Enable nullable reference types

public static string FormatPayload(object input) => input switch
{
    null => "Empty",
    string s => $"Text: {s}",
    int i => $"Number: {i}",
    _ => "Unknown"
};

public static PricingStrategy DetermineStrategy(Order order) => order switch
{
    { Status: OrderStatus.Pending, TotalAmount: > 1000 } => PricingStrategy.Premium,
    { Status: OrderStatus.Pending, TotalAmount: <= 1000 } => PricingStrategy.Standard,
    { Status: OrderStatus.Cancelled } => PricingStrategy.None,
    _ => throw new InvalidOperationException("Unhandled order state")
};

public static string ClassifyScore(double score) => score switch
{
    >= 90 => "A",
    >= 80 => "B",
    >= 70 => "C",
    >= 60 => "D",
    < 60 => "F",
    _ => "Invalid"
};

. Requires record or init-only properties.

public record Point(double X, double Y);

public static string Describe(Point p) => p switch
{
    (0, 0) => "Origin",
    (var x, 0) => $"On X-axis at {x}",
    (0, var y) => $"On Y-axis at {y}",
    _ => $"Quadrant point ({p.X}, {p.Y})"
};

Rationale: Positional patterns leverage Deconstruct methods generated by records. They enable structural matching without exposing internal state. Use them when domain models represent data carriers rather than behavior-rich entities.

Step 5: Tuple Patterns

Match multiple independent values without creating wrapper objects.

public static string RouteRequest(string method, string endpoint) => (method, endpoint) switch
{
    ("GET", "/users") => "FetchUsers",
    ("POST", "/users") => "CreateUser",
    ("DELETE", "/users/{id}") => "DeleteUser",
    _ => "NotFound"
};

Architecture decision: Tuple patterns excel in routing, parsing, and state machine transitions. They avoid allocation overhead compared to creating DTOs for matching purposes. Pair them with switch expressions for pure mapping functions.

Step 6: when Clauses and Advanced Guarding

Use when only when pattern matching alone cannot express the condition. Keep guards side-effect-free.

public static bool IsEligible(User user) => user switch
{
    { Age: >= 18, Role: "admin" } => true,
    { Age: >= 18, Role: "user" } when user.IsVerified => true,
    _ => false
};

Rationale: when clauses compile to conditional branches evaluated after pattern matching. Overuse degrades readability and prevents compiler exhaustiveness analysis. Reserve them for cross-property validation or external state checks that cannot be encoded in the type structure.

Architecture Decisions and Rationale

1. Prefer switch expressions for pure transformations. They enforce value return on all branches, eliminating implicit null or uninitialized state.
2. Align pattern matching with immutable domain models. Positional and property patterns assume stable state. Mutable objects break pattern guarantees across evaluation cycles.
3. Isolate pattern routing in dedicated modules. Group related switches into static dispatch classes or extension methods. This prevents scattering matching logic across business layers.
4. Leverage compiler warnings. Enable <Nullable>enable</Nullable> and treat CS8509 (switch expression not exhaustive) as errors. The compiler is your primary validation layer.
5. Avoid mixing patterns with legacy casting in the same scope. It creates cognitive dissonance and obscures intent. Migrate incrementally: replace is/as blocks with switch expressions, then introduce property/relational patterns.

Pitfall Guide

1. Overusing positional patterns on mutable classes Positional patterns rely on Deconstruct or primary constructors. Applying them to mutable reference types breaks encapsulation and creates stale match states. Restrict positional patterns to record types or value types with deterministic structure.

2. Ignoring exhaustiveness warnings in switch expressions Switch expressions require every possible input to map to a value. Suppressing CS8509 or adding _ => null masks missing domain cases. Instead, model the full state space or throw InvalidOperationException with explicit context.

3. Mixing is patterns with as casting in the same block Combining if (x is Type t) with var y = x as Type in adjacent logic creates redundant type checks and obscures control flow. Standardize on one approach per module. Pattern matching should replace, not coexist with, legacy casting.

4. Neglecting null patterns Forgetting null or not null patterns causes NullReferenceException when matching reference types. Always handle null explicitly or use not null to restrict the match scope. C# 11+ treats null as a first-class pattern; leverage it.

