In this post I describe why foreach sometimes allocates, and show how you can use DynamicMethod and Reflection.Emit to go allocation-free

Andrew Lock's blog focuses on ASP.NET Core development, web APIs, and software architecture. Readers can find tutorials, code samples, and explanations covering topics such as RESTful API design, authentication, and authorization in ASP.NET Core. Additionally, they can learn about dependency injection, middleware, and performance optimization techniques.

.NET Escapades

Enumerating IEnumerable<T> with foreach causes heap allocations due to boxing of struct enumerators. This deep-dive explains why this happens by examining lowered C# code and IL, then demonstrates an advanced technique using Reflection.Emit and DynamicMethod to create allocation-free enumeration. Benchmarks show significant performance improvements on .NET Framework and .NET 9 for custom collection types, though .NET 10's deabstraction optimizations make this approach unnecessary on the latest runtime. The technique is particularly valuable for hot-path code in libraries that support older .NET versions.

Making foreach on an IEnumerable allocation-free using reflection and dynamic methods

Avoiding foreach allocation for known return types

Avoiding foreach allocation when you can't reference the return type