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X++, C# Comparison: Event [AX 2012]

Updated: May 19, 2010

Applies To: 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.

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.

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.

    NoteNote

    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.

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.

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