csharp-modern skill

---
name: csharp-modern
description: |
  Modern C# development with .NET 8+, async patterns, and records. Use when:
  - Writing or reviewing C# code
  - Configuring async/await with ConfigureAwait
  - Using nullable reference types
  - Implementing pattern matching
  - Setting up .NET projects
  Keywords: C#, .NET, async, await, ConfigureAwait, nullable, record,
  pattern matching, xUnit, ValueTask
effort: high
---

# Modern C#

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

## Async Patterns

**Always use `async/await` for I/O. Always pass `CancellationToken`:**
```csharp
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:**
```xml
<Nullable>enable</Nullable>
<TreatWarningsAsErrors>true</TreatWarningsAsErrors>
```

**Explicit nullability:**
```csharp
// 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:**
```csharp
// 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:**
```csharp
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

```xml
<!-- 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