建立ActiveX 動(dòng)態(tài)鏈接庫(kù);;;;使用 VB 5.0，我們要開發(fā)一個(gè)ActiveX 動(dòng)態(tài)鏈接庫(kù)。這個(gè)動(dòng)態(tài)鏈接庫(kù)讓我們能夠連接到SQL數(shù)據(jù)庫(kù)，提取圖片數(shù)據(jù)，并把它們保存成文件，放在目錄里。要建立動(dòng)態(tài)鏈接庫(kù)，請(qǐng)?jiān)诖蜷_VB程序之后執(zhí)行下列步驟：首先：在文件（File）菜單里，選擇新建項(xiàng)目（New Project）。接著，在新建立項(xiàng)目對(duì)話框里，雙擊ActiveX DLL 圖標(biāo)。VB自動(dòng)地向項(xiàng)目里增加一個(gè)類，叫做Class1。然后，在屬性容器里，雙擊名稱（Name）屬性，把它改成clsImageLoader，保持instancing 屬性為缺省值MultiUse不動(dòng)。最后，在項(xiàng)目（Project）菜單里，選擇Project1 屬性，打開項(xiàng)目屬性對(duì)話框。選擇通用（General）選項(xiàng)卡，把項(xiàng)目名稱改為ImageLoader。其它字段可以不管，保持原樣。單擊確定（OK）完成對(duì)ImageLoader 這個(gè)工程的設(shè)置。 ;;;;在類里，寫入了5個(gè)Let屬性，1個(gè)Get屬性（只讀）和兩個(gè)方法。Let屬性傳遞動(dòng)態(tài)鏈接庫(kù)需要的信息，Get屬性提供動(dòng)態(tài)鏈接庫(kù)生成的圖片文件的位置。兩個(gè)方法用來(lái)打開到數(shù)據(jù)庫(kù)的連接，以生成圖片文件。表 A 總結(jié)了這些屬性和方法的作用。當(dāng)然，根據(jù)項(xiàng)目的需要，可以在類里加入更復(fù)雜的功能。 表A: 在ActiveX動(dòng)態(tài)鏈接庫(kù)里的屬性和方法屬性/方法;;;;作用DbName;;打開數(shù)據(jù)庫(kù)連接所需的數(shù)據(jù)庫(kù)名稱。TableName,NameOfImageColumn,;;;;;;打開記錄集所需要的信息。NameOfImageTypeColumn,NameOfImageIdColumn ImageFile (只讀);;;;;;把圖片文件的名稱和位置返回給容器。OpenConnection (方法);打開數(shù)據(jù)庫(kù)連接。CreateTemplateImageFile (方法);;;;;;從數(shù)據(jù)庫(kù)提取圖片數(shù)據(jù)，并把它保存到目錄里。;;;;在類里還有其它的私有子程序，用來(lái)初始化類、終止類、打開記錄集、建立圖片文件，以及處理可能的錯(cuò)誤。注意微軟活動(dòng)數(shù)據(jù)對(duì)象ADO1.5(現(xiàn)在為2.0)被用來(lái)連接SQL數(shù)據(jù)庫(kù)。也可以使用其它數(shù)據(jù)訪問方法，比如遠(yuǎn)程數(shù)據(jù)對(duì)象RDO。在這個(gè)項(xiàng)目里，必須使用ADO。操作方法是，單擊項(xiàng)目（Project）菜單，然后選擇引用（References）菜單 項(xiàng)。在引用對(duì)話框里，選中微軟ActiveX Data Objects 1.5 庫(kù)，然后單擊確定（OK）。在列表 B里，列出了類ImageLoader 完整源代碼。類里使用的變量定義都在代碼內(nèi)部給出，還有其它注釋，幫助理解上下文。列表 B:類 ImageLoader 的完整源代碼Private mAdoConn As New ADODB.ConnectionPrivate mAdoRst As New ADODB.RecordsetPrivate mstrDbName As StringPrivate mstrTableName As StringPrivate mstrImageColumnName As String '圖片字的名稱。Private mstrImageTypeColumnName As String '圖片類型字段的名稱。Private mstrImageIdColumnName As String '圖片ID字段的名稱。Private mstrFileName() As String '數(shù)組，里面包含文件名和路徑。Private mlngImageId() As Long '數(shù)組，里面包含圖片IDPrivate mlngNumberOfFiles As LongConst BLOCKSIZE = 102400Public Property Let DbName(ByVal strVal As String);;mstrDbName = strValEnd PropertyPublic Property Let TableName(ByVal strVal As String);;mstrTableName = strValEnd PropertyPublic Property Let NameOfImageColumn(ByVal strVal As String);;mstrImageColumnName = strValEnd PropertyPublic Property Let NameOfImageTypeColumn(ByVal strVal As String);;mstrImageTypeColumnName = strValEnd PropertyPublic Property Let NameOfImageIdColumn(ByVal strVal As String);;mstrImageIdColumnName = strValEnd PropertyPublic Property Get ImageFile(ByVal ImageId As Integer) As String;;Dim intPos As Integer;;Dim blnFindId As Boolean ;;Dim i As Integer;;blnFindId = False;;For i = 0 To mlngNumberOfFiles - 1;;;;;If