Declaring an Association Class

An association class is a special type of class that defines a relationship between two other classes.

The following procedure describes how to create an association class using MOF code.

To create an association class using MOF code

1. Assign the Association qualifier to your class.

   Although it is possible to create a class with references to objects or classes, using the Association qualifier not only makes it clear that your class is an association class, but, as a best practice, ensures that your class fully functions as an association class.

2. Create two references within the class describing the two object instances you wish to associate together using the ref type.

   The references bind the two objects in the association by containing paths to the objects. Although not required, use the reference properties as key properties as well.

   Although you can create fully qualified or namespace-relative references, WMI has only limited support for cross-namespace references. Specifically, only statically defined objects can reference each other across namespace boundaries; dynamically supported objects cannot reference each other.

   If necessary, use the HasClassRef and Classref qualifiers in conjunction with the object ref type to reference a class.

   WMI supports having one ref reference point to an instance, and the other object reference point to a class. In this case, your association class would describe an association that binds instances to classes.

   The following code example describes the syntax for using HasClassRef and Classref with an object type.

   [HasClassRefs, Association]
   class SomeAssocClass
   {
        [key, classref{ "MyEndpoint", "OtherContainer" }]
        object ref ep1;
        [key] object ref ep2;
   }; 
   

   In the previous example, the ep1 reference can point to the class definitions for either the MyEndpoint class or the OtherContainer class. Note that while you must weakly type the reference class, you cannot weakly type the Classref qualifier itself; doing so would severely reduce the efficiency of the WMI query engine. Weak typing is creating a reference that can contain any data type by using the object keyword and ref data type. To successfully use HasClassRef, you must set the relevant qualifier flavors to propagate to all instances and subclasses.

3. Create any other properties as necessary.

   The following code example shows that WMI does not currently support association classes having less or more than two reference properties.

   [Association : ToInstance] 
   class MyAssocClass
   {
       ClassX ref PathToClassX ;
       ClassY ref PathToClassY ;
   };
   

4. When finished, compile your MOF code with the MOF compiler.

   For more information, see Compiling MOF Files.

The code example in Step 3 defines the MyAssocClass association class. The MyAssocClass class defines a relationship between ClassX and ClassY. The PathToClassX and PathToClassY properties contain object paths to the instances of the classes to be associated. The keyword ToInstance is one of several flavor flags that WMI defines to provide information about the use of a qualifier. The ToInstance keyword indicates that WMI should propagate the Association qualifier to all instances of the association class. By checking this instance qualifier, the client software can determine that an instance belongs to an association class, without having to retrieve the class definition to look for the Association qualifier. For more information, see Describing a Qualifier With a Qualifier Flavor and References.

Related topics

Designing Managed Object Format (MOF) Classes