{"id":1198,"date":"2013-01-18T17:49:06","date_gmt":"2013-01-18T22:49:06","guid":{"rendered":"http:\/\/www.ikriv.com\/blog\/?p=1198"},"modified":"2013-01-18T17:49:06","modified_gmt":"2013-01-18T22:49:06","slug":"mef-though-shalt-not-loadfrom","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=1198","title":{"rendered":"MEF: Though Shalt Not LoadFrom"},"content":{"rendered":"

I was preparing for a presentation on MEF, and I stumbled<\/a> upon this sentence:<\/p>\n

catalog.Catalogs.Add(new DirectoryCatalog(\"C:\\\\SimpleCalculator\\\\SimpleCalculator\\\\Extensions\"));<\/code><\/p>\n

“This absolute path is for debugging purposes only. In a production application, you would use a relative path.<\/i>”<\/p>\n

The article does not explain why it is so, but I sensed it has to do with LoadFrom context<\/a>. Basically, when you load an assembly via absolute path outside of your application directory, it gets placed in a special context that is a source of all kinds of trouble.<\/p>\n

For a moment I hoped that MEF invented some kind of cure for this disease. In DirectoryCatalog<\/code> they create an AssemblyCatalog<\/code> for each file found in the directory, and AssemblyCatalog<\/code> does this:<\/p>\n

<\/p>\n

var assemblyName = AssemblyName.GetAssemblyName(filePath);\nAssembly.Load(assemblyName);<\/pre>\n
<\/code><\/p>\n
Voila? Jackpot? An override of Assembly.Load()<\/code> that takes an absolute path, albeit in a cloaked form!<\/p>\n
Not so fast, folks. Although this is called Assembly.Load()<\/code>, it will still use LoadForm<\/code> context if the path is outside of the application directory. This can be confirmed by writing a small example and enabling LoadFromContext<\/code> debug assistant in Visual Studio (Debug→xceptions→Managed Debuggin Assistants).<\/p>\n
The bottom line: MEF’s DirectoryCatalog<\/code>, as well as Assembly.Load(AssemblyName)<\/code> will still use LoadFrom<\/code> context if the path is outside of the application base. This is exactly why one should use relative paths for DirectoryCatalog<\/code>.<\/p>\n","protected":false},"excerpt":{"rendered":"
I was preparing for a presentation on MEF, and I stumbled upon this sentence: catalog.Catalogs.Add(new DirectoryCatalog(“C:\\\\SimpleCalculator\\\\SimpleCalculator\\\\Extensions”)); “This absolute path is for debugging purposes only. In a production application, you would […]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"Layout":"","footnotes":""},"categories":[3,4,5],"tags":[],"class_list":["entry","author-ikriv","post-1198","post","type-post","status-publish","format-standard","category-dotnet","category-hack","category-dev"],"_links":{"self":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1198","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1198"}],"version-history":[{"count":0,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1198\/revisions"}],"wp:attachment":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1198"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1198"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1198"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}