mlngImageId(i) = ImageId ThenintPos = 5 + Len(ImageId) + 3ImageFile = Right(mstrFileName(i), intPos) 'reformat the location of file.blnFindId = True;;;;;End If;;Next i;;If blnFindId = False Then;;;;;Err.Clear;;;;;Err.Raise vbObjectError + 23, "Get ImageFile", "Can't find image file!";;End IfEnd PropertyPublic Sub OpenConnection()'**********************************************************'作用：打開數(shù)據(jù)庫(kù)連接。'**********************************************************On Error GoTo Error_handler;;If mstrDbName = "" Then GoTo Error_handler;;If mAdoConn.State = adStateOpen Then mAdoConn.Close;;mAdoConn.ConnectionString = "DRIVER={SQL Server};SERVER=(local);UID=sa;PWD=;WSID=JIA;DATABASE=" & mstrDbName;;mAdoConn.ConnectionTimeout = 15;;mAdoConn.OpenExit SubError_handler:;;Call HandleErrorEnd SubPublic Sub CreateTempImageFile(ByVal ImageId As Integer);;Dim strImageType As String;;Dim i As Integer'**********************************************************'作用：打開記錄集，提取二進(jìn)制數(shù)據(jù)，并把數(shù)據(jù)存入文件。注意文件名使用圖片ID生成。'輸入：圖片ID。'**********************************************************If mAdoConn.State = adStateClosed Then Exit Sub;;Call OpenRecordset(ImageId);;If mAdoRst.State = adStateClosed Then Exit SubOn Error GoTo Error_handler;;For i = 0 To mlngNumberOfFiles - 1;;;;;'檢測(cè)圖片文件是否已經(jīng)存在。;;;;;If mlngImageId(i) = ImageId Then Exit Sub;;Next i;;mlngNumberOfFiles = mlngNumberOfFiles + 1;;ReDim Preserve mstrFileName(mlngNumberOfFiles) '改變數(shù)組大小。;;ReDim Preserve mlngImageId(mlngNumberOfFiles) '改變數(shù)組大小。;;mlngImageId(mlngNumberOfFiles - 1) = ImageId;;strImageType = mAdoRst.Fields(mstrImageTypeColumnName) ' 取得圖片類型。;;;;;mstrFileName(mlngNumberOfFiles - 1) = App.Path & "images" & _;;;;"image" & LTrim(Str(ImageId)) & "." & strImageType;;'取得圖片文件名稱和位置。;;Call ReadFromDB(mAdoRst.Fields(mstrImageColumnName), _;;;;;mstrFileName(mlngNumberOfFiles - 1), AdoRst.Fields(mstrImageColumnName).ActualSize);;Exit SubError_handler:;;Call HandleErrorEnd SubPrivate Sub OpenRecordset(ByVal ImageId As Integer)Dim SqlText As String'**********************************************************'作用：打開記錄集。'輸入：圖片ID。'**********************************************************On Error GoTo Error_handler;;If mAdoRst.State = adStateOpen Then mAdoRst.Close;;SqlText = "SELECT " & mstrImageColumnName & "," & _;;;;;mstrImageTypeColumnName & " FROM " & mstrTableName & _;;;;;" WHERE " & mstrImageIdColumnName & "=" & ImageId;;Set mAdoRst.ActiveConnection = mAdoConn;;mAdoRst.Open SqlText, , adOpenStatic, adLockReadOnly 'Open recordset.Exit SubError_handler:;;Call HandleErrorEnd SubPrivate Sub ReadFromDB(fld As ADODB.Field, ByVal DiskFile As String, _;;;;FldSize As Long)Dim NumBlocks As IntegerDim LeftOver As LongDim byteData() As Byte;;;'字節(jié)數(shù)組，用于長(zhǎng)的可變二進(jìn)制數(shù)據(jù)LongVarBinary。