Visual Basic Q&A

Symptoms
The Stream object introduced in ActiveX Data Objects (ADO) 2.5 can be used to greatly simplify the code that needs to be written to access and modify Binary Large Object (BLOB) data in a SQL Server Database. The previous versions of ADO [ 2.0, 2.1, and 2.1 SP2 ] required careful usage of the GetChunk and AppendChunk methods of the Field Object to read and write BLOB data in fixed-size chunks from and to a BLOB column. An alternative to this method now exists with the advent of ADO 2.5. This article includes code samples that demonstrate how the Stream object can be used to program the following common tasks: Save the data stored in a SQL Server Image column to a file on the hard disk.Move the contents of a .gif file to an Image column in a SQL Server table.
Resolution
The following code samples are based on the data stored in the pub_info table in the SQL Server 7.0 pubs sample database. You need to modify the ADO connection string to point to your SQL Server installation. Example 1 : Saving the Data in a SQL Server Image Column to a File on the Hard Disk The code in this example opens a recordset on the pub_info table in the pubs database and saves the binary image data stored in the logo column of the first record to a file on the hard disk, as follows: Open a new Standard EXE Visual Basic project.On the Project menu, click to select References, and then set a reference to the Microsoft ActiveX Data Objects 2.5 Object Library.Place a CommandButton control on Form1.Make the following declarations in the form’s General declarations section:

Dim cn As ADODB.ConnectionDim rs As ADODB.RecordsetDim mstream As ADODB.Stream Cut and paste the following code into the Click event of the CommandButton that you added to the form:

Set cn = New ADODB.Connectioncn.Open “Provider=SQLOLEDB;data Source=<name of your SQL Server>;Initial Catalog=pubs;User Id=<Your Userid>;Password=<Your Password>”Set rs = New ADODB.Recordsetrs.Open “Select * from pub_info”, cn, adOpenKeyset, adLockOptimisticSet mstream = New ADODB.Streammstream.Type = adTypeBinarymstream.Openmstream.Write rs.Fields(“logo”).Valuemstream.SaveToFile “c:\publogo.gif”, adSaveCreateOverWriters.Closecn.Close Save and run the Visual Basic project.Click the CommandButton to save the binary data in the logo column of the first record to the file c:\publogo.gid. Look for this file in Windows Explorer and open it to view the saved image.
The code in this example declares an ADODB Stream object and sets its Type property to adTypeBinary to reflect that this object will be used to work with Binary data. Following this, the binary data stored in the logo column of the first record in the pub_info table is written out to the Stream object by calling its Write method. The Stream object now contains the binary data that is saved to the file by calling its SaveToFile method and passing in the path to the file. The adSaveCreateOverWrite constant passed in as the second parameter causes the SaveToFile method to overwrite the specified file if it already exists.Example 2 : Transfer the Image Stored in a .gif File to an Image Column in a SQL Server Table The code in this example saves an image stored in a .gif file to the logo column in the first record of the pub_info table by overwriting its current contents, as follows: Open a new Standard EXE Visual Basic project.On the Project menu, click to select References, and then set a reference to the Microsoft ActiveX Data Objects 2.5 Object Library.Place a CommandButton on Form1. Make the following declarations in the form’s General declarations section:

Dim cn As ADODB.ConnectionDim rs As ADODB.RecordsetDim mstream As ADODB.Stream Cut and paste the following code in the Click event of the CommandButton that you added to the form:

Set cn = New ADODB.Connectioncn.Open “Provider=SQLOLEDB;data Source=<name of your SQL Server>;Initial Catalog=pubs;User Id=<Your Userid>;Password=<Your Password>”Set rs = New ADODB.Recordsetrs.Open “Select * from pub_info”, cn, adOpenKeyset, adLockOptimisticSet mstream = New ADODB.Streammstream.Type = adTypeBinarymstream.Openmstream.LoadFromFile “<path to .gif file>”rs.Fields(“logo”).Value = mstream.Readrs.Updaters.Closecn.Close Save and run the Visual Basic project.Click on the CommandButton to run the code to stream the contents of the .gif file to the ADO Stream object, and save the data in the Stream to the logo column in the first record of the recordset.Verify that the image in the logo column has been modified by using the code in Example 1.

Symptoms
When you try to insert an empty string into Memo, Text, nText or Blob columns by using the OLE DB .NET data provider, you receive the following exception:

An unhandled exception of type ‘System.InvalidOperationException’ occurred in system.data.dll If you handle this exception within a try-catch block, you receive the following information:

System.InvalidOperationException
System.Data.OleDb.OleDbException: Multiple-Step OLE DB operation generated errors. Check each OLE DB status value, if available. No work was done.
Resolution
To work around this problem: For strings, insert a space that has a length greater than zero (such as ” “), instead of using an empty string.If the database has an Allow Nulls setting, select Allow Nulls for the columns in the database, and treat nulls as empty strings.

Symptoms
In an MFC application, AfxThrowOleDispatchException is used to throw anexception from within an OLE Automation function. When Microsoft VisualBasic is used as the automation client application, it will not be able tocorrectly interpret the error code passed to AfxThrowOleDispatchException.
Resolution
AfxThrowOleDispatchException constructs and throws a COleDispatchExceptionobject, which is used to handle exceptions specific to the OLE IDispatchinterface. The constructor of COleDispatchException incorrectly initializesthe COleDispatchException object, resulting in the Visual Basic clientbeing unable to interpret the error code of the exception.