Comptime

A .NET source generator that executes methods at compile time and serializes their results to C# code. Comptime brings meta-programming capabilities to C#, enabling compile-time code generation and evaluation.

Overview

Comptime allows you to mark methods with the [Comptime] attribute to have them executed during compilation. The return values are serialized into C# source code and used at runtime, eliminating the need for runtime computation of values that can be determined at build time.

This meta-programming approach enables developers to shift expensive computations from runtime to compile time, resulting in faster application startup and execution.

Features

• Compile-time execution: Methods marked with [Comptime] are executed during compilation
• Method parameters: Methods can accept parameters with compile-time constant expressions
• C# serialization: Results are serialized to valid C# code
• Supported return types:
  • Primitive types: int, long, short, byte, sbyte, uint, ulong, ushort, float, double, decimal, bool, char, string
  • Collections: IReadOnlyList<T>, IReadOnlyDictionary<TKey, TValue>, List<T>, Dictionary<TKey, TValue>
  • Note: Arrays are not allowed as return types because they are mutable. Use IReadOnlyList<T> instead.
• Supported argument types: Any expression that doesn't contain variables, including:
  • Literals: 42, "hello", true
  • Collection initializers: new List<int> { 1, 2, 3 }, new[] { "a", "b", "c" }
  • Expressions: 1 + 2, Math.PI * 2
  • Const values and enum members
• Interceptor-based: Uses C# interceptors to replace method calls with pre-computed values

Usage

Basic Usage (Parameterless Methods)

using Comptime;

public static partial class Constants
{
    [Comptime]
    public static IReadOnlyList<int> GetPrimeNumbers()
    {
        // Complex computation that runs at compile time
        var primes = new List<int>();
        for (int i = 2; i <= 100; i++)
        {
            if (IsPrime(i))
                primes.Add(i);
        }
        return primes;
    }
    
    private static bool IsPrime(int n) { /* ... */ }
}

// At runtime, calling GetPrimeNumbers() returns the pre-computed list
var primes = Constants.GetPrimeNumbers(); // Returns [2, 3, 5, 7, 11, ...]

Methods with Parameters

using Comptime;

public static partial class Math
{
    [Comptime]
    public static long Factorial(int n)
    {
        if (n <= 1) return 1;
        long result = 1;
        for (int i = 2; i <= n; i++)
            result *= i;
        return result;
    }

    [Comptime]
    public static int SumList(IReadOnlyList<int> numbers)
    {
        return numbers.Sum();
    }
}

// Each unique argument combination is computed at compile time
var fact5 = Math.Factorial(5);   // Pre-computed: 120
var fact10 = Math.Factorial(10); // Pre-computed: 3628800

// Collection initializers work too!
var sum = Math.SumList(new List<int> { 1, 2, 3, 4, 5 }); // Pre-computed: 15
var sum2 = Math.SumList(new[] { 10, 20, 30 });           // Pre-computed: 60

Generic Methods

using Comptime;

public static partial class Utils
{
    [Comptime]
    public static int CountItems<T>(IReadOnlyList<T> items)
    {
        return items.Count;
    }

    [Comptime]
    public static string JoinStrings(IReadOnlyList<string> strings, string separator)
    {
        return string.Join(separator, strings);
    }
}

var count = Utils.CountItems(new[] { "a", "b", "c" }); // Pre-computed: 3
var joined = Utils.JoinStrings(new[] { "hello", "world" }, " "); // Pre-computed: "hello world"

Requirements

• .NET 8.0 or later
• C# 12 or later (for interceptors support)

Installation

<PackageReference Include="Comptime" Version="1.0.0" />

How It Works

1. The source generator finds methods marked with [Comptime]
2. It identifies all call sites and their arguments
3. For each unique argument combination, it executes the method at compile time
4. The return values are serialized to C# literals/expressions
5. Interceptor methods are generated that return the pre-computed values
6. At runtime, calls to the original methods are intercepted and return the cached values

Diagnostics

| Code | Description | |------|-------------| | COMPTIME001 | Class must be partial | | COMPTIME002 | Method must be static | | COMPTIME004 | Unsupported return type | | COMPTIME005 | Compilation emit failed | | COMPTIME006 | Method execution failed | | COMPTIME007 | Serialization failed | | COMPTIME011 | Array return type not allowed (use IReadOnlyList<T>) | | COMPTIME012 | Argument must be a constant (no variables allowed) |

Limitations

• Methods must be static
• The containing class must be partial
• Return types must be immutable (arrays are not allowed, use IReadOnlyList<T>)
• Method arguments must be compile-time constant expressions (no variables, only literals and expressions of literals)
• Methods cannot have side effects that depend on runtime state

License

MIT License