Microsoft KB Archive/153238

= How To Use GetChunk and AppendChunk Methods of RDO and ADO Object =

Article ID: 153238

Article Last Modified on 7/13/2004

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APPLIES TO


 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition

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This article was previously published under Q153238



SUMMARY
This article describes how to use the GetChunk and AppendChunk methods of an RDO and ADO column object. Included is the code for a working example of how to implement this behavior.

The GetChunk and AppendChunk methods work with the LongVarChar and LongVarBinary column types, also known as TEXT and IMAGE columns, in Microsoft SQL Server. To identify these column types in RDO, use the .Type property that will return the constants rdLongVarChar or rdLongVarBinary, or use the .ChunkRequired property to determine if you need to use the Get/AppendChunk methods to access the column. Each of these column types is commonly referred to as Binary Large Objects (BLOBs), so the term BLOB will be used for the remainder of this article.

Following are some suggestions for using BLOBs with RDO:


 * Using BLOB columns in a SQL table will cause performance degradation at your server and add an extra layer of complexity in your application code that you can avoid. If you are storing files such as Paintbrush (.bmp), Microsoft Word (.doc), or just text (.txt) files, it is more efficient to store these files in your file system than in your table. You can do this by storing the UNC path for the file in a column of your table, then letting your Visual Basic code read the path and handle the file appropriately.
 * When selecting a result set containing BLOB columns, you should place the BLOB columns at the end of the select list. If you usually use the "Select * from table" syntax, you should change this to "Select char1, text1, image1 from table" to explicitly reference each column and place the BLOB columns at the end.
 * When editing a BLOB column using the AppendChunk method, you should select at least one other editable non-BLOB column in your result set and edit the non-BLOB column as well as the BLOB column. If you do not edit the non-BLOB column, RDO will not raise an error but the data may not be saved back to the base table.
 * You cannot bind a BLOB value to a parameter marker because the AppendChunk method is not available on the rdoParameter object. If you want to pass a BLOB as an input parameter to a stored procedure, you will have to utilize the ODBC handle from RDO to process this through ODBC API calls. Fortunately, the ability to do this will be implemented in all future versions of RDO following version 1.0x.
 * If you are trying to display a bitmap image in a Picture control that is stored in a LongVarBinary column, keep in mind that the Picture control in Visual Basic does not have the capability of taking in a stream of bits via Visual Basic Applications (VBA) code. The only way to place a picture into the Picture control through code, or get the bits back out of a Picture control through code, is to use a file on the disk. You can also use the RDC and bind the Picture box to the BLOB column. This works well for reads (displaying the Picture), but updates are unstable in Visual Basic 4.0 due to problems in Visual Basic's binding manager. To perform updates, you should use code, rather than the RDC.

With the ODBC cursor library, it is not possible to use the GetChunk or AppendChunk methods on a resultset returned from a stored procedure. This is because the BLOB data does not come across the pipe with the rest of the resultset. RDO has to go back and use the SQLGetData or SQLPutData ODBC AP functions on the column when you request it with the RDO GetChunk or AppendChunk methods. When a stored procedure creates a result set that is returned to your application, RDO can use the data in the result set, but it cannot go back to the base tables and columns and perform the SQLGetData/SQLP tData because all it knows is that the SQL Statement was something like "{call myproc(...)}", and there is no way for the cursor library to know how to ask for the BLOB data because there is no base table or column expressed there.

With server side cursors, it is possible to get at your BLOB data. The server-side cursor knows the content of the stored procedure and can thus get at the base table. A limitation of this is that you can't create a server-side cursor based on a stored procedure that has anything besides just one single select statement in it (a SQL Server restriction), so it is pretty limiting and doubtful that you would be able to use this as your primary technique.

The fact that users want to update their BLOB column demands that they expose their base tables and create the cursor by using a standard select statement from that base table. This would be true even if you were coding directly to ODBC (not an RDO thing), as well as dbLibary (a proprietary SQL Server API). If you use Jet, you can't update cursors based on stored procedures at all because they always become read-only.



MORE INFORMATION
The following example is divided into three separate procedures, Command1_Click, ColumnToFile, and FileToColumn. ColumnToFile and FileToColumn are two self-contained procedures that you should be able to paste directly into your code if you are moving BLOB data back and forth from your table to files on disk. Each of the procedures accept parameters that can be provided by your application. Command1_Click contains the example code that makes the connection to your database, creates the table CHUNKTABLE if it does not exist, and calls ColumnToFile and FileToColumn procedures with the proper parameters.

 Start a new project in Visual Basic. Form1 is created by default. Add a Command button, Command1, to Form1.  Paste the following code into the General Declarations section of Form1:

