C#

Parse text without allocating with Span<T>

Splitting a line with string.Split creates a new string for every field. For large files, ReadOnlySpan<char> lets you parse the same data with almost no allocations.

Pattern matching that reads like the business rule

Switch expressions, property patterns and list patterns can turn a page of nested ifs into a few lines that read like the requirements. Here are the forms worth knowing.

Test time-dependent code with TimeProvider

Code that calls DateTime.UtcNow directly is hard to test. Since .NET 8, TimeProvider gives you a standard abstraction, and a fake one that you can move forward on demand.

Do you still need ConfigureAwait(false)?

In ASP.NET Core application code, no. In libraries, yes. Here's why the answer depends on where your code runs.

Primary constructors and their surprises

Primary constructors on classes save a lot of boilerplate for dependency injection. Their parameters aren't readonly fields, though, and that has consequences.

Exceptions or results for expected failures?

"Customer not found" isn't exceptional, but most .NET code throws for it anyway. A simple result type makes expected failures part of the method's signature.

Stream results with IAsyncEnumerable

Loading 100,000 rows into a list before sending the first one wastes memory and time. IAsyncEnumerable<T> lets you process and return items as they arrive.

Stop using async void and fire-and-forget

async void can crash your process, and _ = Task.Run(...) in a web request quietly loses errors and uses disposed services. Here's what to do instead.

Value objects with C# records

Replacing a decimal and a string with a Money type removes a whole class of bugs. Records make value objects short to write, with one catch around with expressions.

Turning on nullable reference types in an existing codebase

Nullable reference types catch null reference exceptions at compile time. Switching them on in a large, older project produces thousands of warnings. Here's how to adopt them one step at a time.

ValueTask: when to use it and when not to

ValueTask<T> can save an allocation on hot paths. It also comes with rules that Task doesn't have, and breaking them causes bugs that are hard to trace.

Records vs classes in C#

Records give you value equality, with expressions and a one-line declaration. Here's when that helps, and the two places it quietly bites.