Comptime – C# meta-programming with compile-time code generation and evaluation

Comptime – C# meta-programming with compile-time code generation and evaluation(github.com)

150 points by bj-rn 149 days ago | 66 comments

I started using C# recently for a hobby project writing a game engine on top of Monogame, and I have been very surprised at how nice of a language C# is to use.

It has a clean syntax, decent package management, and basically every language feature I regularly reach for except algebraic data types, which are probably coming eventually.

I think the association of .NET to Microsoft tarnished my expectations.

andix 144 days ago | |

Modern C# and .NET are great. It still suffers from the bad reputation of the Windows-only .NET Framework. It's still a quite heavy platform with a lot of features, but the .NET team invested a lot of time to make it more approachable recently.

With top level Programs and file-based apps[1] it can be used as a scripting language now, just add a shebang (#!/usr/local/share/dotnet/dotnet run) to the first line and make it executable. It will still compile to a temporary file (slow on first run), but it doesn't require a build step anymore for smaller scripts.

[1]: https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals...

actionfromafar 144 days ago | | |

Also, if you compile Ahead Of Time (AOT) you can cut down on the features and get basically as small a subset as you want of the libraries. IMHO C# and dotnet are really starting to become very impressive.

thdrtol 144 days ago | |

I love C# and .NET and use it every day.

Sometimes I wonder if the .NET department is totally separated from the rest of Microsoft. Microsoft is so bad on all fronts I stopped using everything that has to do with it. Windows, Xbox, the Microsoft account experience, the Microsoft store, for me it has been one big trip of frustration.

andix 144 days ago | | |

Microsoft is huge, it's many companies inside one company.

.NET seems to be somewhere close to Azure, but now far away from Windows or the business applications (Office/Teams, Dynamics, Power Platform). Things like GitHub, LinkedIn or Xbox seem to be de facto separate companies.

Edit: .NET used to be tied closely to Windows, which gave it the horrible reputation. The dark age of .NET ;)

vips7L 143 days ago | |

Unions have a proposal: https://github.com/dotnet/csharplang/issues/9662

jauntywundrkind 144 days ago |

Makes me think of Boo language; Boo was so good at metaprogramming and multi-phasr programming! A very fine .NET language that was so far ahead of the curve, with having the tools of that language be usable at runtime.

Alas many of the docs are offline now. But it had great quasiquotes, which let you write code that gets turned into AST that you can then process. Good macros. A programmable compiler pipeline. So much. Alas, obscured now. https://boo-language.github.io/

daeken 144 days ago | |

Boo and Nemerle both were really showing what was possible in .NET back in the early days. I still miss the metaprogramming they had, not to mention their pattern matching (which C# has closed the gap on, but is still way, way short.)

andix 144 days ago | |

The syntax looks a bit like F#. But F# only has a few features that generous people might consider meta programming.

There was a lot of fuss about meta programming around 10-15 years ago, but it never got a lot of traction. Maybe for a good reason? I think a lot of the problems it solved, were also solved by functional programming features that slowly appeared in C# over the years.

CharlieDigital 144 days ago |

C# meta programming game is strong. Source generators are :chefs_kiss:

daeken 144 days ago | |

I love (and heavily use) source generators, but the development experience is godawful. Working with the raw Roslyn types is painful at best and this is compounded by them having to be written against .NET Standard, severely limiting the use of newer .NET functionality.

Eventually I want to write a good baseline library to use for my source generators -- simplifying finding definitions with attributes, mapping types to System.Type, adding some basic pattern matching for structures -- but haven't found a way to do it that's general enough while being very useful.

zigzag312 144 days ago | | |

I agree, .NET Standard limitation unnecessarily complicates development experience. I think it's because some tools (Visual Studio) is still use legacy .NET Framework. I don't understand why they didn't integrate them via out of process architecture into these tools, since source generators didn't exist in the legacy framework anyway.

I sometimes generate code from plain CLI projects (avoiding source generators altogether), as whole debugging and DX is so much better.

pjmlp 144 days ago | | |

Yeah, it is kind of sad that it was the community that had to step up for some T4 like experience instead of string concatenation.

That isn't as cool as Aspire and AI features.

mexicocitinluez 144 days ago | | |

I feel the exact same way. They can be insanely powerful, but the syntax is insanely off-putting.

And the documentation is still pretty sparse.

> Eventually I want to write a good baseline library to use for my source generators -- simplifying finding definitions with attributes, mapping types to System.Type, adding some basic pattern matching for structures -- but haven't found a way to do it that's general enough while being very useful.

I'd use it in a heartbeat. The new ForAttributeWithMetadataName function has been a big help, but everything else feels like a totally different language to me.

olidb 143 days ago | | |

Add "<LangVersion>latest</LangVersion>" to the csproj to get more useful features like pattern matching and raw string literals.

torginus 144 days ago |

While this looks cool, I don't see any code generation capability in the examples or the tests, only compile time evaluation.

