通过IHttpAsyncHandler读取BLOB

Question

我将图像存储在SQL Server 2000数据库(BLOB类型)中。我的ASP.NET应用程序中有一个网页，它需要向最终用户显示大量图像，我希望通过IHttpAsyncHandler处理浏览器对图像的请求。我在代码项目"Asynchronous HTTP Handler for Asynchronous BLOB DataReader."上找到了一个类似的帖子，但由于某些原因，页面在完成"BeginProcessRequest“方法和"EndProcessRequest”方法时冻结，而“EndProcessRequest”方法从未被调用。有没有人可以看看这篇文章，让我知道那篇文章有什么问题？或者我需要做什么来完成这个任务(通过IHttpAsyncHandler读取BLOB)？

这篇文章的源代码如下：

public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
{
    try
    {
        // image to retrieve
        // this needs to change if setup a different path schema in the httpHandles web.config
        string imgId = context.Request.Url.Segments[context.Request.Url.Segments.Length - 1];

        SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["LocalBLOB"].ConnectionString);
        conn.Open();
        SqlCommand cmd = conn.CreateCommand();
        cmd.CommandText = "select image from BLOBTest where imgId='" + imgId + "'";
        // store our Command to be later retrieved by EndProcessRequest
        context.Items.Add("cmd", cmd);

        // start async DB read
        return cmd.BeginExecuteReader(cb, context, 
            CommandBehavior.SequentialAccess |  // doesn't load whole column into memory
            CommandBehavior.SingleRow |         // performance improve since we only want one row
            CommandBehavior.CloseConnection);   // close connection immediately after read
    }
    catch (Exception exc)
    {
        // will return an image with the error text
        this.renderError(context, "ERROR: " + exc.Message);
        context.Response.StatusCode = 500;
        context.Response.End();
        // just to avoid compilation errors
        throw;
    }
}

/// <summary>
/// Provides an end method for an asynchronous process. 
/// </summary>
/// <param name="result">An IAsyncResult that contains information about the status of the process.</param>
public void EndProcessRequest(IAsyncResult result)
{
    HttpContext context = (HttpContext)result.AsyncState;

    try
    {
        // restore used Command
        SqlCommand cmd = (SqlCommand)context.Items["cmd"];
        // retrieve result
        using (SqlDataReader reader = cmd.EndExecuteReader(result))
        {
            this.renderImage(context, reader);
        }
    }
    catch (Exception exc)
    {
        // will return an image with the error text
        this.renderError(context, "ERROR: " + exc.Message);
        context.Response.StatusCode = 500;
    }
}

#endregion

#region IHttpHandler Members

public bool IsReusable
{
    get { return true; }
}

public void ProcessRequest(HttpContext context)
{
    // should never get called
    throw new Exception("The method or operation is not implemented.");
}

#endregion

/// <summary>
/// Render a BLOB field to the Response stream as JPEG
/// </summary>
/// <param name="context">HttpContext of current request</param>
/// <param name="myReader">open SqlDataReader to read BLOB from</param>
private void renderImage(HttpContext context, SqlDataReader myReader)
{
    // Size of the BLOB buffer.
    const int bufferSize = 1024;            
    long startIndex = 0;
    byte[] outbyte = new byte[bufferSize];
    long retval;

    myReader.Read();

    context.Response.Clear();
    context.Response.ContentType = "image/jpeg";

    do
    {
        retval = myReader.GetBytes(0, startIndex, outbyte, 0, bufferSize);
        // reposition the start index
        startIndex += bufferSize;
        // output buffer
        context.Response.BinaryWrite(outbyte);
        context.Response.Flush();
    } while (retval == bufferSize);
}

/// <summary>
/// Renders an Error image
/// </summary>
/// <param name="context">HttpContext of current request</param>
/// <param name="msg">message text to render</param>
private void renderError(HttpContext context, string msg)
{
    context.Response.Clear();
    context.Response.ContentType = "image/jpeg";
    // calculate the image width by message length
    Bitmap bitmap = new Bitmap(7 * msg.Length, 30);
    Graphics g = Graphics.FromImage(bitmap);
    // create a background filler
    g.FillRectangle(new SolidBrush(Color.DarkRed), 0, 0, bitmap.Width, bitmap.Height);
    // draw our message
    g.DrawString(msg, new Font("Tahoma", 10, FontStyle.Bold), new SolidBrush(Color.White), new PointF(5, 5));
    // stream it to the output
    bitmap.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
}

Answer 1

我知道这是一个很晚的回复，但也许这对其他人有帮助。

我希望您的连接字符串具有异步processing=true。

此外，您还需要在RenderImage函数中添加一个内容部署标头。

 context.Response.AddHeader("Content-Disposition", "attachment; filename=imagefilename.extension");

最后，在do while循环之后添加一个Context.Response.End()调用。