如何有效地从数据库 Blob 存储和检索图像？-mysql教程-PHP中文网

如何有效地从数据库 Blob 存储和检索图像？

Mary-Kate Olsen

发布： 2024-11-16 18:28:02

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How to Effectively Store and Retrieve Images from a Database Blob?

将图像存储在数据库 Blob 中并检索它以进行显示

问题

从数据库存储和检索图像是编程中的常见任务。虽然看起来很简单，但如果代码未正确实现，则可能会出现问题。在本例中，用户在将图像保存到数据库并在 Picturebox 控件中显示图像时遇到问题。

解决方案

为了解决该问题，让我们检查用户的代码并识别潜在问题：

保存到数据库：

ImageStream = New System.IO.MemoryStream
PbPicture.Image.Save(ImageStream, System.Drawing.Imaging.ImageFormat.Jpeg)
ReDim rawdata(CInt(ImageStream.Length - 1))

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代码无法使用正确的维度初始化原始数据。要正确存储图像字节，应重新调整大小以匹配图像流的长度：

ReDim rawdata(CInt(ImageStream.Length))

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从数据库检索：

Dim ad As New System.IO.MemoryStream(100000)
Dim im As Image = Image.FromStream(ad) * "error occurs here" (see below)

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错误出现此问题的原因是广告内存流未正确定位或容量不足。以下代码修复了此问题：

ad.Position = 0
Dim im As Image = Image.FromStream(ad)

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将图像保存到数据库的工作代码

Dim filename As String = txtName.Text + ".jpg"
Dim FileSize As UInt32

conn.Close()

Dim mstream As New System.IO.MemoryStream()
PbPicture.Image.Save(mstream, System.Drawing.Imaging.ImageFormat.Jpeg)
Dim arrImage() As Byte = mstream.GetBuffer()

FileSize = mstream.Length
Dim sqlcmd As New MySqlCommand
Dim sql As String
mstream.Close()

sql = "insert into [your table]  (picture, filename, filesize) 
                               VALUES(@File, @FileName, @FileSize)"

Try
    conn.Open()
    With sqlcmd
        .CommandText = sql
        .Connection = conn
        .Parameters.AddWithValue("@FileName", filename)
        .Parameters.AddWithValue("@FileSize", FileSize)
        .Parameters.AddWithValue("@File", arrImage)

        .ExecuteNonQuery()
    End With
Catch ex As Exception
    MsgBox(ex.Message)
Finally
    conn.Close()
End Try

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从数据库检索图像的工作代码

Dim adapter As New MySqlDataAdapter
adapter.SelectCommand = Cmd

data = New DataTable

adapter = New MySqlDataAdapter("select picture from [yourtable]", conn)
commandbuild = New MySqlCommandBuilder(adapter)
adapter.Fill(data)

Dim lb() As Byte = data.Rows(0).Item("picture")
Dim lstr As New System.IO.MemoryStream(lb)
PbPicture.Image = Image.FromStream(lstr)
PbPicture.SizeMode = PictureBoxSizeMode.StretchImage
lstr.Close()

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