What causes "never types are not yet supported in C# output" in clockworklabs/SpacetimeDB?

Remove the Never/empty-enum type from tables, reducer args/returns, and exported type definitions

never types are not yet supported in C# output

Error message

never types are not yet supported in C# output

What it means

When generating C# bindings, the codegen must produce a default-value expression for every algebraic type (crates/codegen/src/csharp.rs:1383). The match has arms for strings, primitives, Result, and struct-like types, but AlgebraicTypeUse::Never hits unimplemented!() because an uninhabited type has no C# default. This is a codegen panic, not a graceful error.

Source

Thrown at crates/codegen/src/csharp.rs:1383

            AlgebraicTypeDef::Product(_) => Some("new()"),
        },
        // See Sum(_) handling above.
        AlgebraicTypeUse::ScheduleAt => Some("null!"),
        AlgebraicTypeUse::Array(_) => Some("new()"),
        // Strings must have explicit default value of "".
        AlgebraicTypeUse::String => Some(r#""""#),
        // Primitives are initialized to zero automatically.
        AlgebraicTypeUse::Primitive(_) => None,
        // Result<,> must be explicitly initialized.
        AlgebraicTypeUse::Result { .. } => Some("default!"),
        // these are structs, they are initialized to zero-filled automatically
        AlgebraicTypeUse::Unit
        | AlgebraicTypeUse::Identity
        | AlgebraicTypeUse::ConnectionId
        | AlgebraicTypeUse::Timestamp
        | AlgebraicTypeUse::TimeDuration
        | AlgebraicTypeUse::Uuid => None,
        AlgebraicTypeUse::Never => unimplemented!("never types are not yet supported in C# output"),
    }
}

struct CsharpAutogen {
    output: CodeIndenter<String>,
}

impl Deref for CsharpAutogen {
    type Target = CodeIndenter<String>;

    fn deref(&self) -> &Self::Target {
        &self.output
    }
}

impl std::ops::DerefMut for CsharpAutogen {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.output

View on GitHub (pinned to 524b4487d9)

Solutions

Remove the Never/empty-enum type from tables, reducer args/returns, and exported type definitions
Replace it with a habitable placeholder (a unit struct or a bool/uint8 tag) if you need the slot
Update the spacetimedb CLI and SDK to matching recent versions in case Never support has landed
As a stopgap, generate bindings for a schema variant that excludes the Never-bearing type

Example fix

// before (Rust module)
#[spacetimedb::table]
pub struct Job { pub status: Status, pub err: Never }
enum Never {}

// after
#[spacetimedb::table]
pub struct Job { pub status: Status, pub err: Option<String> }

Defensive patterns

Strategy: validation

Validate before calling

# In module CI, fail early if the schema contains uninhabited types:
# (run before spacetime generate --lang csharp)
cargo build && cargo test schema_smoke
# schema_smoke: assert every #[table] / reducer-reachable type has at least one variant/value
# e.g. a test that reflectively publishes and asserts no Never in the generated ABI

Prevention

Avoid empty enums / Never placeholders in table columns and reducer signatures
Run spacetime generate for every client language in CI so codegen gaps surface before release
Keep SDK and CLI versions pinned together to avoid new ABI shapes hitting old codegen

When it happens

Trigger: Running spacetime generate --lang csharp (or publishing a module whose schema is also consumed by C# clients) when the module ABI contains a Never type — e.g. an enum with no variants or a Rust '!' that leaked into a table column, reducer signature, or type alias.

Common situations: Declaring 'enum Void {}' as a placeholder column/payload type; SDK/toolchain version skew where the module emits a type shape the installed CLI's codegen cannot lower; experimenting with empty result types.

Related errors

AI-assisted analysis of clockworklabs/SpacetimeDB@524b4487d9 (2026-08-16). Data as JSON: /api/errors/72fe5b1a01197038. Report an issue: GitHub.