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1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">Server Module Languages</span></li><li class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link">C# Quickstart</span></li></ul></nav><div class="tocCollapsible_dqme theme-doc-toc-mobile tocMobile_Z34P"><button type="button" class="clean-btn tocCollapsibleButton_QMSE">On this page</button></div><div class="theme-doc-markdown markdown"><header><h1>C# Module Quickstart</h1></header>
<p>In this tutorial, we&#x27;ll implement a simple chat server as a SpacetimeDB module.</p>
<p>A SpacetimeDB module is code that gets compiled to WebAssembly and is uploaded to SpacetimeDB. This code becomes server-side logic that interfaces directly with the Spacetime relational database.</p>
<p>Each SpacetimeDB module defines a set of tables and a set of reducers.</p>
<p>Each table is defined as a C# <code>class</code> annotated with <code>[SpacetimeDB.Table]</code>, where an instance represents a row, and each field represents a column.
By default, tables are <strong>private</strong>. This means that they are only readable by the table owner, and by server module code.
The <code>[SpacetimeDB.Table(Public = true))]</code> annotation makes a table public. <strong>Public</strong> tables are readable by all users, but can still only be modified by your server module code.</p>
<p>A reducer is a function which traverses and updates the database. Each reducer call runs in its own transaction, and its updates to the database are only committed if the reducer returns successfully. In C#, reducers are defined as functions annotated with <code>[SpacetimeDB.Reducer]</code>. If an exception is thrown, the reducer call fails, the database is not updated, and a failed message is reported to the client.</p>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="install-spacetimedb">Install SpacetimeDB<a href="#install-spacetimedb" class="hash-link" aria-label="Direct link to Install SpacetimeDB" title="Direct link to Install SpacetimeDB" translate="no">​</a></h2>
<p>If you haven&#x27;t already, start by <a href="https://spacetimedb.com/install" target="_blank" rel="noopener noreferrer">installing SpacetimeDB</a>. This will install the <code>spacetime</code> command line interface (CLI), which contains all the functionality for interacting with SpacetimeDB.</p>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="install-net-8">Install .NET 8<a href="#install-net-8" class="hash-link" aria-label="Direct link to Install .NET 8" title="Direct link to Install .NET 8" translate="no">​</a></h2>
<p>Next we need to <a href="https://dotnet.microsoft.com/en-us/download/dotnet/8.0" target="_blank" rel="noopener noreferrer">install .NET 8 SDK</a> so that we can build and publish our module.</p>
<p>You may already have .NET 8 and can be checked:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">dotnet</span><span style="color:#BD93F9"> --list-sdks</span></span></code></pre>
<p>.NET 8.0 is the earliest to have the <code>wasi-experimental</code> workload that we rely on, but requires manual activation:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">dotnet</span><span style="color:#F1FA8C"> workload</span><span style="color:#F1FA8C"> install</span><span style="color:#F1FA8C"> wasi-experimental</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="project-structure">Project structure<a href="#project-structure" class="hash-link" aria-label="Direct link to Project structure" title="Direct link to Project structure" translate="no">​</a></h2>
<p>Create and enter a directory <code>quickstart-chat</code>:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">mkdir</span><span style="color:#F1FA8C"> quickstart-chat</span></span>
<span class="line"><span style="color:#8BE9FD">cd</span><span style="color:#F1FA8C"> quickstart-chat</span></span></code></pre>
<p>Now create <code>server</code>, our module, which runs in the database:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">spacetime</span><span style="color:#F1FA8C"> init</span><span style="color:#BD93F9"> --lang</span><span style="color:#F1FA8C"> csharp</span><span style="color:#F1FA8C"> server</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="declare-imports">Declare imports<a href="#declare-imports" class="hash-link" aria-label="Direct link to Declare imports" title="Direct link to Declare imports" translate="no">​</a></h2>