5. Deep property patterns on large object graphs Nested property patterns ({ Address: { City: "NY", Zip: "10001" } }) compile to sequential property accesses. On large graphs, this increases stack depth and prevents JIT inlining. Flatten matching by extracting relevant values before the switch or using intermediate variables.

6. Misunderstanding pattern precedence Patterns evaluate top-to-bottom. A broad pattern placed early will shadow specific cases. Always order from most specific to most general. The compiler warns on unreachable branches, but manual review is still required for complex relational ranges.

7. Using when clauses for core routing logic when clauses bypass compiler exhaustiveness analysis and execute after pattern matching. Heavy reliance on when degrades readability and prevents static verification. Encode conditions into the type structure or split into multiple switch expressions.

Production Best Practices:

• Enable <AnalysisLevel>latest</AnalysisLevel> and treat pattern-related warnings as errors.
• Use records for all data carriers involved in pattern matching.
• Keep switch expressions under 15 branches; split complex routing into dedicated methods.
• Profile hot-path matching with BenchmarkDotNet to validate JIT optimization.
• Document pattern contracts in XML comments: specify expected types, null behavior, and fallback semantics.

Production Bundle

Action Checklist

• Audit existing is/as casting blocks and replace with switch expressions where a value is returned
• Enable <Nullable>enable</Nullable> and configure CS8509 as an error in .editorconfig
• Convert data-transfer objects to record types to unlock positional and property patterns
• Order switch branches from most specific to most general; verify compiler exhaustiveness
• Replace nested if-else type checks with relational and property patterns
• Isolate pattern routing logic into static dispatch classes or extension methods
• Profile hot-path matching with BenchmarkDotNet to validate performance gains
• Document pattern contracts: expected types, null handling, and fallback behavior

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
Routing domain types to handlers	Switch expression with type patterns	Compiler-enforced exhaustiveness, zero allocation	Low (refactoring time)
Validating nested DTO state	Property patterns	Eliminates casting, keeps validation co-located	Medium (model alignment)
Comparing scalar ranges	Relational patterns	Branchless JIT optimization, readable syntax	Low
Matching multiple independent values	Tuple patterns	Avoids wrapper allocation, clear intent	Low
Complex cross-field validation	when clauses (minimal)	Preserves pattern structure, guards edge cases	Medium (readability trade-off)
Legacy mutable domain models	Type patterns + explicit casting	Maintains compatibility while adopting syntax	High (migration required)

Configuration Template

Copy into .editorconfig to enforce pattern matching standards across the solution:

[*.cs]
# Enable nullable reference types
dotnet_style_require_accessibility_modifiers = always
nullable = enable

# Treat pattern matching warnings as errors
dotnet_diagnostic.CS8509.severity = error
dotnet_diagnostic.CS8524.severity = error
dotnet_diagnostic.CS8600.severity = warning

# Prefer switch expressions over statements for value mapping
csharp_style_pattern_matching_over_is_with_cast_check = true:suggestion
csharp_style_prefer_switch_expression = true:suggestion

# Enforce modern C# version
dotnet_code_style_unused_parameters = all:suggestion

Add to .csproj for compiler optimization:

<PropertyGroup>
  <TargetFramework>net8.0</TargetFramework>
  <Nullable>enable</Nullable>
  <ImplicitUsings>enable</ImplicitUsings>
  <AnalysisLevel>latest</AnalysisLevel>
  <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>

Quick Start Guide

1. Upgrade toolchain: Ensure Visual Studio 2022 (17.8+) or VS Code with C# Dev Kit. Set project target to .NET 8 or higher.
2. Enable nullable and analysis: Add <Nullable>enable</Nullable> and <AnalysisLevel>latest</AnalysisLevel> to your .csproj. Apply the .editorconfig template above.
3. Convert a casting block: Replace an existing if (x is Type t) or var y = x as Type block with a switch expression. Verify CS8509 is resolved.
4. Introduce property/relational patterns: Refactor nested if conditions into property or relational matches. Order branches specific-to-general.
5. Profile and validate: Run dotnet build to confirm zero pattern warnings. Execute unit tests covering null, edge, and fallback cases. Benchmark hot paths if routing logic is performance-critical.