Asp.net 预览图像功能导致文件上载控件重置

“我的图像”上载的代码段：

'Read the file and convert it to Byte Array
 Dim filePath As String = FileUpload1.PostedFile.FileName
 Dim filename As String = Path.GetFileName(filePath)
 Dim ext As String = Path.GetExtension(filename)
 Dim contenttype As String = String.Empty

 'Set the contenttype based on File Extension
 Select Case ext
      Case ".jpg"
           contenttype = "image/jpg"
           Exit Select
      Case ".png"
           contenttype = "image/png"
           Exit Select
 End Select

If Not contenttype Is String.Empty Then

 Dim fs As Stream = FileUpload1.PostedFile.InputStream
 Dim br As New BinaryReader(fs)
 Dim bytes As Byte() = br.ReadBytes(fs.Length)

 'insert the file into database
 cmd.Parameters.Add("@imgName", SqlDbType.VarChar).Value = filename
 cmd.Parameters.Add("@contentType", SqlDbType.VarChar).Value = contenttype
 cmd.Parameters.Add("@data", SqlDbType.Binary).Value = bytes

Else

 cmd.Parameters.Add("@imgName", SqlDbType.VarChar).Value = DBNull.Value
 cmd.Parameters.Add("@contentType", SqlDbType.VarChar).Value = DBNull.Value
 cmd.Parameters.Add("@data", SqlDbType.Binary).Value = DBNull.Value

End If

con.Open()
cmd.ExecuteNonQuery()
con.Close()

预览图像：

Protected Sub preview_btn_Click(sender As Object, e As EventArgs) Handles preview_btn.Click
    Session("ImageBytes") = FileUpload1.FileBytes
    Image1.ImageUrl = "~/Handler1.ashx"

    preview_btn.BackColor = Drawing.Color.Lime
    preview_btn.ForeColor = Drawing.Color.White

    Image1.BorderColor = Drawing.Color.Lime
End Sub

Handler1.ashx

<%@ WebHandler Language="VB" Class="Handler1" %>

Imports System
Imports System.Web

Public Class Handler1 : Implements System.Web.IHttpHandler, System.Web.SessionState.IRequiresSessionState

Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
    If (context.Session("ImageBytes")) IsNot Nothing Then
        Dim image As Byte() = DirectCast(context.Session("ImageBytes"), Byte())
        context.Response.ContentType = "image/jpeg"
        context.Response.BinaryWrite(image)
    End If
End Sub

Public ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
    Get
        Return False
    End Get
End Property

End Class

导入系统
导入系统.Web
公共类句柄1：实现System.Web.IHttpHandler、System.Web.SessionState.IRequiresessionState
Public Sub-ProcessRequest（ByVal上下文作为HttpContext）实现IHttpHandler.ProcessRequest
如果（context.Session（“ImageBytes”）不是空的，那么
Dim image As Byte（）=DirectCast（context.Session（“ImageBytes”），Byte（））
context.Response.ContentType=“image/jpeg”
context.Response.BinaryWrite（图像）
如果结束
端接头
公共只读属性IsReusable（）作为布尔值实现IHttpHandler.IsReusable
得到
返回错误
结束
端属性
末级

预览图像可以工作，但每当我调用预览图像按钮时，

FileUpload

控件就会复位

因此，从

FileUpload

控件的角度来看，用户似乎一开始没有选择任何图像

我尝试先将

FileUpload1.PostedFile.FileName

的值存储在某个变量中，然后手动设置其值，但

FileUpload1.PostedFile.FileName

似乎是只读的，从安全角度看这是有意义的

---

因此，如何解决这个问题？

要快速解决问题，我将执行以下操作：

预览-在会话中保存发布的文件。将Image1图像url设置为handler

处理程序-从会话中获取发布的文件。将图像写入响应

上传-检查文件上传1中是否存在文件，获取它。如果没有，就去拿吧 会议结束后。保存图像。会议结束

以下是我将使用的代码：

编辑：更改代码以修复较大（>50kb）图像的问题

aspx.vb

Protected Sub btnUpload_Click(sender As Object, e As EventArgs) Handles btnUpload.Click
    'Disabled DB operations for test
    'Read the file and convert it to Byte Array
    Dim filePath As String = String.Empty
    Dim filename As String = String.Empty
    Dim ext As String = String.Empty
    Dim contenttype As String = String.Empty
    Dim bytes As Byte()

