What 1 thing in programming has had the largest payoff that you've learned over the past 3 years. Yes, 3 years is an eternity it IT. I'm trying to decide what to learn next and want to see what you've learned that has the most payoff and why.
This could be anything like (but not limited to) coding to an interface, unit testing, TDD, mocking, lambdas, regular expressions, F#, WPF or linq.
Unit Testing with NUnit and TestDriven.NET is about the most useful thing I ever learned.
Where the GAC is physically located, and how it interacts with existing PATH and other environment variables.
Multiple versions of "identical" files on the same machine drove me crazy, until I understood how the GAC enabled such behavior.
Recently XAML, not so recently Lambda, and back in the day Generics. I am in love with XAML for some reason.
But to answer your question directly, the one thing I learned 3 years ago that has significantly improved my skill level is picking up a book on Design Patterns and spending countless hours of going through different patterns. This has helped me write more intelligent and maintainable code. I cannot stress enough the importance of Design Patterns. They have definitely had the largest payoff for me.
Static analysis, performance and memory profiling, design patterns.
I know it is general answer to specific C# question but it does save my day.
Two things stand out as major game-changers:
Part of the stack, although not directly .NET, would be Team Foundation Server. The improvement in process, build qualities, test automation, and issue tracking have allowed us to start focusing more on the code we are writing, as opposed to the draconian way that we had been managing and tracking it.
Everything.
3 years ago I hadn't touched .Net, now you can see my answers for what I've learnt and can do.
Before I used PHP and VB.Net, but had never seen OOP, or even true testing.
Without a doubt, LINQ. It lets me turn complicated gobbleygook code into much simple, easy to manage and maintain, code.
WPF is becoming a fovorite of mine as well. It is a lot harder to master than LINQ, but it has tremendous power for managing the display.
Microsoft enterprise library. Logging is one of the parts I love, since it is some times the only way to get exception messages and error messages, when things are running in production.
Functional programming, still trying to learn that (both in F# and C#).
I like the new way of setting propeties when creating new objects TextBox txtXX = new TextBox(){ Text = "Hello"};
All of these are somewhat interdependent, but: