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Registering a Transient Subscription

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Transient subscriptions request an event for a specific subscriber object that already exists. Transient subscriptions are stored in the COM+ catalog, but the subscription is deleted if the event system or operating system is stopped. Unlike persistent subscriptions, transient subscriptions are tied to a particular object and are stored only in the event system. If there is a problem with the event system or operating system, the subscription disappears. Transient subscriptions can be more efficient than persistent subscriptions, but you must manage their object life cycles.

Transient subscriptions cannot be set by using the Component Services administration tool. You must use the COM+ administrative interfaces to create or update a transient subscription.

Visual Basic

The following procedure describes how to create a transient subscription using Microsoft Visual Basic:

  1. Specify a subscription as transient by adding a new entry to the TransientSubscriptions collection and setting the SubscriberInterface property to the IUnknown interface of the subscriber object. COM+ Events does not create a new instance of the subscriber object when firing an event but instead uses the instance you specify. COM+ Events holds a reference count for the subscriber object until the subscription is removed from the system.

  2. Create a COM+ server application (an .exe, a .dll, or an .ocx file) with an object that implements the interfaces or methods you want to subscribe to.

  3. Create your transient subscription using the following code, by passing in the CLSID of the event class object and the instance of the subscriber object. Using the Component Services administrative tool, you can get the EventCLSID property by right-clicking the COM+ component, selecting Properties, and selecting the General tab.

    Public Function CreateTransientSubscription( _
  ByVal clsid As String, ByVal objref As Object) As String 
    Dim oCOMAdminCatalog As COMAdmin.COMAdminCatalog
    Dim oTSCol As COMAdminCatalogCollection
    Dim oSubscription As ICatalogObject
    Dim objvar As Variant
    On Error GoTo CreateTransientSubscriptionError
    Set oCOMAdminCatalog = CreateObject("COMAdmin.COMAdminCatalog")
    'Gets the TransientSubscriptions collection
    Set oTSCol = oCOMAdminCatalog.GetCollection( _
      "TransientSubscriptions")
    Set oSubscription = oTSCol.Add
    Set objvar = objref
    oSubscription.Value("SubscriberInterface") = objref
    oSubscription.Value("EventCLSID") = clsid
    oSubscription.Value("Name") = "TransientSubscription"
    oTSCol.SaveChanges
    CreateTransientSubscription = oSubscription.Value("ID")
    Set oSubscription = Nothing
    Set oTSCol = Nothing
    Set oCOMAdminCatalog = Nothing
    Set objvar = Nothing
    Exit Function
CreateTransientSubscriptionError:
    CreateTransientSubscription = ""
    Err.Raise Err.Number, "[CreateTransientSubscription]" & _
      Err.Source, Err.Description
End Function

The following example illustrates how to call the CreateTransientSubscription function to subscribe to an interface called IStockTicker, which has a method called UpdateStock.

  1. Create an event class that supports the interface IStockTicker, which has one method, UpdateStock.

  2. In your subscriber project, add a class that implements the IStockTicker interface.

  3. When you want to subscribe execute the following code:

    Dim oMyTicker As Object
Dim sSubscriptionID As String
Set oMyTicker = CreateObject("TheProject.CMyTicker")
sSubscriptionID = CreateTransientSubscription( _
  "{..CLSID for the Event Class..}", oMyTicker)

The CreateTransientSubscription function returns a string, which is a GUID that can be used as a handle or a cookie to revoke the subscription later. To remove the subscription, call the Remove method of COMAdminCatalogCollection on the TransientSubscriptions collection, passing in the index corresponding to the subscription with the GUID previously received.

Registering a Subscription