    If FileUpload1.HasFile Then
        filePath = FileUpload1.PostedFile.FileName
    Else
        If (Session("MyFile") IsNot Nothing AndAlso Session("MyFileName") IsNot Nothing) Then
            filePath = Session("MyFileName").ToString()
            bytes = DirectCast(Session("MyFile"), Byte())
        End If
    End If

    filename = Path.GetFileName(filePath)
    ext = Path.GetExtension(filename)

    'Set the contenttype based on File Extension
    Select Case ext
        Case ".jpg"
            contenttype = "image/jpg"
            Exit Select
        Case ".png"
            contenttype = "image/png"
            Exit Select
    End Select

    If Not contenttype Is String.Empty Then
        If FileUpload1.HasFile Then
            Dim fs As Stream = FileUpload1.PostedFile.InputStream
            Dim br As New BinaryReader(fs)
            bytes = br.ReadBytes(fs.Length)
        End If
        'insert the file into database
        cmd.Parameters.Add("@imgName", SqlDbType.VarChar).Value = filename
        cmd.Parameters.Add("@contentType", SqlDbType.VarChar).Value = contenttype
        cmd.Parameters.Add("@data", SqlDbType.Binary).Value = bytes

    Else

        cmd.Parameters.Add("@imgName", SqlDbType.VarChar).Value = DBNull.Value
        cmd.Parameters.Add("@contentType", SqlDbType.VarChar).Value = DBNull.Value
        cmd.Parameters.Add("@data", SqlDbType.Binary).Value = DBNull.Value

    End If

    con.Open()
    cmd.ExecuteNonQuery()
    con.Close()

    'Cleanup
    Session("MyFile") = Nothing
    Session("MyFileName") = Nothing
End Sub

Protected Sub preview_btn_Click(sender As Object, e As EventArgs) Handles preview_btn.Click
    If FileUpload1.PostedFile IsNot Nothing Then
        Dim file As HttpPostedFile = FileUpload1.PostedFile

        Dim data As Byte() = New [Byte](file.ContentLength - 1) {}
        file.InputStream.Read(data, 0, file.ContentLength)

        Session("MyFile") = data
        Session("MyFileName") = FileUpload1.PostedFile.FileName
        Image1.ImageUrl = "~/Handler1.ashx"

        preview_btn.BackColor = Drawing.Color.Lime
        preview_btn.ForeColor = Drawing.Color.White

        Image1.BorderColor = Drawing.Color.Lime
    End If
End Sub

Handler1.ashx

Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
    If (context.Session("MyFile")) IsNot Nothing Then
        Dim storedImage = TryCast(context.Session("MyFile"), Byte())
        If storedImage IsNot Nothing Then
            context.Response.ContentType = "image/jpeg"
            context.Response.BinaryWrite(storedImage)
        End If
    End If
End Sub

---

希望有帮助

试用后，我得到一个

系统。ObjectDisposedException未经用户代码处理

无法访问已关闭的文件。

在

图像上显示为Byte（）=br.ReadBytes（fs.Length）

第一次尝试预览时，我创建了两个新页面，并复制了

预览\u btn\u单击
代码和
句柄1.ashx
您发布的代码，但仍然遇到相同的问题。您可以下载测试项目并与您的进行比较。或者你可以给你的项目发电子邮件，让我看看。我想我可能发现了问题，但没有找到解决方案。我试着用不同的图像来测试它，有些作品，有些则不然。所有图像都是
.jpg
，只有低于50KB的图像才有效。奇怪…@j_t_fusion-好的，我已经用较大图像的修复程序更新了源代码。还添加了指向我的测试项目的链接。