--- title: Testing internal members with InternalsVisibleTo | daily.dev description: Explains how the InternalsVisibleTo attribute lets a .NET assembly expose its internal members to a specific friend assembly, most commonly a test project.... canonical: https://daily.dev/posts/testing-internal-members-with-internalsvisibleto-omp3oorzh twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Testing internal members with InternalsVisibleTo | daily.dev og:description: Explains how the InternalsVisibleTo attribute lets a .NET assembly expose its internal members to a specific friend assembly, most commonly a test project.... og:url: https://daily.dev/posts/testing-internal-members-with-internalsvisibleto-omp3oorzh og:image: https://api.daily.dev/og/posts/OmP3OoRZH.png og:image:alt: Testing internal members with InternalsVisibleTo 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. # Testing internal members with InternalsVisibleTo **[Code4IT](https://daily.dev/sources/code4it)** · 4 min read · 0 upvotes · 0 comments ## Summary Explains how the InternalsVisibleTo attribute lets a .NET assembly expose its internal members to a specific friend assembly, most commonly a test project. Walks through a simple FluentSumCalculator example with an internal numbers field, shows how to declare the attribute, how AllowMultiple lets you share internals with multiple assemblies, and notes that the attribute applies to the whole assembly rather than individual classes. Also sketches a more realistic scenario involving an internal Markdown-to-HTML transformation class in a DAL layer. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How do I unit test an internal field or method in C# from a separate test project? Add the InternalsVisibleTo assembly attribute above the namespace declaration in the project being tested, naming the test assembly, e.g. [assembly:InternalsVisibleTo("FluentSum.Tests")]. This grants the named assembly access to all internal members of the whole assembly, not just a single class, letting test code reference and assert against internal fields directly. _daily.dev surfaces practical c# testing patterns like this for developers wiring up internal test access._ ### Can InternalsVisibleTo grant internal access to more than one assembly at once? Yes, the attribute has AllowMultiple set to true, so it can be applied more than once in the same file. You can either stack separate attribute declarations for each assembly, such as [assembly:InternalsVisibleTo("AssemblyA")] followed by [assembly:InternalsVisibleTo("AssemblyB")], or combine them in a single bracketed declaration separated by commas. _developers structuring multi-assembly .net solutions can find more testing setup tips via daily.dev._ ## Similar posts on daily.dev - [C\# Source Generator Attributes: Generating Code with ForAttributeWithMetadataName](https://daily.dev/posts/c-source-generator-attributes-generating-code-with-forattributewithmetadataname-fzt75ae1d) · We Are .NET · 1 upvotes · 0 comments - [The best new features of C\# 14](https://daily.dev/posts/the-best-new-features-of-c-14-6gsbbjwv4) · InfoWorld · 46 upvotes · 4 comments --- Tags: [#testing](https://daily.dev/tags/testing), [#.net](https://daily.dev/tags/.net), [#c#](https://daily.dev/tags/c#) [View this post on daily.dev](https://daily.dev/posts/testing-internal-members-with-internalsvisibleto-omp3oorzh) ```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":"Testing internal members with InternalsVisibleTo","url":"https://daily.dev/posts/testing-internal-members-with-internalsvisibleto-omp3oorzh","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/testing-internal-members-with-internalsvisibleto-omp3oorzh"},"datePublished":"2026-10-04T13:07:17.850Z","dateModified":"2026-10-04T13:19:19.887Z","description":"Explains how the InternalsVisibleTo attribute lets a .NET assembly expose its internal members to a specific friend assembly, most commonly a test project....","image":"https://media.daily.dev/image/upload/s--ZrL_HSsR--/f_auto/v1722860399/public/Placeholder%2006","thumbnailUrl":"https://media.daily.dev/image/upload/s--ZrL_HSsR--/f_auto/v1722860399/public/Placeholder%2006","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/testing-internal-members-with-internalsvisibleto-omp3oorzh","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"testing,.net,c#","timeRequired":"PT4M"} {"@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":"Testing internal members with InternalsVisibleTo"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/testing-internal-members-with-internalsvisibleto-omp3oorzh#faq","mainEntity":[{"@type":"Question","name":"How do I unit test an internal field or method in C# from a separate test project?","acceptedAnswer":{"@type":"Answer","text":"Add the InternalsVisibleTo assembly attribute above the namespace declaration in the project being tested, naming the test assembly, e.g. [assembly:InternalsVisibleTo(\"FluentSum.Tests\")]. This grants the named assembly access to all internal members of the whole assembly, not just a single class, letting test code reference and assert against internal fields directly. daily.dev surfaces practical c# testing patterns like this for developers wiring up internal test access."}},{"@type":"Question","name":"Can InternalsVisibleTo grant internal access to more than one assembly at once?","acceptedAnswer":{"@type":"Answer","text":"Yes, the attribute has AllowMultiple set to true, so it can be applied more than once in the same file. You can either stack separate attribute declarations for each assembly, such as [assembly:InternalsVisibleTo(\"AssemblyA\")] followed by [assembly:InternalsVisibleTo(\"AssemblyB\")], or combine them in a single bracketed declaration separated by commas. developers structuring multi-assembly .net solutions can find more testing setup tips via daily.dev."}}]} ```