無法將類型為'System.dbnull'的對象強制轉換為'system.byte []'

Question

請幫助。 無法檢索圖像..即時通訊使用Odbc連接..

    sSql = "select * from Faculty where RFID='" & txtrfid.Text & "'"
            Dim cmd As New OdbcCommand(sSql, con)
            Dim dr As OdbcDataReader = cmd.ExecuteReader()
            If dr.HasRows Then
                dr.Read()
                txtfname.Text = dr("fname").ToString()
                txtlname.Text = dr("lname").ToString()
                txtid.Text = dr("STID").ToString()
                txtposition.Text = dr("Pstion").ToString()
                txtsubject.Text = dr("Subject").ToString()
                Dim bits As Byte() = CType(dr("Pfile"), Byte())'" ERROR HERE!!!""
                Dim memo As New MemoryStream(bits)
                Dim myimg As New Bitmap(memo)
                imgRetrieve.Image = myimg
                dr.Close()

Answer 1

在這種情況下， dr("Pfile")表達式返回一個DBNull值，表示列中沒有任何內容。 該類型與Byte()之間沒有已知的轉換，因此您必須手動執行此操作

Dim data = dr("Pfile")
Dim bits as Byte()
If (TypeOf data is DBNull) Then
  bits = new Byte() { } 
Else
  bits = CType(data, Byte())
End If