When I started making games using Unity, after almost 10 years of C# development, I was very concerned to acknowledge that foreach loops are highly avoided in Unity because they allocate unnecessary memory on the heap. Personally I love the clean syntax of a foreach. It aids readably and clarity and it also increases the abstraction level. However a very clear and neat explanation of the memory issue problem can be found in a blog article posted on Gamasutra by Wendelin Reich.
From Wendelin’s analysis it emerged that the version of the Mono compiler adopted in Unity has a different behaviour from Microsoft implementation. In particular enumerators, which are usually implemented in the .NET framework as mutable value types, are boxed by the compiler, causing an unnecessary generation of garbage. Boxing is the process of converting a value type (allocated on the stack) into a reference type, thus allocating a new instance on the heap.Read More