Dim strData As String;;;;'字符串，用于長(zhǎng)的可變二進(jìn)制數(shù)據(jù)LongVarChar。Dim DestFileNum As IntegerDim pic As VariantDim i As Integer'**********************************************************'作用：提取二進(jìn)制數(shù)據(jù)并把數(shù)據(jù)放入文件。'輸入：圖片字段，文件名/位置和數(shù)據(jù)尺寸。'**********************************************************;;If Len(Dir(DiskFile)) > 0 Then '刪除已經(jīng)存在的目標(biāo)文件。;;;;;Kill DiskFile;;End If;;DestFileNum = FreeFile;;Open DiskFile For Binary As DestFileNum;;NumBlocks = FldSize BLOCKSIZE;;LeftOver = FldSize Mod BLOCKSIZE;;Select Case fld.Type;;Case adLongVarBinary '用于圖片數(shù)據(jù)類型。;;;;;byteData() = fld.GetChunk(LeftOver);;;;;pic = fld.GetChunk(LeftOver);;;;;Put DestFileNum, , byteData();;;;;For i = 1 To NumBlocksbyteData() = fld.GetChunk(BLOCKSIZE)Put DestFileNum, , byteData();;;;;Next i;;Case adLongVarChar '用于文本數(shù)據(jù)類型。;;;;;For i = 1 To NumBlocksstrData = String(BLOCKSIZE, 32)strData = fld.GetChunk(BLOCKSIZE)Put DestFileNum, , strData;;;;;Next i;;;;;strData = String(LeftOver, 32);;;;;strData = fld.GetChunk(LeftOver)Put DestFileNum, , strData;;;;;Case ElseErr.ClearErr.Raise vbObjectError + 22, "Read from DB", "Not a Chunk Required column!";;End Select;;Close DestFileNumEnd SubPrivate Sub HandleError()Dim adoErrs As ADODB.ErrorsDim errLoop As ADODB.ErrorDim strError As StringDim i As Integer'**********************************************************'作用：處理可能的錯(cuò)誤。'**********************************************************;;If mAdoConn.State = adStateClosed Then GoTo Done;;;i = 1;;;Set adoErrs = mAdoConn.Errors;;For Each errLoop In adoErrs;;'枚舉錯(cuò)誤集。;;;;;;;;;;;With errLoopstrError = strError & vbCrLf & ";;;ADO Error;;;#" & .NumberstrError = strError & vbCrLf & ";;;Description;;" & .DescriptionstrError = strError & vbCrLf & ";;;Source;;;;;;;" & .Sourcei = i + 1;;;;;End With;;NextDone:;;Err.Raise vbObjectError + 21, "", strErrorEnd SubPrivate Sub Class_Initialize();;mlngNumberOfFiles = 0End SubPrivate Sub Class_Terminate()Dim i As IntegerOn Error GoTo Error_handler;;;If mAdoRst.State = adStateOpen Then mAdoRst.Close;;'關(guān)閉記錄集。;;;;;;;;;If mAdoConn.State = adStateOpen Then mAdoConn.Close;;'關(guān)閉連接。;;;;;;Set mAdoRst = Nothing;;;;;;Set mAdoConn = NothingExit SubError_handler:;;Call HandleErrorEnd SubError_handler:;;Call HandleErrorEnd Sub下一步是編譯動(dòng)態(tài)鏈接庫(kù)。在文件（File）菜單里，單擊建立（Make） ImageLoader.dll。VB會(huì)詢問保存動(dòng)態(tài)鏈接庫(kù)的位置，然后就編譯項(xiàng)目。在要ASP文件里使用ImageLoader.dll這個(gè)動(dòng)態(tài)鏈接庫(kù)，必須在Windows注冊(cè)表里注冊(cè)它。操作方法是，在NT的啟動(dòng)菜單里，選擇運(yùn)行（Run），然后在輸入框里輸入下面的命令行：regsvr32 localpathImageLoader注意，實(shí)際使用時(shí)要用動(dòng)態(tài)鏈接庫(kù)實(shí)際所在的完整路徑替換上面命令行里的“l(fā)ocalpath”。現(xiàn)在，就可以在程序里，比如ASP文件里使用這個(gè)動(dòng)態(tài)鏈接庫(kù)了。