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The Hidden Cost of yield in C#: What the Compiler Doesn't Tell You

Most C# developers know how to use yield return. Few understand what actually happens after compilation. If you've ever written something like this: public IEnumerable < int > GetNumbers () { yield return 1 ; yield return 2 ; yield return 3 ; } it looks almost magical. No collection. No list allocation. No iterator implementation. Yet somehow the method returns an IEnumerable. So what is really happening? The answer is one of the most elegant compiler transformations in the entire .NET ecosystem. Let's open the hood. The Illusion Most developers imagine the previous code executes like this: Call method ↓ Return 1 ↓ Pause ↓ Return 2 ↓ Pause ↓ Return 3 That isn't what happens. C# methods cannot actually pause execution. Instead, the compiler completely rewrites your method into something entirely different. The Compiler Creates a State Machine Your tiny method becomes a hidden class similar to this: private sealed class GetNumbersIterator : IEnumerable < int >, IEnumerator < int > { private int _state ; private int _current ; public bool MoveNext () { switch ( _state ) { case 0 : _current = 1 ; _state = 1 ; return true ; case 1 : _current = 2 ; _state = 2 ; return true ; case 2 : _current = 3 ; _state = - 1 ; return true ; default : return false ; } } public int Current => _current ; } Your original method no longer exists. Instead, it simply returns: return new GetNumbersIterator (); Every yield return becomes another state inside MoveNext(). Why Local Variables Don't Disappear Consider this code: IEnumerable < int > Squares () { int x = 1 ; while ( x <= 3 ) { yield return x * x ; x ++; } } After the first yield, the method "pauses." But where is x stored? Not on the stack. The original stack frame has already disappeared. Instead, the compiler promotes local variables into fields: private int _x ; The iterator object now owns every variable that must survive between iterations. This is why iterator methods can remember where they left off. Heap Allocation Happens Ma

2026-07-19 原文 →
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I Built an AI App. Eight Months Later, It Became a Skill

When I first wrote about NutriAgent in November 2025, it was a full application. It had a Python backend, a web interface, a Telegram bot, user accounts, Google OAuth, Supabase, and a Google Sheets integration. Recently, I reproduced its core workflow as a skill for my personal AI agent. It took around 15 minutes and two prompts. I didn't build another backend, deploy a service, or implement OAuth again. I explained how I wanted the workflow to behave, tested it, and watched a new row appear in my nutrition spreadsheet. The original application wasn't a mistake. It was how I could deliver that experience with the tools available at the time. Eight months later, the starting point had changed. Eight Months Ago, This Was an App I built NutriAgent because I wanted to track calories and protein without trapping my data inside a nutrition app. I wanted the raw records in a spreadsheet I controlled, where I could create my own reports and eventually connect nutrition with my training data. The first version was a personal n8n workflow. It worked for me, but when a friend wanted to try it, I realized that everything was tied to my accounts. To make it reusable, I rebuilt it in Python and added the parts a real multi-user product needed: authentication, storage, a web interface, Telegram, Google OAuth, conversation history, and account linking. I've already told that story in I Ditched MyFitnessPal and Built an AI Agent to Track My Food , and later wrote about what broke after I used it every day for a month . This article starts after that version. My Gaming PC Became an Agent Box I had a modest gaming PC at home with 16 GB of RAM and a 1 TB drive. Using it through Windows, WSL, and remote desktop from my Mac felt awkward, so I installed Linux and turned it into a remote box for running agents. I'll write about that setup separately. I moved Hermes there from a VPS. Hermes is the personal agent I now run on that machine. It can load reusable skills and use tools connected

2026-07-19 原文 →