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)
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