Language cheatsheet

C# 14 Cheatsheet

Modern C# 14 syntax for strings, nullability, collections, LINQ, records, pattern matching, async code and generics.

Basics

Write a program without a Main method — Top-level statements: the file's statements are the program. Implicit usings bring in common namespaces.
Declare variables — Use var when the type is obvious. const values must be known at compile time.
Insert values into a string — Prefix a string with $ and put expressions in braces, with an optional format.
Write multi-line text — Raw string literals keep quotes and line breaks as written, with no escaping.
Handle null safely — Mark possibly-null reference types with ?. The compiler warns when you dereference them.
Assign only when null — Assign a default if a variable is currently null.
Return several values — Tuples group values without defining a type, and can be deconstructed.
Write a local function — Define a helper inside a method, where only that method can use it.
Get a name as a string — Refer to variables and members without hard-coding their names.

Create collections concisely — Collection expressions build arrays, lists and spans with the same syntax. .. spreads another collection.
Use a dictionary — Add and read values by key. TryGetValue avoids an exception for missing keys.
Keep unique items — A set rejects duplicates, and Add returns false if the item was already there.
Filter and transform — LINQ methods work on any sequence. Nothing runs until you enumerate the result.
Sort by one or more keys — Sort by one key, then break ties with another.
Group items — Group by a key and compute a value for each group.
Find or test elements — FirstOrDefault returns null or default when nothing matches, while First throws.
Total, average and extremes — Compute simple aggregates over a sequence.
Join two sequences — Match items from two sequences by a key.
Generate numbers — Produce a sequence of integers.

Declare an immutable data type — A record gives you value equality, ToString and deconstruction.
Copy a record with changes — Create a modified copy and leave the original unchanged.
Declare a class with a primary constructor — Put constructor parameters on the type declaration and use them in members.
Require properties at creation — Force callers to set a property, and make it settable only during initialization.
Customize an auto-property — C# 14: use the field keyword to reach the compiler-generated backing field.
Choose a value with a switch expression — A compact switch that returns a value, with a default.
Test and unpack a type — Check a type and bind it to a variable in one expression.
Match the shape of a list — Match arrays and lists by length and contents.
Define an enum — A named set of constants, and parsing one from text.
Add a method to an existing type — A static method in a static class, with this on its first parameter.
Constrain a generic type — Require that a type argument has certain capabilities.
Provide a default interface method — An interface can include an implementation that classes may override.

Write an async method — Await I/O without blocking a thread. Async methods return Task or Task<T>.
Run tasks in parallel — Start several tasks and wait for all of them.
Support cancellation — Pass a CancellationToken so callers can stop long operations, for example on a timeout.
Handle exceptions — Catch specific exceptions, filter with when, and clean up in finally.
Dispose a resource automatically — A using declaration disposes the object at the end of the enclosing scope.
Read and write files — Read and write text asynchronously.
Serialize to and from JSON — Convert objects to JSON text and back with System.Text.Json.
Work with dates and times — DateOnly and TimeOnly model a date or a time of day on its own.
Protect shared state — Use a lock to let only one thread run a block at a time.

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