Phase 4 of 30 · Topic 4.1

Type Patterns & Declaration Patterns (`is` vs `as`)

1Concept

Pattern matching `if (obj is TargetType t)` combines type inspection and variable assignment into a single atomic operation, completely obsoleting unsafe `as` casting followed by null checks.

2Architecture Diagram

Legacy:
Object ──> (as Target) ──> Check != null ──> Use target

Modern C# Pattern:
Object ──> `is Target t` ──> Single IL isinst Opcode + Immediate Scope Assignment

3Code Example

C# 13 & .NET 9
using System;

public abstract record PaymentMethod;
public record CreditCard(string CardNumber, decimal Limit) : PaymentMethod;
public record BankTransfer(string Iban, string Swift) : PaymentMethod;

public class TypePatternDemo
{
    public static void ProcessPayment(PaymentMethod payment)
    {
        if (payment is CreditCard { Limit: > 1000 } cc)
        {
            Console.WriteLine($"Approved high-limit Credit Card: {cc.CardNumber[^4..]}");
        }
        else if (payment is BankTransfer bt)
        {
            Console.WriteLine($"Bank Transfer via IBAN: {bt.Iban}");
        }
    }

    public static void Main()
    {
        ProcessPayment(new CreditCard("4111222233334444", 5000m));
        ProcessPayment(new BankTransfer("DE89370400440532013000", "DEUTDEDB"));
    }
}

4Expected Output

Approved high-limit Credit Card: 4444
Bank Transfer via IBAN: DE89370400440532013000

5Key Takeaways

  • ✓`is` with declaration patterns prevents double casting in IL bytecode.
  • ✓Property sub-patterns `{ Limit: > 1000 }` enable recursive deep evaluation.
  • ✓Combine with `not` patterns (e.g. `if (input is not null)`) for idiomatic C#.