Does <Deterministic>true</Deterministic> Enforce a Specific File Order in C# Compilation?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I have observed that the order in which files are passed to the compiler can affect program behavior.

By default, projects use wildcard expansion for files (which is unstable), but when <Deterministic>true</Deterministic> is set, what order does the compiler use?

Documentation on this topic is lacking. If no defined order exists despite observable changes based on file ordering, I intend to file a bug against Roslyn since reordering shouldn’t affect determinism.

Simplest reproduction:

CSHARP
/* a file */
partial class Class1 {
    public static int x = 3;
}

CSHARP
/* another file */
partial class Class1 {
    public static int y = x;
}

CSHARP
/* Program.cs */
Console.WriteLine(Class1.y);

This reproduction works best on a FAT filesystem, where file creation order influences the file list.

Comments 0

•

Answers 2

•

2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

When you enable deterministic compilation in our tools (i.e. via true), the compiler no longer relies on the file system’s “wildcard” ordering (which can be unstable) but instead computes an explicit canonical order for the source files. In practice this means that the compiler sorts the input files by their full path (using ordinal, lexicographical order) and then processes them in that order.

This ordering isn’t something that’s driven by an undocumented “feature”—it is required for the output to be truly bit‐for‐bit reproducible. Even if file creation time or enumeration order on a FAT filesystem (or other filesystems) seem to affect the order during a wildcard expansion, the deterministic build overrides that by sorting the file paths.

So to answer your question: when true is set, the compiler passes files in a stable order computed by sorting the full file paths lexically. If your experiment shows a change in behavior with file order when the deterministic flag is set, that could indicate a bug in the compiler.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

No comments yet.

Answer by PulsarAdventurer926 • 1 month ago

The compiler's behavior indeed depends on the ordering of files. There's some other cases too around partial classes/members, which can have XML doc comments on both which get merged, and the order of the merging depends on the file ordering.

The compiler, as in "the csc.exe process", will ensure that the order it gets the source files passed in via the command line are handed around and processed in the correct order internally. It is absolutely deterministic, given a specific ordering of files -- it's just that the file ordering is effectively an input to the determinism. If you had a case where the same compiler invocation with the same ordering of files gave a different non-deterministic output, we'd absolutely want that bug.

However, the end-to-end tooling makes this murky: as you observed the globbing MSBuild does might depend on your file system, or operating system, etc. And frankly, if your file ordering matters, then you might just want to not use globbing because that's just asking for trouble at some point!

We absolutely have bugs in the IDE where when we create the "internal" compiler for IntelliSense, we might not get the file ordering correct to match the ordering the actual build will use. We haven't prioritized these bugs since they're very hard to hit. Frankly, in the 15 years I've worked on this you might be the winner for the first customer to notice!

No comments yet.

Discussion

No comments yet.

Does <Deterministic>true</Deterministic> Enforce a Specific File Order in C# Compilation?

2 Answers

Discussion

Similar Posts

Why Does ZipArchiveEntry.Open().ReadAsync() Return Fewer Bytes Than Read()?

Why does TarWriter buffer SQL streams and consume excessive memory in C#?

How can I build an in-memory tar file from SQL Server varbinary(max) columns using .NET 8's native Tar API?