
public class Subscriber2
{
public Subscriber2(Shape shape)
{
IShape d = (IShape)shape;
d.OnDraw += d_OnDraw;
}
void d_OnDraw(object sender, EventArgs e)
{
Console.WriteLine("Sub2 receives the IShape event.");
}
}
public class Program
{
static void Main(string[] args)
{
Shape shape = new Shape();
Subscriber1 sub = new Subscriber1(shape);
Subscriber2 sub2 = new Subscriber2(shape);
shape.Draw();
// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
}
/* Output:
Sub1 receives the IDrawingObject event.
Drawing a shape.
Sub2 receives the IShape event.
*/
//</Snippet10>
//<snippet12>
namespace ImplementInterfaceEvents
{
public interface IDrawingObject
{
event EventHandler ShapeChanged;
}
public class MyEventArgs : EventArgs
{
// class members
}
public class Shape : IDrawingObject
{
public event EventHandler? ShapeChanged;
void ChangeShape()
{
// Do something here before the event…
OnShapeChanged(new MyEventArgs(/*arguments*/));
// or do something here after the event.
}
protected virtual void OnShapeChanged(MyEventArgs e)
{
ShapeChanged?.Invoke(this, e);
}
}
}
//</snippet12>