Recipe 7.10 Detect Whether an Object Exists

7.10.1 Problem

You create and delete objects as your application runs. At some point, you need to be able to tell whether an object exists and make decisions based on that fact. But you can't find a function in Access that will tell you if a specific object already exists. Are you missing something? This ought to be a basic part of the product!

7.10.2 Solution

You haven't missed anything: Access really doesn't supply a simple method of determining if a specific object already exists. On the other hand, this is really quite simple, as long as you understand two important concepts: Access's support for DAO Container objects, and the ways you can use error handling to retrieve information. This solution uses these two subjects to provide a function you can call to check for the existence of any object.

Load and run frmTestExist from 07-10.MDB. This form, shown in Figure 7-14, lets you specify an object name and its type and then tells you whether that object exists. Certainly, you wouldn't use this form as-is in any application—its purpose is to demonstrate the acbDoesObjExist function in basExists (07-10.MDB). To make your exploration of frmTestExist easier, Table 7-8 lists the objects that exist in 07-10.MDB. Try entering names that do and don't exist, and get the types right and wrong, to convince yourself that the acbDoesObjExist function does its job correctly.

Figure 7-14. frmTestExist lets you check for the existence of any object in the current database

Table 7-8. The sample objects in 07-10.MDB

Object name	Object type
tblTest	Table
qryTest	Query
frmTest	Form
frmTestExist	Form
basExists	Module

Follow these steps to use acbDoesObjExist in your own applications:

7.10.3 Discussion

The acbDoesObjExist function, shown in full here, checks for the existence of an object by attempting to retrieve that object's Name property. Because every object that exists exposes a Name property, this action can't fail unless the object doesn't exist. In skeleton format, the code works like this:

   Dim strName As String
   On Error Goto acbDoesObjExist_Err

   strName = obj.Name
   acbDoesObjExist = True

acbDoesObjectExist_Exit:
   Exit Function

acbDoesObjectExist_Err:
   acbDoesObjExist = False
   Resume acbDoesObjectExist_Exit

That is, the code sets up an error handler and then attempts to retrieve the Name property of the requested object. If it succeeds, the code falls through, sets the return value to True, and returns. If it triggers an error, the procedure can be assured that the object doesn't exist, and it will return False.

The only other issue is how to convert a string containing the name of the object and an integer containing its type to a real object reference. This is where the Jet engine's Container objects come in handy. The Container collections, supplied by Access so the Jet engine can support security for all the Access objects, contain collections of Document objects (one for each saved object in your database). The Containers collection contains collections named Tables, Forms, Reports, Scripts (that's macros for us users!), and Modules. Except for tables and queries, the code checks in those collections of documents, looking for the document whose name you've supplied. For tables and queries, it's simpler to use the TableDefs and QueryDefs collections directly. Access lumps tables and queries together in the Tables container, but keeps them separate in the TableDefs and QueryDefs collections. If the code looked in the Tables container, it would have to take an extra step to distinguish tables from queries; that step isn't necessary if it uses the collections instead.

The code for acbDoesObjExist is as follows:

Public Function acbDoesObjExist( _
 strObj As String, objectType As AcObjectType)
    Dim db As DAO.Database
    Dim strCon As String
    Dim strName As String
    
    On Error GoTo HandleErr
    
    Set db = CurrentDb( )
    Select Case objectType
        Case acTable
            strName = db.TableDefs(strObj).Name
        Case acQuery
            strName = db.QueryDefs(strObj).Name
        Case acForm, acReport, acMacro, acModule
            Select Case objectType
                Case acForm
                    strCon = "Forms"
                Case acReport
                    strCon = "Reports"
                Case acMacro
                    strCon = "Scripts"
                Case acModule
                    strCon = "Modules"
            End Select
            strName = db.Containers(strCon).Documents(strObj).Name
    End Select
    acbDoesObjExist = True

ExitHere:
    Exit Function
       
HandleErr:
    acbDoesObjExist = False
    Resume ExitHere
End Function

Note that in the Select Case statement, the code first checks to see if you're asking about a table or a query. If so, it looks in the appropriate collection:

Select Case objectType
   Case acTable
      strName = db.TableDefs(strObj).Name
   Case acQuery
      strName = db.QueryDefs(strObj).Name
.
.
.
End Select

If not, it assigns to strCon the name of the container it will need and then attempts to retrieve the Name property of the particular document within the selected container:

Case acForm, acReport, acMacro, acModule
   Select Case objectType
      Case acForm
         strCon = "Forms"
      Case acReport
         strCon = "Reports"
      Case acMacro
         strCon = "Scripts"
      Case acModule
         strCon = "Modules"
   End Select
   strName = db.Containers(strCon).Documents(strObj).Name

7.10.4 See Also

If you haven't done much investigation of DAO in Access, you may find it useful to study the appropriate chapters in the Building Applications manual that ships with Access. Though complete coverage of DAO is beyond the scope of this book, there are several examples using DAO in other chapters, especially Chapter 4 and Chapter 6. In addition, DAO Object Model: The Definitive Reference, by Helen Feddema (O'Reilly), provides complete documentation of the DAO object model.