csharp-server

name	csharp-server
description	SpacetimeDB C# server module SDK reference. Use when writing tables, reducers, or module logic in C#.
license	Apache-2.0
metadata	{"author":"clockworklabs","version":"2.0","role":"server","language":"csharp","cursor_globs":"**/*.cs","cursor_always_apply":true}

SpacetimeDB C# SDK Reference

Imports

using SpacetimeDB;

Module Structure

All tables, types, and reducers go inside a static partial class:

using SpacetimeDB;

public static partial class Module
{
    // Tables, types, and reducers here
}

Tables

[SpacetimeDB.Table(...)] on a public partial struct. Accessor should be PascalCase:

[SpacetimeDB.Table(Accessor = "Entity", Public = true)]
public partial struct Entity
{
    [PrimaryKey]
    [AutoInc]
    public ulong Id;
    public Identity Owner;
    public string Name;
    public bool Active;
}

Options: Accessor = "PascalCase" (recommended), Public = true, Scheduled = nameof(ReducerFn), ScheduledAt = nameof(field), Event = true

ctx.Db accessors use the Accessor name: ctx.Db.Entity, ctx.Db.Record.

Column Types

C# type	Notes
byte / ushort / uint / ulong	unsigned integers
U128 / U256	large unsigned integers (SpacetimeDB types)
sbyte / short / int / long	signed integers
I128 / I256	large signed integers (SpacetimeDB types)
float / double	floats
bool	boolean
string	text
List<T>	list/array
Identity	user identity
ConnectionId	connection handle
Timestamp	server timestamp (microseconds since epoch)
TimeDuration	duration in microseconds
Uuid	UUID

Column Attributes

[PrimaryKey]          // primary key
[AutoInc]             // auto-increment (use 0 as placeholder on insert)
[Unique]              // unique constraint
[SpacetimeDB.Index.BTree]  // btree index (enables .Filter() on this column)

Indexes

Prefer [SpacetimeDB.Index.BTree] inline for single-column. Multi-column uses struct-level:

// Inline (preferred for single-column):
[SpacetimeDB.Index.BTree]
public ulong AuthorId;
// Access: ctx.Db.Post.AuthorId.Filter(authorId)

// Multi-column (struct-level):
[SpacetimeDB.Table(Accessor = "Membership")]
[SpacetimeDB.Index.BTree(Accessor = "ByGroupUser", Columns = ["GroupId", "UserId"])]
public partial struct Membership { public ulong GroupId; public Identity UserId; ... }

When you frequently look up rows by multiple columns, prefer a multi-column index over filtering by one column and looping over the results.

Reducers

[SpacetimeDB.Reducer]
public static void CreateEntity(ReducerContext ctx, string name, int age)
{
    ctx.Db.Entity.Insert(new Entity { Owner = ctx.Sender, Name = name, Age = age, Active = true });
}

// No arguments:
[SpacetimeDB.Reducer]
public static void DoReset(ReducerContext ctx) { ... }

DB Operations

ctx.Db.Entity.Insert(new Entity { Name = "Sample" });             // Insert
ctx.Db.Entity.Id.Find(entityId);                                  // Find by PK → Entity? (nullable)
ctx.Db.Entity.Identity.Find(ctx.Sender);                          // Find by unique column → Entity?
ctx.Db.Item.AuthorId.Filter(authorId);                            // Filter by index → IEnumerable<Item>
ctx.Db.Entity.Iter();                                             // All rows → IEnumerable<Entity>
ctx.Db.Entity.Count;                                              // Count rows
ctx.Db.Entity.Id.Update(existing with { Name = newName });        // Update by PK
ctx.Db.Entity.Id.Delete(entityId);                                // Delete by PK

Note: Filter/Iter return enumerables. Use .ToList() if you need to sort or mutate.

The pattern is ctx.Db.{Accessor}.{ColumnName}.{Method}(value) for all indexed column operations.

Lifecycle Hooks

[SpacetimeDB.Reducer(ReducerKind.Init)]
public static void OnInit(ReducerContext ctx) { ... }

[SpacetimeDB.Reducer(ReducerKind.ClientConnected)]
public static void OnConnect(ReducerContext ctx) { ... }

[SpacetimeDB.Reducer(ReducerKind.ClientDisconnected)]
public static void OnDisconnect(ReducerContext ctx) { ... }

Views

// Anonymous view (same result for all clients):
[SpacetimeDB.View(Accessor = "ActiveUsers", Public = true)]
public static List<Entity> ActiveUsers(AnonymousViewContext ctx)
{
    return ctx.Db.Entity.Iter().Where(e => e.Active).ToList();
}

// Per-user view:
[SpacetimeDB.View(Accessor = "MyProfile", Public = true)]
public static Entity? MyProfile(ViewContext ctx)
{
    return ctx.Db.Entity.Identity.Find(ctx.Sender) as Entity?;
}

Reducer Context API

ReducerContext is the single source of sender identity, deterministic time, and deterministic randomness inside a reducer. Always go through ctx for these. Standard library clocks and random sources are not available in modules.

