[nihilist_t21]

A decade ago puts you at .NET Framework 4.0 or so, and C# version 4. A lot has changed since then. While your old books would still be somewhat relevant, you are missing out on a lot of new features in both C# and .NET. Especially since depreciation of .NET Framework and the transition to .NET 5.

[MarkSweep]

To add to this, C# added async/await in 2012. Depending on what frameworks you use it may be hard to avoid interacting with it (for example, many APIs in UWP are async without a sync version). I feel like other language features added since then you can get away without knowing while writing code, but of course you may encounter them when reading other’s code.

[ygra]

That being said, C# 5 code can be retrofitted with most new features simply by listening to the IDE which suggests refactorings to make use of them.