AGIBuild/dotnet-csharp-modern-patterns

dotnet-csharp-modern-patterns

Modern C# language feature guidance adapted to the project's target framework. Always run [skill:dotnet-version-detection] first to determine TFM and C# version.

Cross-references: [skill:dotnet-csharp-coding-standards] for naming/style conventions, [skill:dotnet-csharp-async-patterns] for async-specific patterns.

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Quick Reference: TFM to C# Version

| TFM | C# | Key Language Features | |-----|----|-----------------------| | net8.0 | 12 | Primary constructors, collection expressions, alias any type | | net9.0 | 13 | params collections, Lock type, partial properties | | net10.0 | 14 | field keyword, extension blocks, nameof unbound generics | | net11.0 | 15 (preview) | Collection expression with() arguments |

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Records

Use records for immutable data transfer objects, value semantics, and domain modeling where equality is based on values rather than identity.

Record Classes (reference type)

// Positional record: concise, immutable, value equality
public record OrderSummary(int OrderId, decimal Total, DateOnly OrderDate);

// With additional members
public record Customer(string Name, string Email)
{
    public string DisplayName => $"{Name} <{Email}>";
}

Record Structs (value type, C# 10+)

// Positional record struct: value type with value semantics
public readonly record struct Point(double X, double Y);

// Mutable record struct (rare -- prefer readonly)
public record struct MutablePoint(double X, double Y);

When to Use Records vs Classes

| Use Case | Prefer | |----------|--------| | DTOs, API responses | record | | Domain value objects (Money, Email) | readonly record struct | | Entities with identity (User, Order) | class | | High-throughput, small data | readonly record struct | | Inheritance needed | record (class-based) |

Non-destructive Mutation

var updated = order with { Total = order.Total + tax };

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Primary Constructors (C# 12+, net8.0+)

Capture constructor parameters directly in the class/struct body. Parameters become available throughout the type but are not fields or properties -- they are captured state.

For Services (DI injection)

public class OrderService(IOrderRepository repo, ILogger<OrderService> logger)
{
    public async Task<Order> GetAsync(int id)
    {
        logger.LogInformation("Fetching order {OrderId}", id);
        return await repo.GetByIdAsync(id);
    }
}

Gotchas

• Primary constructor parameters are mutable captures, not readonly fields. If immutability matters, assign to a readonly field in the body.
• Do not use primary constructors when you need to validate parameters at construction time -- use a traditional constructor with guard clauses instead.
• For records, positional parameters become public properties automatically. For classes/structs, they remain private captures.

// Explicit readonly field when immutability matters
public class Config(string connectionString)
{
    private readonly string _connectionString = connectionString
        ?? throw new ArgumentNullException(nameof(connectionString));
}

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Collection Expressions (C# 12+, net8.0+)

Unified syntax for creating collections with [...].

// Array
int[] numbers = [1, 2, 3];

// List
List<string> names = ["Alice", "Bob"];

// Span
ReadOnlySpan<byte> bytes = [0x00, 0xFF];

// Spread operator
int[] combined = [..first, ..second, 99];

// Empty collection
List<int> empty = [];

Collection Expression with Arguments (C# 15 preview, net11.0+)

Specify capacity, comparers, or other constructor arguments:

// Capacity hint
List<int> nums = [with(capacity: 1000), ..Generate()];

// Custom comparer
HashSet<string> set = [with(comparer: StringComparer.OrdinalIgnoreCase), "Alice", "bob"];

// Dictionary with comparer
Dictionary<string, int> map = [with(comparer: StringComparer.OrdinalIgnoreCase),
    new("key1", 1), new("key2", 2)];

net11.0+ only. Requires <LangVersion>preview</LangVersion>. Do not use on earlier TFMs.

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Pattern Matching

Switch Expressions (C# 8+)

string GetDiscount(Customer customer) => customer switch
{
    { Tier: "Gold", YearsActive: > 5 } => "30%",
    { Tier: "Gold" } => "20%",
    { Tier: "Silver" } => "10%",
    _ => "0%"
};

List Patterns (C# 11+)

bool IsValid(int[] data) => data is [> 0, .., > 0]; // first and last positive

string Describe(int[] values) => values switch
{
    [] => "empty",
    [var single] => $"single: {single}",
    [var first, .., var last] => $"range: {first}..{last}"
};

Type and Property Patterns

decimal CalculateShipping(object package) => package switch
{
    Letter { Weight: < 50 } => 0.50m,
    Parcel { Weight: var w } when w < 1000 => 5.00m + w * 0.01m,
    Parcel { IsOversized: true } => 25.00m,
    _ => 10.00m
};

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required Members (C# 11+)

Force callers to initialize properties at construction via object initializers.

public class UserDto
{
    public required string Name { get; init; }
    public required string Email { get; init; }
    public string? Phone { get; init; }
}

// Compiler enforces Name and Email
var user = new UserDto { Name = "Alice", Email = "[email protected]" };

Useful for DTOs that need to be deserialized (System.Text.Json honors required in .NET 8+).