// Auth: ctx.Sender is the caller's Identity
if (row.Owner != ctx.Sender)
    throw new Exception("unauthorized");

// Server timestamp (deterministic per reducer call)
ctx.Db.Item.Insert(new Item { CreatedAt = ctx.Timestamp, .. });

// Timestamp arithmetic
var expiry = ctx.Timestamp + new TimeDuration(delayMicros);

// Deterministic RNG
int roll = ctx.Rng.Next(1, 7);          // [1, 7): inclusive 1, exclusive 7
double f = ctx.Rng.NextDouble();        // [0.0, 1.0)

// Client: Timestamp → milliseconds since epoch
timestamp.MicrosecondsSinceUnixEpoch / 1000

Scheduled Tables

[SpacetimeDB.Table(
    Accessor = "TickTimer",
    Scheduled = nameof(Tick),
    ScheduledAt = nameof(ScheduledAt),
    Public = true
)]
public partial struct TickTimer
{
    [PrimaryKey]
    [AutoInc]
    public ulong ScheduledId;
    public ScheduleAt ScheduledAt;
}

[SpacetimeDB.Reducer]
public static void Tick(ReducerContext ctx, TickTimer timer)
{
    // timer row is auto-deleted after this reducer runs
}

// One-time: fires once at a specific time
var at = new ScheduleAt.Time(DateTimeOffset.UtcNow.AddSeconds(10));
// Repeating: fires on an interval
var at = new ScheduleAt.Interval(TimeSpan.FromSeconds(5));

ctx.Db.TickTimer.Insert(new TickTimer { ScheduledId = 0, ScheduledAt = at });

Custom Types

[SpacetimeDB.Type]
public enum Status { Online, Away, Offline }

[SpacetimeDB.Type]
public partial struct Point { public float X; public float Y; }

// Tagged enum (discriminated union):
[SpacetimeDB.Type]
public partial record MyUnion : SpacetimeDB.TaggedEnum<(string Text, int Number)>;

Optional Fields

[SpacetimeDB.Table(Accessor = "Player")]
public partial struct Player
{
    [PrimaryKey, AutoInc]
    public ulong Id;
    public string Name;
    public string? Nickname;
    public uint? HighScore;
}

Complete Example

using SpacetimeDB;

[SpacetimeDB.Table(Accessor = "Entity", Public = true)]
public partial struct Entity
{
    [PrimaryKey]
    public Identity Identity;
    public string Name;
    public bool Active;
}

[SpacetimeDB.Table(Accessor = "Record", Public = true)]
public partial struct Record
{
    [PrimaryKey]
    [AutoInc]
    public ulong Id;
    public Identity Owner;
    public uint Value;
    public Timestamp CreatedAt;
}

public static partial class Module
{
    [SpacetimeDB.Reducer(ReducerKind.ClientConnected)]
    public static void OnConnect(ReducerContext ctx)
    {
        var existing = ctx.Db.Entity.Identity.Find(ctx.Sender);
        if (existing is not null)
            ctx.Db.Entity.Identity.Update(existing.Value with { Active = true });
    }

    [SpacetimeDB.Reducer(ReducerKind.ClientDisconnected)]
    public static void OnDisconnect(ReducerContext ctx)
    {
        var existing = ctx.Db.Entity.Identity.Find(ctx.Sender);
        if (existing is not null)
            ctx.Db.Entity.Identity.Update(existing.Value with { Active = false });
    }

    [SpacetimeDB.Reducer]
    public static void CreateEntity(ReducerContext ctx, string name)
    {
        if (ctx.Db.Entity.Identity.Find(ctx.Sender) is not null)
            throw new Exception("already exists");
        ctx.Db.Entity.Insert(new Entity { Identity = ctx.Sender, Name = name, Active = true });
    }

    [SpacetimeDB.Reducer]
    public static void AddRecord(ReducerContext ctx, uint value)
    {
        if (ctx.Db.Entity.Identity.Find(ctx.Sender) is null)
            throw new Exception("not found");
        ctx.Db.Record.Insert(new Record {
            Id = 0,
            Owner = ctx.Sender,
            Value = value,
            CreatedAt = ctx.Timestamp,
        });
    }
}