{"id":419,"date":"2015-02-05T23:37:04","date_gmt":"2015-02-05T15:37:04","guid":{"rendered":"https:\/\/www.techcoil.com\/blog\/?p=419"},"modified":"2018-09-04T22:45:16","modified_gmt":"2018-09-04T14:45:16","slug":"how-to-raise-and-consume-custom-events-in-c","status":"publish","type":"post","link":"https:\/\/www.techcoil.com\/blog\/how-to-raise-and-consume-custom-events-in-c\/","title":{"rendered":"How to raise and consume custom events in C#"},"content":{"rendered":"

When something happens asynchronously, the events mechanisms in C# is a good way for different components of our C# programs to notify one another about it. <\/p>\n

For example, when the report generation component had the finance report ready, it can raise an event to components that had registered interest in that finance report. <\/p>\n

This post documents how we can raise and consume custom events in C#.<\/p>\n

Defining a delegate for components that are interested in the event<\/h2>\n

We first define the delegate that will hold C# methods from components which need to be notified when our report is completed by the report generation module. <\/p>\n

Components which are interested in the report generation module shall provide a method that returns nothing and accepts a string<\/code> as its only input parameter.<\/p>\n

\r\npublic delegate void ReportCompletedHandler(string report);\r\n<\/pre>\n
I tend to put the definition of the delegate in the same source file that defines the component which will fire the event.<\/p>\n
Defining the event object<\/h2>\n
We then define the event object as an instance variable in the class definition of the component which will fire the event.<\/p>\n
\r\npublic event ReportCompletedHandler reportCompleted;\r\n<\/pre>\n
The event keyword defines the delegate that we had defined earlier as a custom event and makes the event available through the variable reportCompleted<\/code>. <\/p>\n
Registering interest in the event<\/h2>\n
To register interest in the event, we first define a method which had the same signature as the delegate, ReportCompletedHandler<\/code>.<\/p>\n
\r\nprivate void displayReport(string report) \r\n{\r\n    Console.WriteLine("Received report: " + report);\r\n}\r\n<\/pre>\n
And then add the method as a delegate to the event:<\/p>\n
\r\nreportCompleted += new ReportCompletedHandler(this.displayReport);\r\n<\/pre>\n
Firing the event<\/h2>\n
When the report is ready, the report generation component will first check whether there are delegates registered to the reportCompleted<\/code> event. If there are interested parties registered to the event, it will fire the event.<\/p>\n
\r\n\r\nString report = "<A sample report>";\r\n\r\nif (reportCompleted != null) \r\n{\r\n    reportCompleted(report);\r\n}\r\n<\/pre>\n
A complete example for this post<\/h2>\n
The delegate and event generator source<\/h3>\n
\r\n\/\/ Define delegate for components that are interested in report completion\r\npublic delegate void ReportCompletedHandler(string report);\r\n\r\n\/\/ The event generator source\r\npublic class ReportGenerator\r\n{\r\n    public event ReportCompletedHandler reportCompleted;\r\n\r\n    public void generateReport()\r\n    {\r\n        \/\/ Fire report completed event with report\r\n        string report = "<A sample report>";\r\n\r\n        if (reportCompleted != null)\r\n        {\r\n            reportCompleted(report);\r\n        }\r\n    }\r\n}\r\n<\/pre>\n
A component that is interested in report completion<\/h3>\n
\r\npublic class ReportDisplayer \r\n{\r\n\r\n    public ReportDisplayer()\r\n    {\r\n        \/\/ Register interest in the completed report\r\n        ReportGenerator.getInstance().reportCompleted += new ReportCompletedHandler(this.displayReport);\r\n    }\r\n\r\n    public void displayReport(string report)\r\n    {\r\n        Console.WriteLine("Diplaying report:");\r\n        Console.WriteLine(report);\r\n    }\r\n\r\n}\r\n<\/pre>\n
Class that contains the main method to start the program<\/h3>\n
\r\nusing System;\r\npublic class Program\r\n{\r\n    public static void Main(string[] args)\r\n    {\r\n\r\n        \/\/ Set up report displayer\r\n        ReportDisplayer reportDisplayer = new ReportDisplayer();\r\n\r\n        \/\/ Trigger report generation\r\n        ReportGenerator.getInstance().generateReport();\r\n\r\n    }\r\n}\r\n<\/pre>\n\n