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field Keyword (C# 14, net10.0+)

Access the compiler-generated backing field directly in property accessors.

public class TemperatureSensor
{
    public double Reading
    {
        get => field;
        set => field = value >= -273.15
            ? value
            : throw new ArgumentOutOfRangeException(nameof(value));
    }
}

Replaces the manual pattern of declaring a private field plus a property with custom logic. Use when you need validation or transformation in a setter without a separate backing field.

net10.0+ only. On earlier TFMs, use a traditional private field.

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Extension Blocks (C# 14, net10.0+)

Group extension members for a type in a single block.

public static class EnumerableExtensions
{
    extension<T>(IEnumerable<T> source) where T : class
    {
        public IEnumerable<T> WhereNotNull()
            => source.Where(x => x is not null);

        public bool IsEmpty()
            => !source.Any();
    }
}

net10.0+ only. On earlier TFMs, use traditional static extension methods.

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Alias Any Type (using, C# 12+, net8.0+)

using Point = (double X, double Y);
using UserId = System.Guid;

Point origin = (0, 0);
UserId id = UserId.NewGuid();

Useful for tuple aliases and domain type aliases without creating a full type.

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params Collections (C# 13, net9.0+)

params now supports additional collection types beyond arrays, including Span<T>, ReadOnlySpan<T>, and types implementing certain collection interfaces.

public void Log(params ReadOnlySpan<string> messages)
{
    foreach (var msg in messages)
        Console.WriteLine(msg);
}

// Callers: compiler may avoid heap allocation with span-based params
Log("hello", "world");

net9.0+ only. On net8.0, params only supports arrays.

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Lock Type (C# 13, net9.0+)

Use System.Threading.Lock instead of object for locking.

private readonly Lock _lock = new();

public void DoWork()
{
    lock (_lock)
    {
        // thread-safe operation
    }
}

Lock provides a Scope-based API for advanced scenarios and is more expressive than lock (object).

net9.0+ only. On net8.0, use private readonly object _gate = new(); and lock (_gate).

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Partial Properties (C# 13, net9.0+)

Partial properties enable source generators to define property signatures that users implement, or vice versa.

// In generated file
public partial class ViewModel
{
    public partial string Name { get; set; }
}

// In user file
public partial class ViewModel
{
    private string _name = "";
    public partial string Name
    {
        get => _name;
        set => SetProperty(ref _name, value);
    }
}

net9.0+ only. See [skill:dotnet-csharp-source-generators] for generator patterns.

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nameof for Unbound Generic Types (C# 14, net10.0+)

string name = nameof(List<>);      // "List"
string name2 = nameof(Dictionary<,>); // "Dictionary"

Useful in logging, diagnostics, and reflection scenarios.

net10.0+ only.

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Polyfill Guidance for Multi-Targeting

When targeting multiple TFMs, newer language features may not compile on older targets. Use these approaches:

1. PolySharp -- Polyfills compiler-required types (IsExternalInit, RequiredMemberAttribute, etc.) so language features like init, required, and record work on older TFMs.
2. Polyfill -- Polyfills runtime APIs (e.g., string.Contains(char) for netstandard2.0).
3. Conditional compilation -- Use #if for features that cannot be polyfilled:

#if NET10_0_OR_GREATER
    // Use field keyword
    public double Value { get => field; set => field = Math.Max(0, value); }
#else
    private double _value;
    public double Value { get => _value; set => _value = Math.Max(0, value); }
#endif

See [skill:dotnet-multi-targeting] for comprehensive polyfill guidance.

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Knowledge Sources

Feature guidance in this skill is grounded in publicly available language design rationale from:

• C# Language Design Notes (Mads Torgersen et al.) -- Design decisions behind each C# version's features. Key rationale relevant to this skill: primary constructors (reducing boilerplate for DI-heavy services), collection expressions (unifying collection initialization syntax), field keyword (eliminating backing field ceremony), and extension blocks (grouping extensions by target type). Each feature balances expressiveness with safety -- e.g., primary constructor parameters are intentionally mutable captures (not readonly) to keep the feature simple; use explicit readonly fields when immutability is needed. Source: https://github.com/dotnet/csharplang/tree/main/meetings
• C# Language Proposals Repository -- Detailed specifications and design rationale for accepted and proposed features. Source: https://github.com/dotnet/csharplang/tree/main/proposals

Note: This skill applies publicly documented design rationale. It does not represent or speak for the named sources.

References

• C# Language Reference
• What's new in C# 12
• What's new in C# 13
• What's new in C# 14
• C# Language Design Notes
• .NET Framework Design Guidelines