--- title: Primary constructors are more than syntactic sugar description: Switching from a regular constructor to a primary constructor in C# changes when fields get initialized relative to the base constructor call. With a regular... canonical: https://daily.dev/posts/primary-constructors-are-more-than-syntactic-sugar-15ikh5h1i twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Primary constructors are more than syntactic sugar | daily.dev og:description: Switching from a regular constructor to a primary constructor in C# changes when fields get initialized relative to the base constructor call. With a regular... og:url: https://daily.dev/posts/primary-constructors-are-more-than-syntactic-sugar-15ikh5h1i og:image: https://api.daily.dev/og/posts/15IKh5h1I.png og:image:alt: Primary constructors are more than syntactic sugar 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. # Primary constructors are more than syntactic sugar **[We Are .NET](https://daily.dev/sources/wearedotnet)** · 3 min read · 2 upvotes · 0 comments ## Summary Switching from a regular constructor to a primary constructor in C# changes when fields get initialized relative to the base constructor call. With a regular constructor, a field is assigned only after the base constructor returns, so a virtual member called from the base constructor sees null. With a primary constructor, the compiler stores the parameter in a hidden field that's assigned before the base constructor runs. Running `new Regular(3)` and `new Primary(3)` against the same base class prints 'Message = count = null' for Regular but 'Message = count = 3' for Primary. The hidden field isn't readonly, and using the parameter both in a member and a field initializer triggers warning CS9124. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does my C# base class constructor see a null value for a field set by a derived class primary constructor parameter? It shouldn't, and that's the point: primary constructors assign their captured parameter to a hidden field before the base constructor runs, so virtual members called from the base constructor see the real value. A regular constructor instead assigns 'this.field' only after 'base(...)' returns, so the base constructor sees null or a default during its own execution. _Developers debugging constructor ordering bugs in C# track compiler behavior details like this on daily.dev._ ### What does the C# compiler generate differently for primary constructors versus regular constructors when a parameter is used in a virtual member? For a regular constructor, the compiler creates a closure class to capture the parameter for a lambda, then assigns the instance field after the base constructor call. For a primary constructor, the compiler instead generates a hidden field like 'P' and assigns it before calling the base constructor, matching the timing of field initializers in C#. _C# developers comparing lowered IL behavior across constructor styles can follow deep dives like this on daily.dev._ ### Is the hidden field generated by a C# primary constructor parameter readonly? No, the generated field such as 'P' is not readonly, so the parameter can still be reassigned inside the class without a compiler error. To get a readonly guarantee, declare the field explicitly with a field initializer like 'private readonly int? count = count;', but avoid also using the parameter in a member or the compiler warns with CS9124. _Teams adopting C# primary constructors can keep track of these readonly gotchas on daily.dev._ ## Similar posts on daily.dev - [Why I Switched to Primary Constructors for DI in C\#](https://daily.dev/posts/why-i-switched-to-primary-constructors-for-di-in-c--ctodiijms) · Milan Jovanović · 53 upvotes · 4 comments - [Simplify constructors with builder pattern](https://daily.dev/posts/simplify-constructors-with-builder-pattern-tz4nra6pk) · Damir's Corner · 0 upvotes · 0 comments - [Closed class hierarchies](https://daily.dev/posts/closed-class-hierarchies-xmmjyy63z) · .NET Escapades · 15 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/primary-constructors-are-more-than-syntactic-sugar-15ikh5h1i) ```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":"Primary constructors are more than syntactic sugar","url":"https://daily.dev/posts/primary-constructors-are-more-than-syntactic-sugar-15ikh5h1i","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/primary-constructors-are-more-than-syntactic-sugar-15ikh5h1i"},"datePublished":"2026-10-26T08:49:19.380Z","dateModified":"2026-10-26T08:50:18.141Z","description":"Switching from a regular constructor to a primary constructor in C# changes when fields get initialized relative to the base constructor call. 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A regular constructor instead assigns 'this.field' only after 'base(...)' returns, so the base constructor sees null or a default during its own execution. Developers debugging constructor ordering bugs in C# track compiler behavior details like this on daily.dev."}},{"@type":"Question","name":"What does the C# compiler generate differently for primary constructors versus regular constructors when a parameter is used in a virtual member?","acceptedAnswer":{"@type":"Answer","text":"For a regular constructor, the compiler creates a closure class to capture the parameter for a lambda, then assigns the instance field after the base constructor call. For a primary constructor, the compiler instead generates a hidden field like '\u003ccount\u003eP' and assigns it before calling the base constructor, matching the timing of field initializers in C#. 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