Tuesday, April 28, 2009

Bang Vs. Dot In DAO

You sometime hear that: "the bang (!) refers to user-defined things and dot (.) refers to Access-defined things." Although that is the standard rule of thumb, it is not exactly correct. More precisely, the bang (!) serves to separate an object from the collection which contains it, and the dot (.) serves to separate one level of the DAO hierarchy from another.

Let me back up.

DAO naming is hierarchical in nature, sort of like the DOS path. And like DOS, you can refer to an object using a fully qualified name or a semi-qualified name. In DOS, a fully qualified name would be like this:

C:\MSOFFICE\ACCESS\TEST.MDB

If you are already in a folder, you can refer to the file by its semi-qualified name:

TEST.MDB
In the same way, you can refer to an Access object by its fully qualified name:

DBEngine.Workspaces(0).Databases! _
[c:\msoffice\access\test.mdb].TableDefs!Table1

or if you assume the default DBEngine (which we almost always do), the default Workspace, and default Database, you can refer to the table by its semi-qualified name:

TableDefs!Table1

If you look at the fully qualified name like this:

DBEngine.
Workspaces(0).
Databases![c:\msoffice\access\test.mdb].
TableDefs!Table1

you can see the DAO hierarchy levels more easily and how the dot separates them. (Much like the "\" in DOS.)

The dot also serves to separate an object from its properties and methods, which can also be thought of as another level in the hierarchy. So I can refer to "TableDefs!Table1.RecordCount". RecordCount being a property of Table1.

The bang (!) separates objects from the collections which hold them, thus it separates "Table1" from the collection "TableDefs" and the object "c:\msoffice\access97\test.mdb" from its collection "Databases".

Since most objects are defined by you, and since levels of DAO hierarchy are defined by Access, we get the rule of thumb named earlier.

DAO Naming Rules:

  1. The dot serves to separate one level of the DAO hierarchy from another in a fully qualified object name.
  2. The dot also serves to separate an object from its methods and properties.  (This, by the way, is the principle use for most people).
  3. The bang serves to separate an object from the collection in which it is contained.
In version DAO 3.0 (Access 2.0), you could use either the bang or the dot to separate an object from its collection. But in DAO 3.5 (Access 97), most objects didn't support the dot for this purpose. DAO 4.0 (Access 2000 and beyond), doesn't support it at all. You have to use the bang and dot properly or you will get a syntax error.

But this isn't really the end of the story.  When using class modules in Access (like Forms and Reports), the Bang and Dot behavior is slightly different.  To find out more, read my post: Bang Vs. Dot in Forms .

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