Private Sub Command1_Click MousePointer = vbHourglass Dim cn As rdoConnection Dim rs As rdoResultset, TempRs As rdoResultset Dim cnstr As String, sqlstr As String cnstr = "Driver={SQLServer};Server=myserver;Database=pubs;Uid=sa;Pwd=" sqlstr = "Select int1, char1, text1, image1 from chunktable"

rdoEnvironments(0).CursorDriver = rdUseServer Set cn = rdoEnvironments(0).OpenConnection( _      "", rdDriverNoPrompt, False, cnstr) On Error Resume Next If cn.rdoTables("chunktable").Updatable Then 'table exists End If    If Err > 0 Then On Error GoTo 0 Debug.Print "Creating new table..." cn.Execute "Create table chunktable(int1 int identity, " & _                 "char1 char(30), text1 text, image1 image)" cn.Execute "create unique index int1index on chunktable(int1)" End If    On Error GoTo 0 Set rs = cn.OpenResultset(Name:=sqlstr, _      Type:=rdOpenDynamic, _       LockType:=rdConcurRowver) If rs.EOF Then rs.AddNew rs("char1") = Now rs.Update rs.Requery End If    Dim currec As Integer currec = rs("int1") rs.Edit FileToColumn rs.rdoColumns("text1"), App.Path & "\README.TXT", 102400 FileToColumn rs.rdoColumns("image1"), App.Path & "\SETUP.BMP", 102400 rs("char1") = Now 'need to update at least one non-BLOB column rs.Update

'this code gets the columnsize of each column Dim text1_len As Long, image1_len As Long If rs("text1").ColumnSize = -1 Then 'the function Datalength is SQL Server specific 'so you may have to change this for your database sqlstr = "Select Datalength(text1) As text1_len, " & _ "Datalength(image1) As image1_len from chunktable " & _ "Where int1=" & currec Set TempRs = cn.OpenResultset(Name:=sqlstr, _        Type:=rdOpenStatic, _         LockType:=rdConcurReadOnly) text1_len = TempRs("text1_len") image1_len = TempRs("image1_len") TempRs.Close Else text1_len = rs("text1").ColumnSize image1_len = rs("image1").ColumnSize End If

ColumnToFile rs.rdoColumns("text1"), App.Path & "\text1.txt", _ 102400, text1_len ColumnToFile rs.rdoColumns("image1"), App.Path & "\image1.bmp", _ 102400, image1_len MousePointer = vbNormal End Sub

Sub ColumnToFile(Col As rdoColumn, ByVal DiskFile As String, _    BlockSize As Long, ColSize As Long) Dim NumBlocks As Integer Dim LeftOver As Long Dim byteData As Byte  'Byte array for LongVarBinary Dim strData As String   'String for LongVarChar Dim DestFileNum As Integer, i As Integer

' Remove any existing destination file If Len(Dir$(DiskFile)) > 0 Then Kill DiskFile End If

DestFileNum = FreeFile Open DiskFile For Binary As DestFileNum

NumBlocks = ColSize \ BlockSize LeftOver = ColSize Mod BlockSize Select Case Col.Type Case rdTypeLONGVARBINARY byteData = Col.GetChunk(LeftOver) Put DestFileNum,, byteData For i = 1 To NumBlocks byteData = Col.GetChunk(BlockSize) Put DestFileNum,, byteData Next i      Case rdTypeLONGVARCHAR For i = 1 To NumBlocks strData = String(BlockSize, 32) strData = Col.GetChunk(BlockSize) Put DestFileNum,, strData Next i        strData = String(LeftOver, 32) strData = Col.GetChunk(LeftOver) Put DestFileNum,, strData Case Else MsgBox "Not a ChunkRequired column." End Select Close DestFileNum

End Sub

Sub FileToColumn(Col As rdoColumn, DiskFile As String, _  BlockSize As Long) 'moves a disk file to a ChunkRequired column in the table 'A Byte array is used to avoid a UNICODE string Dim byteData As Byte  'Byte array for LongVarBinary Dim strData As String   'String for LongVarChar Dim NumBlocks As Integer Dim filelength As Long Dim LeftOver As Long Dim SourceFile As Integer Dim i As Integer SourceFile = FreeFile Open DiskFile For Binary Access Read As SourceFile filelength = LOF(SourceFile) ' Get the length of the file If filelength = 0 Then Close SourceFile MsgBox DiskFile & " empty or not found." Else ' Calculate number of blocks to read and left over bytes NumBlocks = filelength \ BlockSize LeftOver = filelength Mod BlockSize Col.AppendChunk Null

Select Case Col.Type Case rdTypeLONGVARCHAR ' Read the 'left over' amount of LONGVARCHAR data strData = String(LeftOver, " ") Get SourceFile,, strData Col.AppendChunk strData strData = String(BlockSize, " ") For i = 1 To NumBlocks Get SourceFile,, strData Col.AppendChunk strData Next i          Close SourceFile Case rdTypeLONGVARBINARY ' Read the left over amount of LONGVARBINARY data ReDim byteData(0, LeftOver) Get SourceFile,, byteData Col.AppendChunk byteData ReDim byteData(0, BlockSize) For i = 1 To NumBlocks Get SourceFile,, byteData Col.AppendChunk byteData Next i          Close SourceFile Case Else MsgBox "not a chunkrequired column." End Select End If

End Sub  You will need to change the Server, Database, UID, and PWD values in the cnstr variable in order to connect to your database. The code in the Command1_Click event expects to find two files named README.TXT and SETUP.BMP in the current directory. These files are usually found in the Windows directory. You can either move these files to your current directory or change the path to match another bitmap and text file on your hard drive. Press the F5 key to start the program.</li> Click the Command1 button to execute the RDO code. The code will automatically create a table named chunktable, if it does not already exist, and move the text and bitmap files into and out of the BLOB columns.</li></ol>

For information on ADO object, click the link below:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/off2000/html/acmthAppendChunkADOX.asp

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