例外:无法从依赖项生成etag

我有一个IHttpHandlerASP.NET,它提供一些图像.处理程序按以下方式设置ETAG:

context.Response.AddFileDependency(filename);                
context.Response.Cache.SetLastModifiedFromFileDependencies();
context.Response.Cache.SetETagFromFileDependencies();
context.Response.Cache.SetCacheability(HttpCacheability.Public);
context.Response.Cache.SetMaxAge(new TimeSpan(999,0,0,0));
context.Response.Cache.SetSlidingExpiration(true);
context.Response.Cache.SetValidUntilExpires(true);                
context.Response.Cache.VaryByParams["*"] = true;
byte[] buffer = File.ReadAllBytes(filename);
context.Response.ContentType = MimeMapping.GetMimeMapping(mi.Mi_filename);
context.Response.StatusCode = 200;
context.Response.BinaryWrite(buffer);

有时System.Web.HttpException在IIS7下得到一个Exception,它说:

无法从依赖项生成etag.其中一个依赖项无法生成唯一ID.

但我无法重现问题(我知道我无法使用ASP.NET内部测试Web服务器测试它).有谁知道为什么会发生这种情况以及我可以采取哪些措施来防止这种情况发生？

我无法解释为什么会发生这种情况,但我发现让线程在非常短的时间内休眠允许文件依赖管理器"将它放在一起"并在尝试之前确认新生成的文件实际存在从中创建一个eTag.

......

// BUG: For some reason, even though the cache file has definitely been created at this stage, the file dependency manager
// seems to require a bit of time to "register" that the file exists before we add it to the list of dependencies.
// If we don't tell the thread to sleep, we will get an error when it generates the eTag (no idea why this happens - can't find anything on the web).
// If the cache file already existed when this request began, then there is no error.
Thread.Sleep(5);
context.Response.AddFileDependency(filename);                
context.Response.Cache.SetLastModifiedFromFileDependencies();
context.Response.Cache.SetETagFromFileDependencies();
context.Response.Cache.SetCacheability(HttpCacheability.Public);
context.Response.Cache.SetMaxAge(new TimeSpan(999,0,0,0));
context.Response.Cache.SetSlidingExpiration(true);
context.Response.Cache.SetValidUntilExpires(true);                
context.Response.Cache.VaryByParams["*"] = true;
byte[] buffer = File.ReadAllBytes(filename);
context.Response.ContentType = MimeMapping.GetMimeMapping(mi.Mi_filename);
context.Response.StatusCode = 200;
context.Response.BinaryWrite(buffer);