--- title: Writing a .NET Garbage Collector in C# - Part 9: Frozen... description: This installment of a series on writing a custom .NET garbage collector in C# explains frozen segments and the NonGC heap, which store immortal objects like... canonical: https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Writing a .NET Garbage Collector in C# - Part 9: Frozen segments and new allocation strategy | daily.dev og:description: This installment of a series on writing a custom .NET garbage collector in C# explains frozen segments and the NonGC heap, which store immortal objects like... og:url: https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu og:image: https://api.daily.dev/og/posts/wuBvY9Lsu.png og:image:alt: Writing a .NET Garbage Collector in C# - Part 9: Frozen segments and new allocation strategy og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Writing a .NET Garbage Collector in C# - Part 9: Frozen segments and new allocation strategy **[Kevin Gosse \(minidump.net\)](https://daily.dev/sources/minidump)** · 14 min read · 0 upvotes · 0 comments ## Summary This installment of a series on writing a custom .NET garbage collector in C# explains frozen segments and the NonGC heap, which store immortal objects like string literals so the GC can skip scanning them. It walks through a real bug where marking objects in frozen segments corrupts their method-table pointer during sweep, then explores fixes: walking frozen segments to unmark (costly), checking IsInFrozenSegment per object (too slow), and finally a better approach - reserving a huge contiguous virtual address range (2 TB via VirtualAlloc/mmap) so any address outside that range is known not to be GC-managed. A new NativeAllocator replacing NativeMemory.Alloc is implemented, committing memory on demand and exposing a cheap O(1) IsInRange check used during mark phase, avoiding the need to ever mark or unmark frozen segment objects. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does marking a string literal during garbage collection crash the program when it's later checked with 'is string'? The crash happens because the string literal lives in a frozen segment outside the normal managed heap, so when the mark phase sets a mark bit on its method-table pointer, the sweep phase never walks that segment to unmark it afterward. The object is left with a corrupted method-table pointer, so any later type check on it reads garbage and crashes. _Developers debugging GC-related crashes with frozen segments or string literals can find implementation walkthroughs like this on daily.dev._ ### How can a garbage collector cheaply check if an object pointer belongs to the managed heap versus unmanaged memory? Reserve a single huge contiguous virtual address range upfront (for example 2 TB via VirtualAlloc on Windows or mmap on Linux) and commit pages from it on demand as objects are allocated. Then checking whether a pointer is managed becomes a simple comparison against the reserved range's lower and upper bounds, an O(1) test, instead of searching through separate frozen segments. _Engineers designing memory allocators or GC internals can track techniques like this via daily.dev._ ## Similar posts on daily.dev - [The Fasting Developer: Memory Discipline and GC Awareness in .NET](https://daily.dev/posts/the-fasting-developer-memory-discipline-and-gc-awareness-in-net-5yq85h552) · We Are .NET · 12 upvotes · 0 comments - [Win16 Memory Management](https://daily.dev/posts/win16-memory-management-iurgwfefx) · Lobsters · 0 upvotes · 0 comments - [Internals for Interns](https://daily.dev/posts/internals-for-interns-fg0xibngw) · Hacker News · 0 upvotes · 0 comments --- Tags: [#.net](https://daily.dev/tags/.net), [#c#](https://daily.dev/tags/c#) [View this post on daily.dev](https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Writing a .NET Garbage Collector in C# - Part 9: Frozen segments and new allocation strategy","url":"https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu"},"datePublished":"2026-10-04T13:06:42.821Z","dateModified":"2026-10-04T13:15:38.623Z","description":"This installment of a series on writing a custom .NET garbage collector in C# explains frozen segments and the NonGC heap, which store immortal objects like...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/a93e8b78d12e43a93d826c59371147d0?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/a93e8b78d12e43a93d826c59371147d0?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Kevin Gosse (minidump.net)","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Kevin Gosse (minidump.net)","logo":"https://media.daily.dev/image/upload/s--8KHn1aFB--/f_auto,q_auto/v1791119046/logos/minidump?_a=BAMAMicg0","url":"https://daily.dev/sources/minidump"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":".net,c#","timeRequired":"PT14M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Kevin Gosse (minidump.net)","item":"https://daily.dev/sources/minidump"},{"@type":"ListItem","position":3,"name":"Writing a .NET Garbage Collector in C# - Part 9: Frozen segments and new allocation strategy"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/writing-a-net-garbage-collector-in-c---part-9-frozen-segments-and-new-allocation-strategy-wubvy9lsu#faq","mainEntity":[{"@type":"Question","name":"Why does marking a string literal during garbage collection crash the program when it's later checked with 'is string'?","acceptedAnswer":{"@type":"Answer","text":"The crash happens because the string literal lives in a frozen segment outside the normal managed heap, so when the mark phase sets a mark bit on its method-table pointer, the sweep phase never walks that segment to unmark it afterward. 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