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C#

687 problems50 sections📅 2026 curriculum

Learn C# by typing real, idiomatic code.

Type through everyday C#: LINQ queries, async/await, properties and records, pattern matching, and generics. Every snippet is idiomatic, compiling C#, and an AI tutor explains each line — how a LINQ chain defers execution, why a record gives you value equality — so you learn the .NET way of doing things as you build typing speed.

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Run C# on your machine

1. Install the toolchain

brew install dotnet # or see docs

2. Save your code as Program.cs, then compile & run

dotnet run

Official install docs ↗

progress by section

1Fundamentals0/12start →

Hello, C#Constants for loop foreach loop if / else if / else String interpolation Numeric types Ternary operator var and explicit types while loop Methods switch expression

2Strings In Depth0/18start →

Concatenation Strings are immutable The char type Compare strings Contains and friends Index and iterate chars Interpolation with format Join Replace Split Substring Trim and blank checks Verbatim strings StringBuilder Composite formatting Raw string literals Reverse via char array Span<char>

3Values & References0/14start →

Passing a reference type Class is a reference type Struct is a value type Passing a value type null vs default Nullable references out parameters ref parameters Boxing Class reference equality in parameters Record value equality Nondestructive mutation ref returns

4Slices & Arrays0/15Unlock →

Arrays Contains Array.Reverse Index from the end Array.IndexOf Range operator Array.Sort Array.Copy Fill and Chunk List.GetRange Order by a key LINQ Take and Skip Array.BinarySearch Memory<T>Span<T>

5Data structures0/12Unlock →

Arrays List<T> basics Collection expressions Tuples Dictionary word count Iterate a Dictionary HashSet dedup Queue<T>Spread with [..]Stack<T>Nested collections Records as data

6Iterators0/20Unlock →

7Traits0/20Unlock →

8Generics0/20Unlock →

9Lifetimes0/20Unlock →

10Closures0/20Unlock →

11Pattern Matching0/20Unlock →

12Error Handling0/20Unlock →

13Option & Result0/20Unlock →

14Conversions0/15Unlock →

int.Parse bool to a number char and int Numeric cast String and char array Hex with a format string int.TryParse The Convert class Cast truncates a double Convert.ToString with a base BitConverter round-trip checked overflow Parse a span unchecked wrap Implement IParsable<T>

15Collection Conversions0/19Unlock →

Distinct Keys and values to lists Enumerable.Range To a SortedSet List to a Queue ToArray ToHashSet drops duplicates ToList Except Flatten with SelectMany Frequency map Intersect Range with Select ToDictionary Union Zip two lists to a map DistinctBy GroupBy Invert a dictionary

16Math0/14Unlock →

Math constants Float functions Integer arithmetic Max, Min, Clamp Rounding Logarithms & exp Trigonometry Decimal precision DivRem & non-negative modulo Factorial with BigInteger Greatest common divisor Primality test Mean and median Standard deviation

17Encoding0/22Unlock →

18Random0/21Unlock →

19Data I/O0/18Unlock →

Parse CSV into rows Tab-separated values Build a CSV string Select one column CSV into a number matrix Split off the header Serialize to JSON Whitespace-separated matrix Read a data file Skip blanks and comments Print an aligned table Write CSV to a file Append a row Navigate JSON with JsonNode Parse JSON into an object Parse key=value config Round-trip write then read Drop unparseable rows

20Linear Algebra0/21Unlock →

21Text Processing0/16Unlock →

Word count Count occurrences Replace text Reverse word order Strip all whitespace Count vowels Zero-pad a number Pad and align Capitalize the first letter Character frequency Text elements Longest word Number the lines Title case Truncate with an ellipsis Levenshtein distance

22Date & Time0/13Unlock →

Day of the week Leap year test Add and subtract dates Seconds to HH:MM:SS Time a block of work TimeSpan arithmetic TimeSpan units UNIX timestamp round trip UNIX timestamp now Break a TimeSpan into parts Parse a clock string Parse and format dates Time zone conversion

23Memory0/12Unlock →

Free early with Dispose Swap with ref Reset to default Replace and return old Size of a type Boxing and unboxing Class vs struct GC generations readonly struct ArrayPool<T>Span + stackalloc Weak references

24Modules0/20Unlock →

25Macros0/20Unlock →

26Testing0/20Unlock →

27Algorithms0/8Unlock →

Greatest common divisor Binary search Palindrome check Fibonacci with memoization Merge two sorted arrays Two sum Quicksort Fisher-Yates with an LCG

28Sorting Algorithms0/12Unlock →

Array.Sort & List.Sort Check if sorted OrderBy (LINQ)Bubble sort Sort by key and descending Insertion sort Selection sort Counting sort A custom IComparer Merge sort Heapsort Quicksort

