{"action":"created","comment":{"url":"https://api.github.com/repos/dotnet/roslyn/pulls/comments/49354457","id":49354457,"diff_hunk":"@@ -24,6 +24,9 @@ namespace Microsoft.CodeAnalysis\n     /// </summary>\n     internal sealed class PEModule : IDisposable\n     {\n+        // We need to store reference for to keep the metadata alive while symbols have reference to PEModule.\n+        private readonly ModuleMetadata _owner;","path":"src/Compilers/Core/Portable/MetadataReader/PEModule.cs","position":5,"original_position":5,"commit_id":"8d43adf28bf223a682130c8539414b8d75563625","original_commit_id":"8d43adf28bf223a682130c8539414b8d75563625","user":{"login":"tmat","id":41759,"avatar_url":"https://avatars.githubusercontent.com/u/41759?v=3","gravatar_id":"","url":"https://api.github.com/users/tmat","html_url":"https://github.com/tmat","followers_url":"https://api.github.com/users/tmat/followers","following_url":"https://api.github.com/users/tmat/following{/other_user}","gists_url":"https://api.github.com/users/tmat/gists{/gist_id}","starred_url":"https://api.github.com/users/tmat/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/tmat/subscriptions","organizations_url":"https://api.github.com/users/tmat/orgs","repos_url":"https://api.github.com/users/tmat/repos","events_url":"https://api.github.com/users/tmat/events{/privacy}","received_events_url":"https://api.github.com/users/tmat/received_events","type":"User","site_admin":false},"body":"I think it's likely always been a problem but we don't work too much with standalone .netmodules so we haven't hit it. It's be good to add a dedicated test that creates a netmodule, only holds on the corresponding PE symbol (and not the metadata) calls GC.Collect and then accesses some members of the symbol. 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It is used by numerous IDE features to quickly go from Name-to-Symbol (for example with features like Add-Using).  SymbolTreeInfo keeps both exact indices (i.e. the searched name must exactly match the symbol name) as well as fuzzy indices (i.e. the searched name needs to be 'close enough' to the symbol name).  \r\n\r\nThis change addresses two problems we were having with the current system around SymbolTreeInfo.\r\n\r\nFirst:\r\nThe current implementation of SymbolTreeInfo does no synchronization when clients call in to create one.  This means if we have N threads trying to get the same SymbolTreeInfo we'll perform the computation for it N times, keeping only the last result.  This is pretty wasteful.  And, in my own personal testing, was happening a lot.\r\n\r\nNow, we only allow a single SymbolTreeInfo to be created at a time, so that we don't waste any CPU producing and then throwing away work.\r\n\r\n--\r\n\r\nSecond:\r\nWe were caching SymbolTreeInfos in a way that was causing them to be created and thrown out far more often than we would have liked.  For metadata dlls we want the lifetime of the SymbolTreeInfo to be as long as the metadata we've loaded for them.  In the IDE we load this metadata once and keep it around as long as the file hasn't changed on disk.  The introduction of the Metadata ID works to allow us to represent that lifetime, and associate our indices appropriately.  i.e. as long as a MetadataReference is still producing and caching (in the IDE case) Metadata with the same ID, then our indices for that dll will stay alive.  When the IDE dumps this metadata, then we'll dump the index as 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