Source Generators and Compile-Time Code Generation

Source generators are C# compiler plugins that run during compilation and generate additional C# source files. They enable compile-time code generation that eliminates runtime reflection costs. Instead of a JSON serialiser discovering properties at runtime through reflection, a source generator can generate the serialisation code at compile time — the properties are read directly, no reflection required. ASP.NET Core and .NET 8 use source generators extensively: for JSON serialisation, regex compilation, logging, and dependency injection. Understanding how to consume source generators (even without writing your own) is increasingly important for modern .NET development.

JsonSerializerContext — Compile-Time JSON #

// ── Without source generation — runtime reflection ─────────────────────────
string json = JsonSerializer.Serialize(post);   // uses reflection to find properties

// ── With source generation — compile-time code ────────────────────────────
// Define a JsonSerializerContext that enumerates the types to support
[JsonSerializable(typeof(Post))]
[JsonSerializable(typeof(List<Post>))]
[JsonSerializable(typeof(PostDto))]
[JsonSerializable(typeof(CreatePostRequest))]
public partial class BlogJsonContext : JsonSerializerContext { }

// Program.cs — register the context with ASP.NET Core
builder.Services
    .AddControllers()
    .AddJsonOptions(opts =>
        opts.JsonSerializerOptions.TypeInfoResolver = BlogJsonContext.Default);

// Or directly in serialisation code:
string json2 = JsonSerializer.Serialize(post, BlogJsonContext.Default.Post);
Post? post2   = JsonSerializer.Deserialize(json2, BlogJsonContext.Default.Post);

// Benefits:
// - No reflection at runtime — property access is direct compiled code
// - AOT (Ahead-of-Time) compilation compatible
// - Trimming-safe (unused code can be stripped by the linker)
// - Faster cold-start performance

Regex Source Generator (C# 11 / .NET 7+) #

// ── Traditional regex — compiled at runtime ────────────────────────────────
private static readonly Regex EmailRegex = new(
    @"^[^@\s]+@[^@\s]+\.[^@\s]+$",
    RegexOptions.Compiled | RegexOptions.IgnoreCase);

// ── Source-generated regex — compiled at build time ────────────────────────
// Generates a NFA/DFA state machine at compile time — fastest possible matching
public partial class Validators
{
    [GeneratedRegex(@"^[^@\s]+@[^@\s]+\.[^@\s]+$", RegexOptions.IgnoreCase)]
    private static partial Regex EmailRegex();
}

bool isValid = Validators.EmailRegex().IsMatch("alice@example.com");   // no compile cost

LoggerMessage Source Generator #

// ── Traditional logging — boxes parameters, string interpolation overhead ───
_logger.LogInformation("User {UserId} logged in from {IpAddress}", userId, ipAddress);

// ── Source-generated LoggerMessage — zero allocation, maximum performance ───
public static partial class LogMessages
{
    [LoggerMessage(Level = LogLevel.Information, Message = "User {UserId} logged in from {IpAddress}")]
    public static partial void UserLoggedIn(ILogger logger, string userId, string ipAddress);
}

// Call — no boxing, no string allocation, no conditional check on IsEnabled:
LogMessages.UserLoggedIn(_logger, user.Id, request.RemoteIpAddress?.ToString() ?? "unknown");
// The source generator emits: if (!logger.IsEnabled(LogLevel.Information)) return;

Common Mistakes #

Mistake 1 — Forgetting partial keyword on source-generated class #

❌ Wrong — compile error: cannot extend a non-partial class:

public class BlogJsonContext : JsonSerializerContext { }   // missing partial!

✅ Correct:

public partial class BlogJsonContext : JsonSerializerContext { }   // ✓

Mistake 2 — Not including all serialised types in JsonSerializerContext #

❌ Wrong — serialising a type not registered in the context falls back to reflection silently.

✅ Correct — add a [JsonSerializable(typeof(T))] for every type you serialise, including collection variants like List<PostDto>.