csharp-conventions

C# Conventions (stack-agnostic, C# 10+ / .NET 6+)

This skill encodes idioms that reduce bugs and improve readability in any C# codebase. Apply alongside the active framework plugin's conventions skill (e.g., aspnet-core-plugin:aspnet-conventions).

Detection

Project is C# 10+ / .NET 6+ when:

• .csproj has <TargetFramework>net6.0</TargetFramework> or higher (net7.0, net8.0, net9.0, net10.0).
• global.json pins sdk.version to 6.0.x or higher.

Read the .csproj <TargetFramework> before making any version-specific decisions.

Nullable reference types — eliminate null surprises

Enable in every project. New projects get it by default from dotnet new; older projects need a one-time migration.

<!-- .csproj: enable project-wide -->
<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

// Non-nullable: compiler guarantees non-null, no null check needed
public string Name { get; }

// Nullable: caller must check before dereferencing
public string? MiddleName { get; }

// Null-forgiving operator — use only when you have proven non-null
var definitelySet = _cache[key]!;

// Null-conditional + null-coalescing
string display = user?.FullName ?? "Guest";

Never silence nullable warnings with ! without a comment explaining why the value is guaranteed non-null. Prefer redesigning the API to avoid the need.

Records — prefer for value objects and DTOs

Use record (class) for immutable reference-type value objects; readonly record struct for small value types.

// Immutable DTO — all positional parameters become init-only properties
public record Money(decimal Amount, string Currency)
{
    // Compact validation in the record body
    public Money
    {
        if (Amount < 0) throw new ArgumentOutOfRangeException(nameof(Amount), "Amount must be non-negative.");
        ArgumentException.ThrowIfNullOrWhiteSpace(Currency);
    }

    public Money Add(Money other)
    {
        if (Currency != other.Currency) throw new InvalidOperationException("Currency mismatch.");
        return this with { Amount = Amount + other.Amount };
    }
}

// Small stack-allocated value type
public readonly record struct Point(double X, double Y);