ViewState的奇怪行为

Question

我有一个Asp.Net页面（名为'PostAD'），允许用户上传最多4张图片。 文件上传按钮功能如下：

protected void btnUpload_Click(object sender, EventArgs e)
{
    if ((ViewState["Img1"] != null) && (ViewState["Img2"] != null) && (ViewState["Img3"] != null) && (ViewState["Img4"] != null))
    {
        lblUploadMsg.Text = "You cannot upload more than 4 pictures";
        return;
    }
    if (FileUpload1.HasFile)
    {
        //FileUpload1.Attributes.Clear();
        string fileExtension = System.IO.Path.GetExtension(FileUpload1.FileName);
        if (fileExtension.ToLower() == ".jpg")
        {
            int fileSize = FileUpload1.PostedFile.ContentLength;

            if (FileUpload1.PostedFile.ContentLength < 2097152)
            {

                //FileUpload1.SaveAs(Server.MapPath("~/Temp/" + FileUpload1.FileName));
                //Response.Write("Successfully Done");

                string sp = Server.MapPath("~/ItemPictures/");
                String fn = Guid.NewGuid().ToString() + FileUpload1.FileName.Substring(FileUpload1.FileName.LastIndexOf("."));
                if (sp.EndsWith("\\") == false)
                    sp += "\\";
                sp += fn;
                FileUpload1.PostedFile.SaveAs(sp);
                lblUploadMsg.ForeColor = System.Drawing.Color.Green;
                lblUploadMsg.Text = "Picture Uploaded successfully. You can upload upto 4 pictures";

                if (ViewState["Img1"] == null)
                {
                    ViewState["Img1"] = "~/ItemPictures/" + fn;
                }
                else if (ViewState["Img2"] == null)
                {
                    ViewState["Img2"] = "~/ItemPictures/" + fn;
                }
                else if (ViewState["Img3"] == null)
                {
                    ViewState["Img3"] = "~/ItemPictures/" + fn;
                }
                else if (ViewState["Img4"] == null)
                {
                    ViewState["Img4"] = "~/ItemPictures/" + fn;
                }
            }
            else
            {
                lblUploadMsg.Text = "Maximum 2MB files are allowed";
            }
        }
        else
        {
            lblUploadMsg.Text = "Only JPG files are allowed";
        }
    }
    else
    {
        lblUploadMsg.Text = "No File was Selected";
    }
    ShowAvailblImgs();
}

我有四个在页面加载时不可见的Asp.Net图像。 为了显示它们，我有以下代码。

private void ShowAvailblImgs()
{
    if (ViewState["Img1"] != null)
    {
        //The string URL variable is used just to show what value ViewState["image1"] currently has.
        string URL = (string)ViewState["img1"];
        Response.Write(URL);
        Image1.ImageUrl = (string)ViewState["img1"];
        Image1.Width = 130;
        Image1.Height = 130;
        Image1.Visible = true;
    }
    else
        Image1.Visible = false;
    if (ViewState["Img2"] != null)
    {
        Image2.ImageUrl = (string)ViewState["img2"];
        Image2.Width = 130;
        Image2.Height = 130;
        Image2.Visible = true;
    }
    else
        Image2.Visible = false;
    if (ViewState["Img3"] != null)
    {
        Image3.ImageUrl = (string)ViewState["img3"];
        Image3.Width = 130;
        Image3.Height = 130;
        Image3.Visible = true;
    }
    else
        Image3.Visible = false;
    if (ViewState["Img4"] != null)
    {
        Image4.ImageUrl = (string)ViewState["img4"];
        Image4.Width = 130;
        Image4.Height = 130;
        Image4.Visible = true;
    }
    else
        Image4.Visible = false;
}

我对ViewState变量有非常奇怪的行为。 加载图像后，它们不会显示在Asp.Net图像控件中。 而是显示空图像区域。 虽然我使用的URL变量打印图像的确切路径。 保存图像（实际上是空白图像区域）后，它会保存我的.aspx页面。 我使用的Session变量工作正常，但由于某些原因，我想使用ViewState变量。

Answer 1

我不想说错话，但是，从文档中读到，就是这样写的

“Viewstate用于在往返之间保留页面和控制值” http://msdn.microsoft.com/en-us/library/ms178198(v=vs.85).aspx

您是否尝试将URL放入变量，然后分配给ViewState [“imgX”]，并尝试执行另一个Postback并查看ViewState [“imgX”]是否包含URL？

Answer 2

ViewState很好。 您对ViewState索引字符串使用不同的大小写，因此它们不引用相同的ViewState属性。 “Img1”不等于“img1”。

ViewState["Img1"] != null)
    {
        Image2.ImageUrl = (string)ViewState["img1"];

我建议使用常量作为值名称，如下所示。

    const string image1 = "img1";
    const string image2 = "img2";
    const string image3 = "img3";
    const string image4 = "img4";

或者参考我的博客文章，使用扩展方法创建强类型的伪属性。 http://coding.grax.com/2013/06/simple-strongly-typed-pattern-for.html