14 处理大型对象

14.1 什么是LOB、BLoB和CLOB

14.1.1 LOB的存储位置

14.1.2 处理LOB

14.1.3 读取BLOB数据

 

代码

< ?xml version = " 1.0 "  encoding = " utf-8 "  ? >
< configuration >
   < connectionStrings >
     < add name = " NewString "  
         connectionString = " Data Source=.\SQLEXPRESS;
         AttachDbFilename = |DataDirectory| \ northwnd.mdf;
         Integrated Security = True ; User Instance = True "
         providerName = " system.data.SqlClient " />
   </ connectionStrings >
</ configuration >

 

从数据库获取照片并保存到文件中

 

代码

Imports  System.Configuration
Imports  System.Data.SqlClient
Imports  System.IO

Public   Class  Form1

     Private   Sub  Button1_Click( ByVal  sender  As  System.Object,
                               ByVal  e  As  System.EventArgs)  Handles  Button1.Click
         Const  employeeIdColumn  As   Integer   =   0
         Const  employeePhotoColumn  As   Integer   =   1
         ' bufferSize must be bigger than oleOffset
         Const  bufferSize  As   Integer   =   100
         Dim  buffer(bufferSize)  As   Byte
         Dim  byteCountRead  As   Integer
         Dim  currentIndex  As   Long   =   0

         Dim  nwSetting  As  ConnectionStringSettings  =
            ConfigurationManager.ConnectionStrings( " NwString " )
         Using  cn  As   New  SqlConnection
            cn.ConnectionString  =  nwSetting.ConnectionString
            cn.Open()

             Using  cmd  As  SqlCommand  =  cn.CreateCommand
                cmd.CommandText  =   " SELECT EmployeeID, Photo FROM Employees "
                 Dim  rdr  As  SqlDataReader  =
                    cmd.ExecuteReader(CommandBehavior.SequentialAccess)
                 While  rdr.Read
                     Dim  employeeId  As   Integer   =  rdr.GetInt32(employeeIdColumn)
                     Dim  fileName  As   String   =   " c:\employee "   &
                        employeeId.ToString.PadLeft( 2 ,  " 0 " c)  &   " .bin "
                     ' create a file to hold the output.
                     Using  fs  As   New  FileStream(
                        fileName, FileMode.OpenOrCreate, FileAccess.Write)
                        currentIndex  =   0
                        byteCountRead  =   CInt (rdr.GetBytes(
                                employeePhotoColumn, currentIndex, buffer,  0 , bufferSize))
                         While  byteCountRead  <>   0
                            fs.Write(buffer,  0 , byteCountRead)
                            currentIndex  +=  byteCountRead
                            byteCountRead  =   CInt (rdr.GetBytes(
                                    employeePhotoColumn, currentIndex, buffer,  0 , bufferSize))
                         End   While
                     End   Using
                 End   While
             End   Using
         End   Using
        MessageBox.Show( " Done " )
     End Sub
End Class

用字节数组将数据写入文件

代码

Private   Sub  Button2_Click( ByVal  sender  As  System.Object,
                       ByVal  e  As  System.EventArgs)  Handles  Button2.Click
     Const  oleOffset  As   Integer   =   78
     Const  oleTypeStart  As   Integer   =   20
     Const  oleTypeLength  As   Integer   =   12

     Const  employeeIdColumn  As   Integer   =   0
     Const  employeePhotoColumn  As   Integer   =   1
     Const  bufferSize  As   Integer   =   100   ' bufferSize must be bigger than oleOffset
     Dim  buffer(bufferSize)  As   Byte
     Dim  bufferStart  As   Integer   =   0
     Dim  byteCountRead  As   Integer
     Dim  currentIndex  As   Long   =   0

     Dim  nwSetting  As  ConnectionStringSettings  =
        ConfigurationManager.ConnectionStrings( " NwString " )
     Using  cn  As   New  SqlConnection
        cn.ConnectionString  =  nwSetting.ConnectionString
        cn.Open()

