问无法使用OleDbDataReader读取blob类型

EN

Dim PictureCol As Integer = 0
Dim DestFilePath = "C:\testBlob.dat"
Dim Conn As New OleDbConnection

Conn.ConnectionString = GetConnectionString()
Conn.Open()
Dim cmd As New OleDbCommand("SELECT d.document_file from library_documents d d.CATEGORY_ID=1 ", Conn)

Dim dr As OleDbDataReader = cmd.ExecuteReader

dr.Read()
Dim b(dr.GetBytes(PictureCol, 0, Nothing, 0, Integer.MaxValue) - 1) As Byte
dr.GetBytes(PictureCol, 0, b, 0, b.Length)
dr.Close()
Conn.Close()
Dim fs As New System.IO.FileStream(DestFilePath, IO.FileMode.Create, IO.FileAccess.Write)
fs.Write(b, 0, b.Length)
fs.Close()