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. |
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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:
| 1. | Open a new Standard EXE Visual Basic project. |
| 2. | On the Project menu, click to select References, and then set a reference to the Microsoft ActiveX Data Objects 2.5 Object Library. |
| 3. | Place a CommandButton control on Form1. |
| 4. | Make the following declarations in the form's General
declarations section:
Dim cn As ADODB.Connection
Dim rs As ADODB.Recordset
Dim mstream As ADODB.Stream
|
| 5. | Cut and paste the following code into the Click event of the CommandButton that you added to the form:
Set cn = New ADODB.Connection
cn.Open "Provider=SQLOLEDB;data Source=<name of your SQL Server>;
Initial Catalog=pubs;User Id=<Your Userid>;Password=<Your Password>"
Set rs = New ADODB.Recordset
rs.Open "Select * from pub_info", cn, adOpenKeyset, adLockOptimistic
Set mstream = New ADODB.Stream
mstream.Type = adTypeBinary
mstream.Open
mstream.Write rs.Fields("logo").Value
mstream.SaveToFile "c:\publogo.gif", adSaveCreateOverWrite
rs.Close
cn.Close
|
| 6. | Save and run the Visual Basic project. |
| 7. | 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:
| 1. | Open a new Standard EXE Visual Basic project. |
| 2. | On the Project menu, click to select References, and then set a reference to the Microsoft ActiveX Data Objects 2.5 Object Library. |
| 3. | Place a CommandButton on Form1. |
| 4. | Make the following declarations in the form's General
declarations section:
Dim cn As ADODB.Connection
Dim rs As ADODB.Recordset
Dim mstream As ADODB.Stream
|
| 5. | Cut and paste the following code in the Click event of the CommandButton that you added to the form:
Set cn = New ADODB.Connection
cn.Open "Provider=SQLOLEDB;data Source=<name of your SQL Server>;
Initial Catalog=pubs;User Id=<Your Userid>;Password=<Your Password>"
Set rs = New ADODB.Recordset
rs.Open "Select * from pub_info", cn, adOpenKeyset, adLockOptimistic
Set mstream = New ADODB.Stream
mstream.Type = adTypeBinary
mstream.Open
mstream.LoadFromFile "<path to .gif file>"
rs.Fields("logo").Value = mstream.Read
rs.Update
rs.Close
cn.Close
|
| 6. | Save and run the Visual Basic project. |
| 7. | 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. |
| 8. | Verify that the image in the logo column has been modified
by using the code in Example 1. |
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For additional information, click the following article number
to view the article in the Microsoft Knowledge Base:
308042 (http://support.microsoft.com/kb/308042/EN-US/)
How To Read and Write BLOB Data by Using ADO.NET with Visual Basic .NET
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