<p><code>spacetime init</code> generated a few files:</p>
<ol>
<li>Open <code>server/StdbModule.csproj</code> to generate a .sln file for intellisense/validation support.</li>
<li>Open <code>server/Lib.cs</code>, a trivial module.</li>
<li>Clear it out, so we can write a new module that&#x27;s still pretty simple: a bare-bones chat server.</li>
</ol>
<p>To start, we&#x27;ll need to add <code>SpacetimeDB</code> to our using statements. This will give us access to everything we need to author our SpacetimeDB server module.</p>
<p>To the top of <code>server/Lib.cs</code>, add some imports we&#x27;ll be using:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#FF79C6">using</span><span style="color:#8BE9FD;font-style:italic"> SpacetimeDB</span><span style="color:#F8F8F2">;</span></span></code></pre>
<p>We also need to create our static module class which all of the module code will live in. In <code>server/Lib.cs</code>, add:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> partial</span><span style="color:#FF79C6"> class</span><span style="color:#8BE9FD"> Module</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="define-tables">Define tables<a href="#define-tables" class="hash-link" aria-label="Direct link to Define tables" title="Direct link to Define tables" translate="no">​</a></h2>
<p>To get our chat server running, we&#x27;ll need to store two kinds of data: information about each user, and records of all the messages that have been sent.</p>
<p>For each <code>User</code>, we&#x27;ll store their <code>Identity</code>, an optional name they can set to identify themselves to other users, and whether they&#x27;re online or not. We&#x27;ll designate the <code>Identity</code> as our primary key, which enforces that it must be unique, indexes it for faster lookup, and allows clients to track updates.</p>
<p>In <code>server/Lib.cs</code>, add the definition of the table <code>User</code> to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#F8F8F2">[</span><span style="color:#8BE9FD;font-style:italic">Table</span><span style="color:#F8F8F2">(Name </span><span style="color:#FF79C6">=</span><span style="color:#E9F284"> &quot;</span><span style="color:#F1FA8C">user</span><span style="color:#E9F284">&quot;</span><span style="color:#F8F8F2">, Public </span><span style="color:#FF79C6">=</span><span style="color:#BD93F9"> true</span><span style="color:#F8F8F2">)]</span></span>
<span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> partial</span><span style="color:#FF79C6"> class</span><span style="color:#8BE9FD"> User</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#F8F8F2"> [</span><span style="color:#8BE9FD;font-style:italic">PrimaryKey</span><span style="color:#F8F8F2">]</span></span>
<span class="line"><span style="color:#FF79C6"> public</span><span style="color:#8BE9FD;font-style:italic"> Identity</span><span style="color:#F8F8F2"> Identity;</span></span>
<span class="line"><span style="color:#FF79C6"> public</span><span style="color:#FF79C6"> string</span><span style="color:#F8F8F2">? Name;</span></span>
<span class="line"><span style="color:#FF79C6"> public</span><span style="color:#FF79C6"> bool</span><span style="color:#F8F8F2"> Online;</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<p>For each <code>Message</code>, we&#x27;ll store the <code>Identity</code> of the user who sent it, the <code>Timestamp</code> when it was sent, and the text of the message.</p>
<p>In <code>server/Lib.cs</code>, add the definition of the table <code>Message</code> to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#F8F8F2">[</span><span style="color:#8BE9FD;font-style:italic">Table</span><span style="color:#F8F8F2">(Name </span><span style="color:#FF79C6">=</span><span style="color:#E9F284"> &quot;</span><span style="color:#F1FA8C">message</span><span style="color:#E9F284">&quot;</span><span style="color:#F8F8F2">, Public </span><span style="color:#FF79C6">=</span><span style="color:#BD93F9"> true</span><span style="color:#F8F8F2">)]</span></span>
<span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> partial</span><span style="color:#FF79C6"> class</span><span style="color:#8BE9FD"> Message</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#FF79C6"> public</span><span style="color:#8BE9FD;font-style:italic"> Identity</span><span style="color:#F8F8F2"> Sender;</span></span>
