X++, C# Comparison: Event

Article
05/18/2015

Applies To: Microsoft Dynamics AX 2012 R3, Microsoft Dynamics AX 2012 R2, Microsoft Dynamics AX 2012 Feature Pack, Microsoft Dynamics AX 2012

There are some differences in how X++ and C# implement the event design pattern. For more information, see Event Terminology and Keywords.

Comparison of Events between X++ and C#

There are differences in the way delegates are used for events in X++ versus C#.

Item	X++	C#	Comment
delegate	In X++, a delegate can be declared only as a member on a class. A delegate cannot be a member on a table. All delegates are instance members of their class, not static members. No access modifier can be used on a delegate declaration, because all delegates are protected members. Therefore, the event can be raised only by code within the same class where the delegate is a member. However, the one exception to the private nature of a delegate is that code outside their class can operate on the delegates by using the += and -= operators.	In C#, each delegate is a type, just as every class is a type. A delegate is declared independently of any class. Without the event keyword, you can have a delegate as a parameter type on a method, just as you can have a class as a parameter type. You can construct an instance of a delegate to pass in for the parameter value.	In X++, each class is a type, but no delegate is a type. You cannot construct an instance of a delegate. No delegate can be a parameter for a method. But you can create a class that has a delegate member, and you can pass instances of the class as parameter values. For more information, see X++ Keywords.
event	In X++ code, an event is one of the following: An explicit call to a delegate. The start or end of a method. There is no event keyword in X++.	In C#, the event keyword is used to declare a delegate type as a member of a class. The effect of the event keyword is to make the delegate protected, yet still accessible for the += and -= operators. You can subscribe event handler methods to an event by using the += operator. A delegate can be useful without the event keyword, as a technique for passing a function pointer as a parameter into a method.	The automatic events that occur before the start of a method, and after the end of a method, can be subscribed to only by using the AOT.
+= and -= operators	In X++, you use the += operator to subscribe methods to a delegate. The -= operator unsubscribes a method from a delegate.	In C#, you use the += operator to subscribe methods to an event, or to a delegate that is not used with the event keyword.	The delegate contains a reference to all the objects that have methods subscribed to the delegate. Those objects are not eligible for garbage collection while delegate holds those references.
eventHandler	In X++, the eventHandler keyword is required when you use either the += or -= operator to subscribe or unsubscribe a method from a delegate.	System.EventHandler is a delegate type in the .NET Framework.	This term is used differently in X++ than it is in C# or the .NET Framework. For more information, see X++ Keywords.

X++ and C# Code Examples

This section contains an X++ code example for the event design pattern. It also contains a C# code sample for the same design pattern.

X++ Example

The important things to notice in the X++ example are the following:

The XppClass has a delegate member that is named myDelegate.

Note

The AOT contains a node for the delegate. The node is located at AOT > Classes > XppClass > myDelegate. Several event handler nodes can be located under the myDelegate node. Event handlers that are represented by nodes in the AOT cannot be removed by the -= operator during run time.
The {} braces at the end of the delegate declaration are required, but they cannot have any code in them.
The XppClass has two methods whose parameter signatures are compatible with the delegate. One method is static.
The two compatible methods are added to the delegate with the += operator and the eventHandler keyword. These statements do not call the event handler methods, the statements only add the methods to the delegate.
The event is raised by one call to the delegate.
The parameter value that passed in to the delegate is received by each event handler method.
The short X++ job at the top of the example starts the test.

    // X++
    // Simple job to start the delegate event test.
    static void DelegateEventTestJob()
    {
        XppClass::runTheTest("The information from the X++ job.");
    }
    
    
    // The X++ class that contains the delegate and the event handlers.
    class XppClass
    {
        delegate void myDelegate(str _information)
        {
        }
    
        public void myEventSubscriberMethod2(str _information)
        {
            info("X++, hello from instance event handler 2: " + _information);
        }
    
        static public void myEventSubscriberMethod3(str _information)
        {
            info("X++, hello from static event handler 3: " + _information);
        }
    
        static public void runTheTest(str _stringFromJob)
        {
            XppClass myXppClass = new XppClass();
    
            // Subscribe two event handler methods to the delegate.
            myXppClass.myDelegate += eventHandler
                    (myXppClass.myEventSubscriberMethod2);
            myXppClass.myDelegate += eventHandler
                    (XppClass::myEventSubscriberMethod3);
    
            // Raise the event by calling the delegate one time,
            // which calls all the subscribed event handler methods.
            myXppClass.myDelegate(_stringFromJob);
        }
    }

The output from the previous X++ job is as follows:

X++, hello from static event handler 3: The information from the X++ job.

X++, hello from instance event handler 2: The information from the X++ job.

C# Sample

This section contains a C# code sample for the event design pattern of the previous X++ sample.

// C#
using System;

// Define the delegate type named MyDelegate.
public delegate void MyDelegate(string _information);

public class CsClass
{
    protected event MyDelegate MyEvent;

    static public void Main()
    {
        CsClass myCsClass = new CsClass();

        // Subscribe two event handler methods to the delegate.
        myCsClass.MyEvent += new MyDelegate
                (myCsClass.MyEventSubscriberMethod2);
        myCsClass.MyEvent += new MyDelegate
                (CsClass.MyEventSubscriberMethod3);

        // Raise the event by calling the event one time, which
        // then calls all the subscribed event handler methods.
        myCsClass.MyEvent("The information from the C# Main.");
    }

    public void MyEventSubscriberMethod2(string _information)
    {
        Console.WriteLine("C#, hello from instance event handler 2: " + _information);
    }

    static public void MyEventSubscriberMethod3(string _information)
    {
        Console.WriteLine("C#, hello from static event handler 3: " + _information);
    }
}

The output from the previous C# sample is as follows:

>> CsClass.exe

C#, hello from instance event handler 2: The information from the C# Main.

C#, hello from static event handler 3: The information from the C# Main.

Events and the AOT

Microsoft Dynamics AX has other event systems that apply only to items in the AOT. For more information, see Event Handler Nodes in the AOT.