29Expression Parsing0/6Unlock →

Evaluate postfix (RPN)Tokenize an expression Evaluate infix Recursive-descent calculator Right-associative power Shunting-yard

30Idioms0/9Unlock →

Expression-bodied members LINQ pipelines Null-coalescing idioms Tuples Records Switch expressions Target-typed new and collections using declarations Pattern matching

31OOP & Traits0/8Unlock →

Encapsulation Composition over inheritance Inheritance: virtual/override Primary constructor Records Static members Abstract class & sealed Interface polymorphism

32Trait Objects0/14Unlock →

Default method via interface Return an interface Take an interface parameter An array of interfaces A List of Func Generic vs interface dispatch Fixed vs varying sequence A List of interfaces Pattern-match dispatch Shared reference Interface inheritance Abstract base class Downcast with is A handler registry

33Design Patterns0/14Unlock →

Prototype Factory Method Adapter Builder Lazy Initialization Singleton Strategy Command Decorator Dependency Injection Observer Object Pool State Actor

34Async0/3Unlock →

async and await Task.WhenAll Cancellation and streams

35File I/O0/6Unlock →

Read a file Read all lines Write a file Append to a file Read lines with StreamReader Buffered writing with StreamWriter

36Filesystem0/18Unlock →

Does a path exist?Change the extension Build a path Copy a file Create nested directories Files vs directories List a directory Rename or move File metadata Path components Remove files and trees Sorted listing Write to the temp dir Filter by extension Absolute path Last modified time Total size of a tree Walk the tree

37Networking0/16Unlock →

Parse the request line Route by path Classify an IP address Parse a URL Resolve a hostname Bind an ephemeral port Build an HTTP response Read a line from a stream Shut down a connection TCP client Read timeout UDP receive UDP send A task per connection TCP echo server Host a web page

38Concurrency0/7Unlock →

Run work on a Task Shared state with lock Start a Thread Channel message passing Parallel.For ConcurrentDictionary Atomic increment

39Unsafe & FFI0/14Unlock →

An unsafe method Dereference a pointer Call a C function Export to native code Pin memory with fixed A function pointer Unmanaged memory Marshal a C string stackalloc a Span Marshal a struct MemoryMarshal.Cast Pointer arithmetic Reinterpret the bits A union with StructLayout

40Commands0/14Unlock →

Command-line arguments Skip the program name Env var with a default Read an environment variable Runtime info via AppContext Current directory Iterate all env vars Set and remove a variable Path to this executable Exit codes Run a command Pass env to a child Command exit status Read a line from stdin

41Array0/10Unlock →

Contains Duplicate Two Sum Best Time to Buy and Sell Stock Maximum Subarray Container With Most Water Find Minimum in Rotated Sorted Array Product of Array Except Self Maximum Product Subarray Search in Rotated Sorted Array 3Sum

42Binary0/5Unlock →

Missing Number Number of 1 Bits Counting Bits Reverse Bits Sum of Two Integers

43String0/10Unlock →

Valid Anagram Valid Palindrome Valid Parentheses Encode and Decode Strings Group Anagrams Longest Substring Without Repeating Characters Longest Repeating Character Replacement Longest Palindromic Substring Palindromic Substrings Minimum Window Substring

44Matrix0/4Unlock →

Rotate Image Set Matrix Zeroes Spiral Matrix Word Search

45Interval0/5Unlock →

Meeting Rooms Merge Intervals Insert Interval Non-overlapping Intervals Meeting Rooms II

46Linked List0/6Unlock →

Reverse Linked List Linked List Cycle Merge Two Sorted Lists Remove Nth Node From End Merge K Sorted Lists Reorder List

47Tree0/14Unlock →

Maximum Depth of Binary Tree Invert Binary Tree Same Tree Lowest Common Ancestor of a BST Binary Tree Level Order Traversal Subtree of Another Tree Implement Trie (Prefix Tree)Kth Smallest Element in a BST Validate Binary Search Tree Add and Search Word Construct Binary Tree from Preorder and Inorder Binary Tree Maximum Path Sum Serialize and Deserialize Binary Tree Word Search II

48Graph0/8Unlock →

Number of Connected Components Number of Islands Clone Graph Course Schedule Longest Consecutive Sequence Graph Valid Tree Alien Dictionary Pacific Atlantic Water Flow

49Heap0/3Unlock →

Kth Largest Element in an Array Top K Frequent Elements Find Median from Data Stream

50Dynamic Programming0/11Unlock →

Climbing Stairs House Robber Unique Paths Coin Change House Robber II Jump Game Combination Sum IV Decode Ways Longest Common Subsequence Longest Increasing Subsequence Word Break