Much of what a developer opens on a Monday morning was written in C#. The Q&A page that unblocks a broken build, the chat client carrying a company’s morning stand-up, the app that tracks a parcel across two continents: each of those is built on .NET, Azure, or both, and the engineering teams behind them have published the details.

Six of the seven applications below come from companies an engineer would recognize by name. The seventh is the one you have not heard of, and the one to read for. Every entry covers what the application does, which part of the Microsoft stack it uses, and one detail worth carrying into a technology decision, with the source linked at the point of each claim. A companion post covers the same ground for applications written in Java.

Stack Overflow documented its technology stack before most of the industry had heard of the site. A September 2008 post from the team listed C#, ASP.NET MVC, SQL Server, and LINQ to SQL, and the application has stayed on Microsoft’s platform since, moving to ASP.NET Core as .NET itself moved.

The scaling decisions are the part worth studying. In February 2016, architecture lead Nick Craver published a full breakdown of the infrastructure: roughly 1.3 billion page views a month, served by nine web servers. The team tuned a small number of large machines rather than spreading the application across hundreds of small ones.

Those figures describe a 2016 design philosophy and not a current server inventory. The philosophy is what aged well. A profiled, cache-aware .NET application absorbs a great deal of traffic on modest hardware, a useful thing to recall before a team splits a working monolith into services.

Microsoft last published a Teams user figure in its FY24 Q1 earnings in October 2023: more than 320 million monthly active users. The backend serving them runs on .NET 6 and Azure Communication Services, and Microsoft’s engineering team wrote up the migration on the .NET blog.

Moving the backend off .NET Framework cut Azure compute costs by 29%. CPU utilization and service latency improved by 30% to 50%, depending on the service. Platform vendors seldom publish before-and-after arithmetic on their own products at that level of detail, which makes the Teams write-up one of the more useful documents available on what a framework upgrade returns at scale.

Azure Communication Services adds a second lesson. The calling, chat, and telephony infrastructure behind Teams is published as an Azure product, so a team adding a communication feature to its own application can call the same services Microsoft runs its platform on, with the media handling already solved.

The documented migration covers backend infrastructure, and the claim stops there. Microsoft has not published equivalent details about the desktop and mobile clients, so the .NET story in Teams belongs to the services behind the app.

GitHub kept its own infrastructure for seven years after Microsoft acquired it for $7.5 billion in 2018. That changed in October 2025, when CTO Vladimir Fedorov told staff the platform would put an Azure migration ahead of feature work, citing capacity limits in its Virginia data center against demand driven by Copilot and other AI features.

Progress now appears in GitHub’s monthly availability reports. The July 2026 report put monolith read traffic served from Azure Central US at a 52.75% peak on 28 July, Git traffic in Azure at 47%, and a second Central US replica in place for 29% of all repositories. The stated targets are 70% of read traffic and 30% of write traffic in Central US this quarter, and production traffic out of GitHub’s own data centers by the end of 2026.

Source: GitHub availability report, July 2026.

The framing matters for anyone citing this example. GitHub is moving to Azure and has not finished the move. Its application code is a Ruby on Rails codebase, so Azure is the destination infrastructure and stays outside the development stack. Hosting on Azure and building on .NET are separate facts, and that difference runs through the rest of this list.

GoDaddy carries more than 62 million registered domains for roughly 20 million customers. Its documented technology stack, according to the company’s StackShare profile, includes .NET, ASP.NET, Microsoft Azure, and Microsoft IIS. StackShare entries are self-reported and can lag behind engineering reality, so the profile reads best as a record of what the company has said about its stack.

GoDaddy sits on both sides of the distinction drawn in the previous section: the Microsoft platform in the application layer through .NET and ASP.NET, Azure in the infrastructure layer.

For a registrar, an outage takes customer websites and customer email down in the same minute. That raises the cost of a platform change and rewards a stack the engineering organization already knows how to operate at three in the morning.

UPS moves more than 19 million packages a day across more than 220 countries and territories, with about $1 billion a year going into technology. Two parts of that estate are documented in public.

The first is UPS Mobile, which appears on Microsoft’s Xamarin customer showcase. Customers use it to track shipments, create labels, find drop-off points, and estimate costs, and it was built with Xamarin and C# on a codebase shared across iOS and Android.

The second is quieter evidence. UPS engineering job listings, including senior full-stack C# developer roles, place C# and .NET inside the company’s engineering organization.

Together those sources support a narrower claim than the one that circulates online. UPS has not published the languages behind its routing systems, so the accurate version is a Microsoft-documented mobile application plus hiring evidence of C# and .NET in production engineering.

FlightPulse puts flight data and safety analytics in the hands of the people flying the aircraft. GE Digital built the application with .NET Core, Azure, and Visual Studio, and Microsoft features it on the official .NET customer showcase.

Flight data volume grows with every sensor added to an airframe, and the analysis has to reach a pilot in a form that is usable between two legs of a duty day. That puts the workload on cloud infrastructure and the interface on a device in someone’s flight bag.

Aviation software also answers to requirements few product teams encounter: traceable data lineage, and review processes tied to formal safety programs. A number that cannot be explained in that setting is a number that cannot ship.

.NET Core operating inside those constraints makes an argument about the platform that traffic figures do not. Stack Overflow shows what the stack handles under load. FlightPulse shows what it handles under scrutiny.

ProTitleUSA runs nationwide title search for the real estate and mortgage industry. Its platform was a .NET Framework WebForms application, the kind that keeps running long after the ecosystem around it has moved on, with the cost showing up in how long each new feature takes.

Softwarium rebuilt the front end in Angular against a .NET 6 Web API, containerized the platform with Docker, and deployed it through CI/CD. The migration ran with zero downtime on a live, business-critical system. API response times came out 30% faster, and the resulting architecture gave the platform room for the AI and OCR work that title search invites.

Title search is a document-heavy business running against legal deadlines. A stalled search delays a closing, so the rebuild had to happen underneath a system that stayed in use throughout.

Seven applications, one platform, and seven different reasons for choosing it. Developer Q&A, enterprise communication, domain registration, package logistics, aviation analytics, and title search each carry their own performance ceilings and compliance obligations, and the engineering answer changes with them.

One habit carries over from this list into any stack evaluation. Separating what an application is written in from where it is deployed changes what a technology claim actually supports. GitHub and GoDaddy both appear on Azure, and one of the two writes its application in .NET.

Softwarium has delivered on .NET, C#, and Azure across energy, supply chain, legal and real estate, cybersecurity, healthcare, and SaaS, with Azure Certified Engineers on the team.