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C# Client SDK Quick Start

In this guide we'll show you how to get up and running with a simple SpacetimeDB app with a client written in C#.

We'll implement a command-line client for the module created in our Rust or C# Module Quickstart guides. Ensure you followed one of these guides before continuing.

Project structure

Enter the directory quickstart-chat you created in the Rust Module Quickstart or C# Module Quickstart guides:

cd quickstart-chat

Within it, create a new C# console application project called client using either Visual Studio, Rider or the .NET CLI:

dotnet new console -o client

Open the project in your IDE of choice.

Add the NuGet package for the C# SpacetimeDB SDK

Add the SpacetimeDB.ClientSDK NuGet package using Visual Studio or Rider NuGet Package Manager or via the .NET CLI:

dotnet add package SpacetimeDB.ClientSDK

Generate your module types

The spacetime CLI's generate command will generate client-side interfaces for the tables, reducers and types defined in your server module.

In your quickstart-chat directory, run:

mkdir -p client/module_bindings
spacetime generate --lang csharp --out-dir client/module_bindings --project-path server

Take a look inside client/module_bindings. The CLI should have generated five files:

module_bindings
├── Message.cs
├── ReducerEvent.cs
├── SendMessageReducer.cs
├── SetNameReducer.cs
└── User.cs

Add imports to Program.cs

Open client/Program.cs and add the following imports:

using SpacetimeDB;
using SpacetimeDB.Types;
using System.Collections.Concurrent;

We will also need to create some global variables that will be explained when we use them later. Add the following to the top of Program.cs:

// our local client SpacetimeDB identity
Identity? local_identity = null;

// declare a thread safe queue to store commands in format (command, args)
ConcurrentQueue<(string,string)> input_queue = new ConcurrentQueue<(string, string)>();

// declare a threadsafe cancel token to cancel the process loop
CancellationTokenSource cancel_token = new CancellationTokenSource();

Define Main function

We'll work outside-in, first defining our Main function at a high level, then implementing each behavior it needs. We need Main to do several things:

  1. Initialize the AuthToken module, which loads and stores our authentication token to/from local storage.
  2. Create the SpacetimeDBClient instance.
  3. Register callbacks on any events we want to handle. These will print to standard output messages received from the database and updates about users' names and online statuses.
  4. Start our processing thread which connects to the SpacetimeDB module, updates the SpacetimeDB client and processes commands that come in from the input loop running in the main thread.
  5. Start the input loop, which reads commands from standard input and sends them to the processing thread.
  6. When the input loop exits, stop the processing thread and wait for it to exit.
void Main()
{
    AuthToken.Init(".spacetime_csharp_quickstart");

    RegisterCallbacks();

    // spawn a thread to call process updates and process commands
    var thread = new Thread(ProcessThread);
    thread.Start();

    InputLoop();

    // this signals the ProcessThread to stop
    cancel_token.Cancel();
    thread.Join();
}

Register callbacks

We need to handle several sorts of events:

  1. onConnect: When we connect, we will call Subscribe to tell the module what tables we care about.
  2. onIdentityReceived: When we receive our credentials, we'll use the AuthToken module to save our token so that the next time we connect, we can re-authenticate as the same user.
  3. onSubscriptionApplied: When we get the onSubscriptionApplied callback, that means our local client cache has been fully populated. At this time we'll print the user menu.
  4. User.OnInsert: When a new user joins, we'll print a message introducing them.
  5. User.OnUpdate: When a user is updated, we'll print their new name, or declare their new online status.
  6. Message.OnInsert: When we receive a new message, we'll print it.
  7. Reducer.OnSetNameEvent: If the server rejects our attempt to set our name, we'll print an error.
  8. Reducer.OnSendMessageEvent: If the server rejects a message we send, we'll print an error.
void RegisterCallbacks()
{
    SpacetimeDBClient.instance.onConnect += OnConnect;
    SpacetimeDBClient.instance.onIdentityReceived += OnIdentityReceived;
    SpacetimeDBClient.instance.onSubscriptionApplied += OnSubscriptionApplied;

    User.OnInsert += User_OnInsert;
    User.OnUpdate += User_OnUpdate;

    Message.OnInsert += Message_OnInsert;

    Reducer.OnSetNameEvent += Reducer_OnSetNameEvent;
    Reducer.OnSendMessageEvent += Reducer_OnSendMessageEvent;
}

Notify about new users

For each table, we can register on-insert and on-delete callbacks to be run whenever a subscribed row is inserted or deleted. We register these callbacks using the OnInsert and OnDelete methods, which are automatically generated for each table by spacetime generate.

These callbacks can fire in two contexts:

  • After a reducer runs, when the client's cache is updated about changes to subscribed rows.
  • After calling subscribe, when the client's cache is initialized with all existing matching rows.

This second case means that, even though the module only ever inserts online users, the client's User.OnInsert callbacks may be invoked with users who are offline. We'll only notify about online users.

OnInsert and OnDelete callbacks take two arguments: the altered row, and a ReducerEvent. This will be null for rows inserted when initializing the cache for a subscription. ReducerEvent is an enum autogenerated by spacetime generate with a variant for each reducer defined by the module. For now, we can ignore this argument.

Whenever we want to print a user, if they have set a name, we'll use that. If they haven't set a name, we'll instead print the first 8 bytes of their identity, encoded as hexadecimal. We'll define a function UserNameOrIdentity to handle this.

string UserNameOrIdentity(User user) => user.Name ?? user.Identity.ToString()!.Substring(0, 8);

void User_OnInsert(User insertedValue, ReducerEvent? dbEvent)
{
    if (insertedValue.Online)
    {
        Console.WriteLine($"{UserNameOrIdentity(insertedValue)} is online");
    }
}

Notify about updated users

Because we declared a primary key column in our User table, we can also register on-update callbacks. These run whenever a row is replaced by a row with the same primary key, like our module's User::update_by_identity calls. We register these callbacks using the OnUpdate method, which is automatically implemented by spacetime generate for any table with a primary key column.

OnUpdate callbacks take three arguments: the old row, the new row, and a ReducerEvent.

In our module, users can be updated for three reasons:

  1. They've set their name using the SetName reducer.
  2. They're an existing user re-connecting, so their Online has been set to true.
  3. They've disconnected, so their Online has been set to false.

We'll print an appropriate message in each of these cases.

void User_OnUpdate(User oldValue, User newValue, ReducerEvent dbEvent)
{
    if (oldValue.Name != newValue.Name)
    {
        Console.WriteLine($"{UserNameOrIdentity(oldValue)} renamed to {newValue.Name}");
    }

    if (oldValue.Online == newValue.Online)
        return;

    if (newValue.Online)
    {
        Console.WriteLine($"{UserNameOrIdentity(newValue)} connected.");
    }
    else
    {
        Console.WriteLine($"{UserNameOrIdentity(newValue)} disconnected.");
    }
}

Print messages

When we receive a new message, we'll print it to standard output, along with the name of the user who sent it. Keep in mind that we only want to do this for new messages, i.e. those inserted by a SendMessage reducer invocation. We have to handle the backlog we receive when our subscription is initialized separately, to ensure they're printed in the correct order. To that effect, our OnInsert callback will check if its ReducerEvent argument is not null, and only print in that case.

To find the User based on the message's Sender identity, we'll use User::FindByIdentity, which behaves like the same function on the server.

We'll print the user's name or identity in the same way as we did when notifying about User table events, but here we have to handle the case where we don't find a matching User row. This can happen when the module owner sends a message using the CLI's spacetime call. In this case, we'll print unknown.

void PrintMessage(Message message)
{
    var sender = User.FindByIdentity(message.Sender);
    var senderName = "unknown";
    if (sender != null)
    {
        senderName = UserNameOrIdentity(sender);
    }

    Console.WriteLine($"{senderName}: {message.Text}");
}

void Message_OnInsert(Message insertedValue, ReducerEvent? dbEvent)
{
    if (dbEvent != null)
    {
        PrintMessage(insertedValue);
    }
}

Warn if our name was rejected

We can also register callbacks to run each time a reducer is invoked. We register these callbacks using the OnReducerEvent method of the Reducer namespace, which is automatically implemented for each reducer by spacetime generate.

Each reducer callback takes one fixed argument:

The ReducerEvent that triggered the callback. It contains several fields. The ones we care about are:

  1. The Identity of the client that called the reducer.
  2. The Status of the reducer run, one of Committed, Failed or OutOfEnergy.
  3. The error message, if any, that the reducer returned.

It also takes a variable amount of additional arguments that match the reducer's arguments.

These callbacks will be invoked in one of two cases:

  1. If the reducer was successful and altered any of our subscribed rows.
  2. If we requested an invocation which failed.

Note that a status of Failed or OutOfEnergy implies that the caller identity is our own identity.

We already handle successful SetName invocations using our User.OnUpdate callback, but if the module rejects a user's chosen name, we'd like that user's client to let them know. We define a function Reducer_OnSetNameEvent as a Reducer.OnSetNameEvent callback which checks if the reducer failed, and if it did, prints an error message including the rejected name.

