--- title: C# Tip: use yield return to return one item at the time description: Explains the C# yield return keyword as a way to return collection items lazily, one at a time, instead of building an entire list upfront. Compares code and... canonical: https://daily.dev/posts/c-tip-use-yield-return-to-return-one-item-at-the-time-scytm2qmx twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: C# Tip: use yield return to return one item at the time | daily.dev og:description: Explains the C# yield return keyword as a way to return collection items lazily, one at a time, instead of building an entire list upfront. Compares code and... og:url: https://daily.dev/posts/c-tip-use-yield-return-to-return-one-item-at-the-time-scytm2qmx og:image: https://api.daily.dev/og/posts/sCYtM2QMX.png og:image:alt: C# Tip: use yield return to return one item at the time 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. # C# Tip: use yield return to return one item at the time **[Code4IT](https://daily.dev/sources/code4it)** · 5 min read · 0 upvotes · 0 comments ## Summary Explains the C# yield return keyword as a way to return collection items lazily, one at a time, instead of building an entire list upfront. Compares code and console output of a list-based approach versus a yield-based approach, highlighting performance and memory benefits. Notes that yield only works with IEnumerable/IEnumerator return types, and shows a real-world use case with NUnit's TestCaseSource attribute for generating test cases lazily. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What is the difference between returning a List and using yield return in C#? Returning a List builds the entire collection in memory before any item is used by the caller, while yield return produces items one at a time as the caller iterates, interleaving creation and consumption. This makes yield return faster and more memory-efficient when only part of a large collection is needed, since items are generated lazily rather than all at once. _Developers choosing between eager and lazy collection patterns can explore C# yield return examples on daily.dev._ ### Which return types support the yield return keyword in C#? The yield keyword only works in methods returning IAsyncEnumerable, IEnumerable, IEnumerable, IEnumerator, or IEnumerator. It cannot be used in a method that returns a concrete type like List, since the compiler reports that such a type is not an iterator interface type. _Anyone debugging iterator compile errors can dig into yield return constraints on daily.dev._ ### How does NUnit use yield return for TestCaseSource test cases? NUnit's TestCaseSource attribute points to a class implementing IEnumerable whose GetEnumerator method uses yield return to supply one test case object array at a time, rather than building a full list of cases upfront. This lets the test runner pull test cases lazily, as shown in NUnit's own documentation example with divide test cases. _Developers writing data-driven NUnit tests can find yield-based TestCaseSource patterns on daily.dev._ ## Similar posts on daily.dev - [Demystifying async/await in C\#: The Hidden State Machine](https://daily.dev/posts/demystifying-async-await-in-c-the-hidden-state-machine-ej0npqym4) · C\# Corner · 0 upvotes · 0 comments - [Looking at Unity finally made me understand the point of C\+\+ coroutines · Mathieu Ropert](https://daily.dev/posts/looking-at-unity-finally-made-me-understand-the-point-of-c-coroutines-mathieu-ropert-lyk7zdbtl) · 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/c-tip-use-yield-return-to-return-one-item-at-the-time-scytm2qmx) ```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":"C# Tip: use yield return to return one item at the time","url":"https://daily.dev/posts/c-tip-use-yield-return-to-return-one-item-at-the-time-scytm2qmx","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/c-tip-use-yield-return-to-return-one-item-at-the-time-scytm2qmx"},"datePublished":"2026-10-04T13:06:40.483Z","dateModified":"2026-10-04T13:19:49.647Z","description":"Explains the C# yield return keyword as a way to return collection items lazily, one at a time, instead of building an entire list upfront. 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