--- title: 5 things you didn't know about Guid in C# | daily.dev description: A rundown of lesser-known Guid behaviors in C#: Guids are 128-bit (16-byte) values with astronomically low collision odds; Guid is a struct (value type), so... canonical: https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: 5 things you didn't know about Guid in C# | daily.dev og:description: A rundown of lesser-known Guid behaviors in C#: Guids are 128-bit (16-byte) values with astronomically low collision odds; Guid is a struct (value type), so... og:url: https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x og:image: https://api.daily.dev/og/posts/e37251p8X.png og:image:alt: 5 things you didn't know about Guid in C# 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. # 5 things you didn't know about Guid in C# **[Code4IT](https://daily.dev/sources/code4it)** · 5 min read · 0 upvotes · 0 comments ## Summary A rundown of lesser-known Guid behaviors in C#: Guids are 128-bit (16-byte) values with astronomically low collision odds; Guid is a struct (value type), so passing it to a method without `ref` won't mutate the caller's copy; Guids can be created via Guid.NewGuid(), Guid.Empty, Guid.Parse/TryParse, a string constructor, or a 16-byte array constructor; ToString() supports multiple formats (D, N, B, P, X) and Guid.ParseExact enforces a specific format when parsing; and despite being conceptually 16 bytes, sizeof(Guid) doesn't compile outside an unsafe context because the CLR may add padding, so Marshal.SizeOf() or an unsafe block is needed to get the actual allocated size. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does sizeof(Guid) not compile in C#? Guid does not have a predefined size recognized by the compiler, so sizeof(Guid) only compiles inside an unsafe context. This is because while a Guid's logical size is 16 bytes, the memory the CLR actually allocates isn't guaranteed to be constant, since some architectures add padding. Use an unsafe block with sizeof(Guid), or call Marshal.SizeOf() from System.Runtime.InteropServices instead. _Developers debugging low-level memory quirks in C# often need a quick reference like daily.dev for Guid internals._ ### What is the difference between Guid.Parse and Guid.ParseExact in C#? Guid.Parse converts a string to a Guid as long as it represents a valid Guid in any recognized format, while Guid.ParseExact only succeeds if the string matches a specified format exactly, such as 'P' for parentheses or 'N' for digits only. For example, ParseExact with format 'P' accepts '(e10deb88-171b-4c34-81f7-05fc17d16316)' but rejects the same string under format 'N'. _Picking the right parsing method matters when validating Guid formats, something daily.dev readers refine through shared C# tips._ ### Does passing a Guid to a method in C# change the original value? No, because Guid is a struct (value type) in C#, passing it to a method by default passes a copy, so reassigning it inside the method does not affect the caller's variable. Only using the ref keyword allows the method to modify the original Guid value, as demonstrated by comparing updateGuid versus updateGuidRef behavior with Guid.NewGuid(). _Understanding value versus reference semantics helps C# developers avoid subtle bugs daily.dev's community often discusses._ ## Similar posts on daily.dev - [C\# Reflection: The Complete .NET 10 Guide](https://daily.dev/posts/c-reflection-the-complete-net-10-guide-xjtayoxme) · We Are .NET · 2 upvotes · 0 comments - [The hidden trap of fixed buffers in C\#](https://daily.dev/posts/the-hidden-trap-of-fixed-buffers-in-c--87ydebi3d) · Kevin Gosse \(minidump.net\) · 4 upvotes · 0 comments - [Pointers in C\# and Memory Safety: Span vs. C\# 16 unsafe](https://daily.dev/posts/pointers-in-c-and-memory-safety-span-vs-c-16-unsafe-9nbddfw8p) · NDepend Blog · 9 upvotes · 0 comments - [15 Advanced C\# Mistakes You’re Probably Still Making](https://daily.dev/posts/15-advanced-c-mistakes-you-re-probably-still-making-ctxsuyhsf) · ITNEXT · 36 upvotes · 2 comments --- Tags: [#.net](https://daily.dev/tags/.net), [#c#](https://daily.dev/tags/c#) [View this post on daily.dev](https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x) ```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":"5 things you didn't know about Guid in C#","url":"https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x"},"datePublished":"2026-10-04T13:05:42.333Z","dateModified":"2026-10-04T13:11:50.042Z","description":"A rundown of lesser-known Guid behaviors in C#: Guids are 128-bit (16-byte) values with astronomically low collision odds; Guid is a struct (value type), so...","image":"https://media.daily.dev/image/upload/s--1KxV4ohY--/f_auto/v1722860400/public/Placeholder%2007","thumbnailUrl":"https://media.daily.dev/image/upload/s--1KxV4ohY--/f_auto/v1722860400/public/Placeholder%2007","isAccessibleForFree":true,"articleSection":"Code4IT","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":"Code4IT","logo":"https://media.daily.dev/image/upload/s--OaaG4G10--/f_auto,q_auto/v1791119049/logos/code4it?_a=BAMAMicg0","url":"https://daily.dev/sources/code4it"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":".net,c#","timeRequired":"PT5M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Code4IT","item":"https://daily.dev/sources/code4it"},{"@type":"ListItem","position":3,"name":"5 things you didn't know about Guid in C#"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/5-things-you-didn-t-know-about-guid-in-c--e37251p8x#faq","mainEntity":[{"@type":"Question","name":"Why does sizeof(Guid) not compile in C#?","acceptedAnswer":{"@type":"Answer","text":"Guid does not have a predefined size recognized by the compiler, so sizeof(Guid) only compiles inside an unsafe context. This is because while a Guid's logical size is 16 bytes, the memory the CLR actually allocates isn't guaranteed to be constant, since some architectures add padding. Use an unsafe block with sizeof(Guid), or call Marshal.SizeOf() from System.Runtime.InteropServices instead. Developers debugging low-level memory quirks in C# often need a quick reference like daily.dev for Guid internals."}},{"@type":"Question","name":"What is the difference between Guid.Parse and Guid.ParseExact in C#?","acceptedAnswer":{"@type":"Answer","text":"Guid.Parse converts a string to a Guid as long as it represents a valid Guid in any recognized format, while Guid.ParseExact only succeeds if the string matches a specified format exactly, such as 'P' for parentheses or 'N' for digits only. For example, ParseExact with format 'P' accepts '(e10deb88-171b-4c34-81f7-05fc17d16316)' but rejects the same string under format 'N'. 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