Jon Jagger
Table of Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Notes DownloadECMA-334 C# Language Specificationpreviousnextprevious at this levelnext at this level 17 Classesprevious at this levelnext at this level 17.7 Events Paragraph 11 An event is a member that enables an object or class to provide notifications. 2 Clients can attach executable code for events by supplying event handlers. Paragraph 21 Events are declared using event-declarations: event-declaration : attributesopt event-modifiersopt event type variable-declarators ; attributesopt event-modifiersopt event type member-name { event-accessor-declarations } event-modifiers : event-modifier event-modifiers event-modifier event-modifier : new public protected internal private static virtual sealed override abstract extern event-accessor-declarations : add-accessor-declaration remove-accessor-declaration remove-accessor-declaration add-accessor-declaration add-accessor-declaration : attributesopt add block remove-accessor-declaration : attributesopt remove block Paragraph 31 An event-declaration may include a set of attributes (§24) and a valid combination of the four access modifiers (§17.2.3), the new (§17.2.2), static (§17.5.2, §17.7.3), virtual (§17.5.3, §17.7.4), override (§17.5.4, §17.7.4), sealed (§17.5.5), abstract (§17.5.6, §17.7.4), and extern modifiers. Paragraph 41 Event declarations are subject to the same rules as method declarations (§17.5) with regard to valid combinations of modifiers. Paragraph 51 The type of an event declaration must be a delegate-type (§11.2), and that delegate-type must be at least as accessible as the event itself (§10.5.4). Paragraph 61 An event declaration may include event-accessor-declarations. 2 However, if it does not, for non-extern, non-abstract events, the compiler shall supply them automatically (§17.7.1); for extern events, the accessors are provided externally. Paragraph 71 An event declaration that omits event-accessor-declarations defines one or more events-one for each of the variable-declarators. 2 The attributes and modifiers apply to all of the members declared by such an event-declaration. Paragraph 81 It is a compile-time error for an event-declaration to include both the abstract modifier and brace-delimited event-accessor-declarations. Paragraph 91 When an event declaration includes an extern modifier, the event is said to be an external event. 2 Because an external event declaration provides no actual implementation, it is an error for it to include both the extern modifier and event-accessor-declarations. Paragraph 101 An event can be used as the left-hand operand of the += and -= operators (§14.13.3). 2 These operators are used, respectively, to attach event handlers to, or to remove event handlers from an event, and the access modifiers of the event control the contexts in which such operations are permitted. Paragraph 111 Since += and -= are the only operations that are permitted on an event outside the type that declares the event, external code can add and remove handlers for an event, but cannot in any other way obtain or modify the underlying list of event handlers. Paragraph 121 In an operation of the form x += y or x -= y, when x is an event and the reference takes place outside the type that contains the declaration of x, the result of the operation has type void (as opposed to having the type of x, with the value of x after the assignment). 2 This rule prohibits external code from indirectly examining the underlying delegate of an event. [Example: The following example shows how event handlers are attached to instances of the Button class:
public delegate void EventHandler(object sender, EventArgs e);  
public class Button: Control  
   public event EventHandler Click;  
public class LoginDialog: Form  
   Button OkButton;  
   Button CancelButton;  
   public LoginDialog() {  
      OkButton = new Button(...);  
      OkButton.Click += new EventHandler(OkButtonClick);  
      CancelButton = new Button(...);  
      CancelButton.Click += new EventHandler(CancelButtonClick);  
   void OkButtonClick(object sender, EventArgs e) {  
      // Handle OkButton.Click event  
   void CancelButtonClick(object sender, EventArgs e) {  
      // Handle CancelButton.Click event  
Here, the LoginDialog instance constructor creates two Button instances and attaches event handlers to the Click events. end example]
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