         Using  cmd  As  SqlCommand  =  cn.CreateCommand
            cmd.CommandText  =   " SELECT EmployeeID, Photo FROM Employees "
             Dim  rdr  As  SqlDataReader  =
                cmd.ExecuteReader(CommandBehavior.SequentialAccess)
             While  rdr.Read
                 Dim  employeeId  As   Integer   =  rdr.GetInt32(employeeIdColumn)
                 Dim  fileName  As   String   =   " c:\employee "   &
                    employeeId.ToString.PadLeft( 2 ,  " 0 " c)  &   " .bmp "
                 ' create a file to hold the output.
                 Using  fs  As   New  FileStream(
                    fileName, FileMode.OpenOrCreate, FileAccess.Write)
                    currentIndex  =   0
                     ' read until we have the oleheader, if possible
                     While  currentIndex  <  oleOffset
                        byteCountRead  =   CInt (
                            rdr.GetBytes(employeePhotoColumn, currentIndex,
                                         buffer,  CInt (currentIndex), bufferSize  -   CInt (currentIndex)))
                         If  byteCountRead  =   0   Then   Exit   While
                        currentIndex  +=  byteCountRead
                     End   While
                    byteCountRead  =   CInt (currentIndex)
                     ' process oleheader, if it exists
                     If  byteCountRead  >=  oleOffset  Then
                         Dim  type  As   String   =  Encoding.ASCII.GetString(
                            buffer, oleTypeStart, oleTypeLength)
                         If  type  =   " Bitmap Image "   Then
                            bufferStart  =  oleOffset
                            byteCountRead  =  byteCountRead  -  oleOffset
                         End   If
                     End   If

                     While  byteCountRead  <>   0
                        fs.Write(buffer, bufferStart, byteCountRead)
                        bufferStart  =   0
                        byteCountRead  =   CInt (
                            rdr.GetBytes(employeePhotoColumn, currentIndex, buffer,  0 , bufferSize))
                        currentIndex  +=  byteCountRead
                     End   While
                 End   Using
             End   While
         End   Using
     End   Using
    MessageBox.Show( " Done " )
End Sub

 

14.1.4 写入BLOB数据

 

代码

Private   Sub  Button3_Click( ByVal  sender  As  System.Object,
                           ByVal  e  As  System.EventArgs)  Handles  Button3.Click
     Const  bufferSize  As   Integer   =   100
     Dim  buffer(bufferSize)  As   Byte
     Dim  currentIndex  As   Long   =   0
     Dim  photoPtr()  As   Byte
     Dim  nwSetting  As  ConnectionStringSettings  =
        ConfigurationManager.ConnectionStrings( " NwString " )
     Using  cn  As   New  SqlConnection
        cn.ConnectionString  =  nwSetting.ConnectionString
        cn.Open()
         Using  cmd  As  SqlCommand  =  cn.CreateCommand
            cmd.CommandText  =   " SELECT TEXTPTR(Photo) FROM Employees WHERE EmployeeId=1 "
            photoPtr  =   CType (cmd.ExecuteScalar,  Byte ())
         End   Using
         Using  cmd  As  SqlCommand  =  cn.CreateCommand
            cmd.CommandText  =   " UPDATETEXT Employees.Photo @Pointer @Offset null @Data "
             Dim  ptrParm  As  SqlParameter  =  cmd.Parameters.Add( " @Pointer " , SqlDbType.Binary,  16 )
            ptrParm.Value  =  photoPtr
             Dim  photoParm  As  SqlParameter  =  cmd.Parameters.Add( " @Data " , SqlDbType.Image)
             Dim  offsetParm  As  SqlParameter  =  cmd.Parameters.Add( " @Offset " , SqlDbType.Int)
            offsetParm.Value  =   0
             Using  fs  As   New  FileStream( " Girl.gif " , FileMode.Open, FileAccess.Read)
                 Dim  count  As   Integer   =  fs.Read(buffer,  0 , bufferSize)
                 While  count  <>   0
                    photoParm.Value  =  buffer
                    photoParm.Size  =  count
                    cmd.ExecuteNonQuery()
                    currentIndex  +=  count
                    offsetParm.Value  =  currentIndex
                    count  =  fs.Read(buffer,  0 , bufferSize)
                 End   While
             End   Using
         End   Using
     End   Using
    MessageBox.Show( " Done " )
End Sub