We'll test both that our identity matches the sender and that the status is Failed, even though the latter implies the former, for demonstration purposes.

void Reducer_OnSetNameEvent(ReducerEvent reducerEvent, string name)
{
    bool localIdentityFailedToChangeName =
        reducerEvent.Identity == local_identity &&
        reducerEvent.Status == ClientApi.Event.Types.Status.Failed;

    if (localIdentityFailedToChangeName)
    {
        Console.Write($"Failed to change name to {name}");
    }
}

Warn if our message was rejected

We handle warnings on rejected messages the same way as rejected names, though the types and the error message are different.

void Reducer_OnSendMessageEvent(ReducerEvent reducerEvent, string text)
{
    bool localIdentityFailedToSendMessage =
        reducerEvent.Identity == local_identity &&
        reducerEvent.Status == ClientApi.Event.Types.Status.Failed;

    if (localIdentityFailedToSendMessage)
    {
        Console.Write($"Failed to send message {text}");
    }
}

Connect callback

Once we are connected, we can send our subscription to the SpacetimeDB module. SpacetimeDB is set up so that each client subscribes via SQL queries to some subset of the database, and is notified about changes only to that subset. For complex apps with large databases, judicious subscriptions can save each client significant network bandwidth, memory and computation compared. For example, in BitCraft, each player's client subscribes only to the entities in the "chunk" of the world where that player currently resides, rather than the entire game world. Our app is much simpler than BitCraft, so we'll just subscribe to the whole database.

void OnConnect()
{
    SpacetimeDBClient.instance.Subscribe(new List<string>
    {
        "SELECT * FROM User", "SELECT * FROM Message"
    });
}

OnIdentityReceived callback

This callback is executed when we receive our credentials from the SpacetimeDB module. We'll use the AuthToken module to save our token to local storage, so that we can re-authenticate as the same user the next time we connect. We'll also store the identity in a global variable local_identity so that we can use it to check if we are the sender of a message or name change. This callback also notifies us of our client's Address, an opaque identifier SpacetimeDB modules can use to distinguish connections by the same Identity, but we won't use it in our app.

void OnIdentityReceived(string authToken, Identity identity, Address _address)
{
    local_identity = identity;
    AuthToken.SaveToken(authToken);
}

OnSubscriptionApplied callback

Once our subscription is applied, we'll print all the previously sent messages. We'll define a function PrintMessagesInOrder to do this. PrintMessagesInOrder calls the automatically generated Iter function on our Message table, which returns an iterator over all rows in the table. We'll use the OrderBy method on the iterator to sort the messages by their Sent timestamp.

void PrintMessagesInOrder()
{
    foreach (Message message in Message.Iter().OrderBy(item => item.Sent))
    {
        PrintMessage(message);
    }
}

void OnSubscriptionApplied()
{
    Console.WriteLine("Connected");
    PrintMessagesInOrder();
}

Process thread

Since the input loop will be blocking, we'll run our processing code in a separate thread. This thread will:

  1. Connect to the module. We'll store the SpacetimeDB host name and our module name in constants HOST and DB_NAME. We will also store if SSL is enabled in a constant called SSL_ENABLED. This only needs to be true if we are using SpacetimeDB Cloud. Replace <module-name> with the name you chose when publishing your module during the module quickstart.

Connect takes an auth token, which is null for a new connection, or a stored string for a returning user. We are going to use the optional AuthToken module which uses local storage to store the auth token. If you want to use your own way to associate an auth token with a user, you can pass in your own auth token here.

  1. Loop until the thread is signaled to exit, calling Update on the SpacetimeDBClient to process any updates received from the module, and ProcessCommand to process any commands received from the input loop.

  2. Finally, Close the connection to the module.

const string HOST = "http://localhost:3000";
const string DBNAME = "module";

void ProcessThread()
{
    SpacetimeDBClient.instance.Connect(AuthToken.Token, HOST, DBNAME);

    // loop until cancellation token
    while (!cancel_token.IsCancellationRequested)
    {
        SpacetimeDBClient.instance.Update();

        ProcessCommands();

        Thread.Sleep(100);
    }

    SpacetimeDBClient.instance.Close();
}

Input loop and ProcessCommands

The input loop will read commands from standard input and send them to the processing thread using the input queue. The ProcessCommands function is called every 100ms by the processing thread to process any pending commands.

Supported Commands:

  1. Send a message: message, send the message to the module by calling Reducer.SendMessage which is automatically generated by spacetime generate.

  2. Set name: name, will send the new name to the module by calling Reducer.SetName which is automatically generated by spacetime generate.

void InputLoop()
{
    while (true)
    {
        var input = Console.ReadLine();
        if (input == null)
        {
            break;
        }

        if (input.StartsWith("/name "))
        {
            input_queue.Enqueue(("name", input.Substring(6)));
            continue;
        }
        else
        {
            input_queue.Enqueue(("message", input));
        }
    }
}

void ProcessCommands()
{
    // process input queue commands
    while (input_queue.TryDequeue(out var command))
    {
        switch (command.Item1)
        {
            case "message":
                Reducer.SendMessage(command.Item2);
                break;
            case "name":
                Reducer.SetName(command.Item2);
                break;
        }
    }
}

Run the client

Finally we just need to add a call to Main in Program.cs:

Main();

Now, we can run the client by hitting start in Visual Studio or Rider; or by running the following command in the client directory:

dotnet run --project client

What's next?

Congratulations! You've built a simple chat app using SpacetimeDB. You can look at the C# SDK Reference for more information about the client SDK. If you are interested in developing in the Unity game engine, check out our Unity3d Comprehensive Tutorial and BitcraftMini game example.

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