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需要向最终用户显示很多图像，我想通过 IHttpAsyncHandler 处理浏览器对图像的请求。我在 codeproject “异步 BLOB DataReader 的异步 HTTP 处理程序”上找到了一个类似的帖子。 但是由于某些原因，页面在“BeginProcessRequest”方法完成后冻结，“EndProcessRequest”方法永远不会被调用。任何人都可以看看这个帖子，让我知道那个帖子有什么问题吗？或者我需要做什么来完成这个任务（通过 IHttpAsyncHandler 读取 BLOB）？
这篇文章中的源代码如下，
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);
}