Pattern Matching Deep Dive — is, switch Expressions and Patterns

Pattern matching in C# lets you test a value against a shape, type, or condition and bind variables in the same expression. It replaces verbose if (x is SomeType t) { ... } else if (x is OtherType o) { ... } chains with expressive, compiler-verified switch expressions. Since C# 7, patterns have expanded with every release — C# 9 added relational and logical patterns, C# 10 added extended property patterns, C# 11 added list patterns. The result is a rich vocabulary for expressing complex matching logic that reads almost like a specification.

Pattern Types Reference #

object value = "Hello, World!";

// ── Type pattern — tests the runtime type ─────────────────────────────────
if (value is string s)
    Console.WriteLine($"String of length {s.Length}");

// ── Constant pattern — tests equality ─────────────────────────────────────
if (value is "Hello, World!") Console.WriteLine("Exact match");
if (value is null)            Console.WriteLine("Is null");
if (value is not null)        Console.WriteLine("Is not null");

// ── Relational patterns — numeric comparison ───────────────────────────────
int score = 85;
string grade = score switch
{
    >= 90           => "A",
    >= 80 and < 90  => "B",
    >= 70 and < 80  => "C",
    >= 60           => "D",
    _               => "F",
};

// ── Logical patterns — and, or, not ────────────────────────────────────────
bool isWeekend = DateTime.Now.DayOfWeek switch
{
    DayOfWeek.Saturday or DayOfWeek.Sunday => true,
    _                                       => false,
};

// ── Property pattern — match on object properties ─────────────────────────
var user = new User { Role = "Admin", IsActive = true };
string access = user switch
{
    { Role: "Admin",  IsActive: true  } => "Full access",
    { Role: "Editor", IsActive: true  } => "Edit access",
    { IsActive: false }                  => "Account inactive",
    _                                    => "Read only",
};

// ── Extended property pattern (C# 10) — nested properties ─────────────────
string region = order switch
{
    { ShippingAddress.Country: "US", ShippingAddress.State: "CA" } => "California",
    { ShippingAddress.Country: "US" }                              => "United States",
    { ShippingAddress.Country: "UK" }                              => "United Kingdom",
    _                                                               => "International",
};

Positional and List Patterns #

// ── Positional pattern — deconstruct and match ────────────────────────────
// Works with records, tuples, and types with Deconstruct methods
record Point(int X, int Y);

string quadrant = new Point(3, -2) switch
{
    (> 0, > 0) => "Q1",    // both positive
    (< 0, > 0) => "Q2",    // x negative, y positive
    (< 0, < 0) => "Q3",    // both negative
    (> 0, < 0) => "Q4",    // x positive, y negative
    (0, _)     => "Y-axis",
    (_, 0)     => "X-axis",
    _          => "Origin",
};

// ── List patterns (C# 11) — match on collection structure ─────────────────
int[] numbers = { 1, 2, 3, 4, 5 };

string description = numbers switch
{
    []              => "empty",
    [var single]    => $"one element: {single}",
    [var first, .., var last] => $"starts with {first}, ends with {last}",
};
// "starts with 1, ends with 5"

// Match specific element values
bool isPrimePair = numbers switch
{
    [2, 3, ..] => true,    // starts with 2 and 3
    _          => false,
};

// ── Guard clause with when ─────────────────────────────────────────────────
string classify = score switch
{
    var s when s >= 90 && s <= 100 => "Excellent",
    var s when s >= 70              => "Good",
    var s when s >= 0               => "Poor",
    _                               => throw new ArgumentOutOfRangeException(),
};

Common Mistakes #

Mistake 1 — Putting less-specific patterns before more-specific ones (unreachable arms) #

❌ Wrong — discard arm matches everything, specific arms are never reached:

string result = user switch
{
    _                               => "default",   // matches everything!
    { Role: "Admin", IsActive: true } => "Admin",   // unreachable!
};

✅ Correct — most specific first, discard last.

Mistake 2 — Missing the discard arm in non-exhaustive switches (throws at runtime) #

❌ Wrong — SwitchExpressionException if no arm matches:

string result = status switch { 200 => "OK", 404 => "Not Found" };  // what about 500?

✅ Correct — always include a final _ => ... arm for safety.