This is more like constrexpr than a macro system.

smcnc 144 days ago |

I think Zig really shines here: https://ziglang.org/documentation/master/#comptime

pjmlp 144 days ago | |

D was already doing it in 2010 thereabouts, an then there is the whole reader macros in Lisp and Scheme.

jibal 144 days ago | | |

And Nim, which has multiple levels of metaprogramming.

Tiberium 144 days ago | |

There's also Nim, it shines even more in compile time and has proper compile time macros :)

Zambyte 144 days ago | |

A key difference is that in this C# package, `[Comptime]` is an attribute (annotation? not sure on the C# term) applied to methods. In Zig, the `comptime` keyword can be applied to pretty much any expression. In the C# package, if you want to do factorial at runtime and at compile time, (I think, from reading the README) you need to define the same function twice, one with `[Comptime]` and once without. Contrast this to Zig, where if you have a regular runtime factorial function, you can just execute it at compile time like:

    const x = comptime factorial(n);

Another limitation of the C# package is it only works with primitive types and collections. Zig comptime works on any arbitrary types.

hahn-kev 144 days ago | | |

You don't. The way it works is that it intercepts the call site when the input args are constant. If they're not then it won't be replaced and it will call the original method. C# source generators can't replace method definitions, however a call site can be changed to another method via source generators.

estimator7292 144 days ago | | |

Decorations in [square brackets] are Attributes in C#

pyrolistical 144 days ago | |

I’m annoyed they called it comptime when it isn’t the same as Zig’s more powerful comptime.

You can think of zig’s comptime as partial evaluation. Zig doesnt have a runtime type reflection system, but with comptime it makes it feel like you do.

orphea 144 days ago |

  • Supported return types:
     ◦ Collections: ..., List<T>, ...
     ◦ Note: Arrays are not allowed as return types because they are mutable. Use IReadOnlyList<T> instead.

I don't understand. Why is List<T> allowed then if it's mutable?

andix 144 days ago | |

Array also implements IReadOnlyList if I'm not mistaken.

I think C# doesn't really have immutable collections, they just can be typecasted to IReadonly* to hide the mutable operations. But they can always be typecasted back to their mutable base implementation.

The only real immutable collections I know of, are F#s linked lists.

maltalex 144 days ago | | |

Immutable collections exit, they were just added later. See System.Collections.Immutable:

https://learn.microsoft.com/en-us/dotnet/api/system.collecti...

orphea 144 days ago | | |

Adding to the sibling comment, there are also Frozen{Dictionary,Set}: https://learn.microsoft.com/en-us/dotnet/api/system.collecti...

randomNumber7 144 days ago | | |

One man's constant is another man's variable.

mfro 144 days ago |

This seems like the kind of feature that should be built into MSBuild.

rubenvanwyk 144 days ago |

How is Comptime different from default AOT compilation? I assume it can work with third-party libraries that AOT don’t work with yet?

andix 144 days ago | |

It's a code generator that runs during compile time. It's a source generator that adds some generated code files to the project. So it runs way before AOT or JIT. Once AOT/JIT run, Comptime is already invisible to them, they only see the generated code from Comptime.

betaporter 144 days ago | |

I believe this is like `constexpr` for C#.

faithlv 144 days ago |

This is amazing, after using Zig, I started to write exactly this for C# too but never really had motivation to finish it.

andix 144 days ago |

Does this finally allow reading string literals from files and include them into the binaries?

I've seen a Go project that was heavily using https://pkg.go.dev/embed for loading some template files, and got a bit jealous. Back in the days of .NET Framework it was common to compile file contents into resource files, but it was always quite cumbersome.

eterm 144 days ago | |

C# / .NET has always had that abililty.

  <EmbeddedResource Include="/path/to/file.txt" />

Then you can read that file from the assembly with:

  assembly.GetManifestResourceStream

I'm not sure what's awkward about that, but it's nothing new.

Discoverability for this is in visual studio, if you go to file properties there's a drop-down where you choose between Content, EmbeddedResource, Ignore, etc.

That determines what happens in compile, whether it is copied to output directory, compiled into the binary, etc.

andix 144 days ago | | |

The Stream is awkward. It needs to be processed either once at startup or lazy on access, or every time it's used (allocations!). Also discovering multiple files is awkward.

floucky 144 days ago | |

Can't you already do that with embedded resources?

andix 144 days ago | | |

Yes, you can. But it's a bit cumbersome. The API to read them is not really intuitive, they are only accessible as a Stream, so they need to be either read every time into a string (new allocation, slow), or you need a helper that reads them once and keeps them in memory. I think there are also a lot of gotchas around naming and listing them.

In modern code I don't see them that often anymore.

mgaunard 144 days ago |

So it's like C++ consteval?