Modern C#

.NET 8+, nullable enabled, async-first, records for data.

Async Patterns

Always use async/await for I/O. Always pass CancellationToken :

public async Task<User?> GetUserAsync( int id, CancellationToken cancellationToken = default) { using var connection = await _factory .CreateConnectionAsync(cancellationToken) .ConfigureAwait(false);

return await connection
    .QuerySingleOrDefaultAsync<User>(sql, cancellationToken: cancellationToken)
    .ConfigureAwait(false);

}

ConfigureAwait(false) in library code. Never block on async (.Result , .Wait() ).

Nullable Reference Types

Enable project-wide. Treat warnings as errors:

<Nullable>enable</Nullable> <TreatWarningsAsErrors>true</TreatWarningsAsErrors>

Explicit nullability:

// Nullable when user might not exist Task<User?> GetByIdAsync(int id);

// Non-nullable with exception on not found Task<User> GetRequiredByIdAsync(int id);

// Handle nulls explicitly if (user is not null) { ProcessUser(user); } var name = user?.Name ?? "Anonymous";

Records & Immutability

Records for DTOs and value types:

// DTO public record CreateOrderRequest( string CustomerId, IReadOnlyList<OrderItemDto> Items);

// Domain entity public record class Order { public string Id { get; init; } public OrderStatus Status { get; init; }

public Order Ship() => this with { Status = OrderStatus.Shipped };

}

// Value type (<16 bytes) public readonly record struct Money(decimal Amount, string Currency);

Never expose mutable collections. Use IReadOnlyList<T> .

Pattern Matching

Switch expressions over if-else chains:

public decimal CalculateDiscount(object discount) => discount switch { decimal amount => amount, int percentage => percentage / 100m, string code => GetDiscountForCode(code), _ => throw new ArgumentException("Unsupported type") };

public string GetShipping(Order order) => order switch { { TotalAmount: > 100, Customer.IsPremium: true } => "Free Express", { TotalAmount: > 100 } => "Free Standard", _ => "Standard" };

Project Setup

<!-- Directory.Build.props --> <PropertyGroup> <TargetFramework>net8.0</TargetFramework> <LangVersion>12.0</LangVersion> <Nullable>enable</Nullable> <TreatWarningsAsErrors>true</TreatWarningsAsErrors> <EnableNETAnalyzers>true</EnableNETAnalyzers> <AnalysisLevel>latest-recommended</AnalysisLevel> </PropertyGroup>

Anti-Patterns

• Blocking on async (.Result , .Wait() )

• async void outside event handlers

• Missing ConfigureAwait(false) in libraries

• null! without documented justification

• Mutable DTOs with public setters

• Switch statements over switch expressions

• Legacy .csproj or packages.config