Question

我在ASP.NET中有一个IHttpHandler来提供一些图片。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);

有时在IIS7下获得System.Web.HttpException异常，其中说：

无法从依赖项生成etag。其中一个依赖项无法生成唯一ID。

但是我无法重现这个问题（我知道我无法使用ASP.NET内部测试Web服务器测试它）。有没有人知道为什么会发生这种情况以及我可以做些什么来阻止这种情况？

Answer 1

我无法解释为什么会发生这种情况，但我发现让线程在很短的时间内保持睡眠状态允许文件依赖管理器“让它在一起”并确认在尝试从中创建eTag之前，新生成的文件实际存在。

...<code to generate "filename" goes here>...

// 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);

例外：无法从依赖项生成etag

1 个答案: