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The information in this article applies to:
- Microsoft Visual Basic programming system for Windows, version 3.0
SUMMARY
Chapter 20 in the "Programmer's Guide" explains how to use a data control
in Microsoft Visual Basic version 3.0 for Windows. You may want to use an
object variable such as Snapshot instead of using the data control. Chapter
20 does not explain the limitations of using the data control, so this
article lists those limitations for you.
MORE INFORMATION
Because a data control is a special type of Dynaset, its limitations are
similar to those of Dynasets. Here are the limitations of data controls:
- You cannot use a QueryDef requiring a Parameter in the RecordSource
property of the data control.
- Using a data control along with other bound controls uses System
Resources (memory). When you build larger programs, you may want to
look at other programming methods (Database objects don't require
controls, therefore you don't use System Resources) to display your
database data.
- Not every method and property specific to the Table object can be
performed by the data control. Here are two such cases:
- You cannot take advantage of the Index property of the Table object
to display your database data in a specific indexed order with the
data control. This technique, described in the example shown in the
Help file topic "Index Property (Data Access)," works only with the
Table object, not the data control. As an alternative, you can use an
ORDER BY clause in an SQL statement, as in this example:
Data1.RecordSource = "Select * From Publishers Order By PubID"
The ORDER BY clause technique is also more flexible than the Index
property technique. Using the ORDER BY clause, you can sort on any
field, and no specified index is required.
- You cannot use a Seek method on your database data for a specific
record with the data control. The Seek method can only be used by the
Table object. You can, however, perform a FindFirst method with the
data control.
- You cannot use the Sort property on a specific database record with the
data control. The Sort property technique is specific to a Dynaset or
Snapshot object. The following example proves this limitation:
Data1.Recordset.Sort = "City DESC" '** No error occurs
Data1.Refresh '** No change in order occurs
If you try to sort the Publishers table by City, nothing happens. But if
you use an ORDER BY clause in an SQL statement, as in the following
example, you will see the database data displayed in descending order
by the City names:
Data1.RecordSource = "Select * From Publishers Order By City DESC"
Data1.Refresh
- A data control is bound to one, single form -- the form on which it
resides. Therefore, when the form that contains the data control is
not loaded, you cannot refer to the data control from another form.
- You cannot perform a FileCopy statement on a database while a form that
contains a data control is loaded. A "Permission Denied" error occurs if
you try to use the FileCopy statement to make a backup of your database
while a form containing a data control is loaded in memory. To prevent
this error, first close or unload the form that contains the data
control. Then run the FileCopy statement to make a database backup.
- You cannot use a data control to display the results of a parameter
query. It is possible to use the Database object of the data control to
open and run a querydef which requires parameters. For example:
Sub Command1_Click ()
Dim db As Database
Dim Qd As QueryDef
Dim Sn As Snapshot
Data1.DatabaseName = "c:\vb\biblio.mdb"
Data1.RecordSource = "Authors"
Data1.Refresh
' Open the "By date" query
Set Qd = Data1.Database.OpenQueryDef("By date")
' Set the value of the dp parameter
Qd!dp = 1991
' create a snapshot off of querydef
Set Sn = Qd.CreateSnapshot()
Sn.MoveFirst
Do Until Sn.EOF
For i = 1 To Sn.Fields.Count - 1
Print Sn(i) & " "; 'Display results of query
Next
Print
Sn.MoveNext
Loop
Sn.Close
Qd.Close
db.Close
End Sub
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