PRB: Problems Reusing ADO Command Object on Multiple Recordsets

ID: Q197449

The information in this article applies to:

SYMPTOMS

When using a single command object to open multiple recordset objects, with one or more of those recordset objects open at the same time, you will encounter unexpected behavior. This is most notable when opening stored procedures with output or return parameters.


CAUSE

Consider the ActiveX Data Objects (ADO) code written in Visual Basic for Applications below: 


Dim cn As New ADODB.Connection
Dim cm As New ADODB.Command
Dim rs1 As New ADODB.Recordset
Dim rs2 As ADODB.Recordset
Dim rs3 As New ADODB.Recordset

cn.Open "DSN=Pubs;UID=sa;PWD=;"

On Error Resume Next
cn.Execute "drop procedure dbo.sp_AdoTest"
cn.Execute "create proc sp_AdoTest ( @InParam int ) as " & _
           "select * FROM Authors WHERE State <> 'CA' " & _
           "return @InParam + 10"

cn.Execute "drop procedure dbo.sp_AdoTest2"
cn.Execute "create proc sp_AdoTest2  as " & _
           "select * FROM Authors WHERE State <> 'CA' " & _
           "return 10"


Set cm.ActiveConnection = cn
cm.CommandType = adCmdStoredProc
cm.CommandText = "sp_AdoTest"
cm.Parameters.Refresh
cm.Parameters(1).Value = 10
Set rs1 = cm.Execute

Debug.Print cm.Parameters.Count
At this point, the Parameters collection for the command object has two objects, and you can reference them as you would normally. However, consider the following code, which destroys (resets) the Parameters collection: 

cm.CommandText = "sp_AdoTest2"
cm.Parameters.Refresh
Set rs1 = cm.Execute

Debug.Print cm.Parameters.Count
At this point, the Parameters collection has been reset to contain only one Parameter object. However, the original Recordset (rs1), which has not been closed or released, still has a reference to an internal buffer that stores the value of each parameter that was originally created. When the original recordset is closed, it attempts to release the buffer. However, the second recordset's Parameters collection may be using the same memory location for its own buffers, and if both recordsets are trying to release the same memory location, you could get an Access Violation. 

rs2.Close
rs1.Close

cn.Close
The Access Violation has been confirmed to occur (at random intervals) when using a forward only cursor with a parameterized stored procedure using the OLE DB Provider for ODBC to the SQL Server ODBC Driver. It may occur with other ODBC Drivers or OLE DB Providers as well.

The Access Violation has been fixed in a future release of ADO. However, recycling the same Command Object for multiple, concurrently open Recordsets is not a safe practice. Any savings in performance or memory allocation you receive in minimizing the number of created objects will not be compensated for by increased risk of your code causing a failure within ADO. The Command Object is not designed or intended for this kind of utilization.


STATUS

This behavior is by design.

Additional query words: 


Keywords          : kbADO kbADO100 kbADO200 kbADO201 kbDatabase 
Version           : WINDOWS:1.0,1.5,2.0,2.1
Platform          : WINDOWS 
Issue type        : kbprb

Last Reviewed: July 29, 1999