{"id":3563,"date":"2013-08-08T07:00:00","date_gmt":"2013-08-08T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2013\/08\/08\/on-partially-constructed-objects-additional-remarks-not-as-interesting\/"},"modified":"2013-08-08T07:00:00","modified_gmt":"2013-08-08T07:00:00","slug":"on-partially-constructed-objects-additional-remarks-not-as-interesting","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20130808-00\/?p=3563","title":{"rendered":"On partially-constructed objects, additional remarks, not as interesting"},"content":{"rendered":"

\nDon’t worry.\nOur\n\nlong national nightmare<\/a>\nof CLR week is almost over.\n<\/p>\n

\nI had originally planned to write an article about partially-constructed\nobjects, but in the time since I queued up the topic (back in November 2005)\nto the time I got around to writing it up, I found that\nJoe Duffy had already written it for me!\n<\/p>\n

\n\nOn partially-constructed objects<\/b>\n<\/a>\n<\/p>\n

\nRead it.\n<\/p>\n

\nOkay, here are some follow-up remarks.\n<\/p>\n

\nOne place where people get caught out by partially-constructed objects\nis when they try to maintain a cache of objects\n(perhaps with a little bit of\n\nWeak­Reference<\/code> action<\/a>)\nand stash the objects into the cache before they are fully constructed:\n<\/p>\n

\nclass SomeClass {\n public SomeClass(...) {\n  cache.Add(this);\n  AdditionalConstructionWork();\n }\n}\n<\/pre>\n
\nIf the Additional­Construction­Work<\/code> takes\nan exception,\nthen you end up with a partially-constructed object in your cache.\n(Mind you, you had one all along, but now it’s a persistent condition\nas opposed to a transient one.)\n<\/p>\n
\nYou might think to fix the problem by reordering the operations:\n<\/p>\n
\nclass SomeClass {\n public SomeClass(...) {\n  AdditionalConstructionWork();\n  \/\/ add to cache only after construction ran to completion\n  cache.Add(this);\n }\n}\n<\/pre>\n
\nBut that still doesn’t work once you have derived classes:\n<\/p>\n
\nclass Derived : SomeClass {\n public Derived(...) : base(...) {\n  AdditionalConstruction(); \/\/ oops, what if this throws?\n }\n}\n<\/pre>\n
\nThe base constructor runs first,\nit successfully constructs the base object,\nand then puts it in the cache.\nAnd then the derived constructor runs and encounters an exception.\nYou’re back in the same boat with a partially-constructed object\nin the cache.\n<\/p>\n
\nYou want to wait until the object is fully constructed\nbecause you add it to your cache.\n<\/p>\n
\nclass SomeClass {\n static public SomeClass Create(...) {\n  SomeClass c = new SomeClass(...);\n  Register(c);\n  return c;\n }\n protected static void Register(SomeClass c) { cache.Add(c); }\n protected SomeClass(...) { ... }\n}\nclass Derived : SomeClass {\n static public Derived Create(...) {\n  Derived d = new Derived(...);\n  Register(d);\n  return d;\n }\n public Derived(...) : base(...) { ... }\n}\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Don’t worry. Our long national nightmare of CLR week is almost over. I had originally planned to write an article about partially-constructed objects, but in the time since I queued up the topic (back in November 2005) to the time I got around to writing it up, I found that Joe Duffy had already written […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-3563","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Don’t worry. Our long national nightmare of CLR week is almost over. I had originally planned to write an article about partially-constructed objects, but in the time since I queued up the topic (back in November 2005) to the time I got around to writing it up, I found that Joe Duffy had already written […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/3563","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=3563"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/3563\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=3563"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=3563"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=3563"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}