<span class="line"><span style="color:#FF79C6"> public</span><span style="color:#8BE9FD;font-style:italic"> Timestamp</span><span style="color:#F8F8F2"> Sent;</span></span>
<span class="line"><span style="color:#FF79C6"> public</span><span style="color:#FF79C6"> string</span><span style="color:#F8F8F2"> Text </span><span style="color:#FF79C6">=</span><span style="color:#E9F284"> &quot;&quot;</span><span style="color:#F8F8F2">;</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="set-users-names">Set users&#x27; names<a href="#set-users-names" class="hash-link" aria-label="Direct link to Set users&#x27; names" title="Direct link to Set users&#x27; names" translate="no">​</a></h2>
<p>We want to allow users to set their names, because <code>Identity</code> is not a terribly user-friendly identifier. To that effect, we define a reducer <code>SetName</code> which clients can invoke to set their <code>User.Name</code>. It will validate the caller&#x27;s chosen name, using a function <code>ValidateName</code> which we&#x27;ll define next, then look up the <code>User</code> record for the caller and update it to store the validated name. If the name fails the validation, the reducer will fail.</p>
<p>Each reducer must accept as its first argument a <code>ReducerContext</code>, which includes contextual data such as the <code>Sender</code> which contains the Identity of the client that called the reducer, and the <code>Timestamp</code> when it was invoked. For now, we only need the <code>Sender</code>.</p>
<p>It&#x27;s also possible to call <code>SetName</code> via the SpacetimeDB CLI&#x27;s <code>spacetime call</code> command without a connection, in which case no <code>User</code> record will exist for the caller. We&#x27;ll return an error in this case, but you could alter the reducer to insert a <code>User</code> row for the module owner. You&#x27;ll have to decide whether the module owner is always online or always offline, though.</p>
<p>In <code>server/Lib.cs</code>, add to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#F8F8F2">[</span><span style="color:#8BE9FD;font-style:italic">Reducer</span><span style="color:#F8F8F2">]</span></span>
<span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> void</span><span style="color:#50FA7B"> SetName</span><span style="color:#F8F8F2">(</span><span style="color:#8BE9FD;font-style:italic">ReducerContext</span><span style="color:#FFB86C;font-style:italic"> ctx</span><span style="color:#F8F8F2">, </span><span style="color:#FF79C6">string</span><span style="color:#FFB86C;font-style:italic"> name</span><span style="color:#F8F8F2">)</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#F8F8F2"> name </span><span style="color:#FF79C6">=</span><span style="color:#50FA7B"> ValidateName</span><span style="color:#F8F8F2">(name);</span></span>
<span class="line"></span>
<span class="line"><span style="color:#FF79C6"> if</span><span style="color:#F8F8F2"> (ctx.Db.user.Identity.</span><span style="color:#50FA7B">Find</span><span style="color:#F8F8F2">(ctx.Sender) </span><span style="color:#FF79C6">is</span><span style="color:#8BE9FD;font-style:italic"> User</span><span style="color:#F8F8F2"> user)</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#F8F8F2"> user.Name </span><span style="color:#FF79C6">=</span><span style="color:#F8F8F2"> name;</span></span>
<span class="line"><span style="color:#F8F8F2"> ctx.Db.user.Identity.</span><span style="color:#50FA7B">Update</span><span style="color:#F8F8F2">(user);</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<p>For now, we&#x27;ll just do a bare minimum of validation, rejecting the empty name. You could extend this in various ways, like:</p>
<ul>
<li>Comparing against a blacklist for moderation purposes.</li>
<li>Unicode-normalizing names.</li>
<li>Rejecting names that contain non-printable characters, or removing characters or replacing them with a placeholder.</li>
<li>Rejecting or truncating long names.</li>
<li>Rejecting duplicate names.</li>
</ul>
<p>In <code>server/Lib.cs</code>, add to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#6272A4">/// Takes a name and checks if it&#x27;s acceptable as a user&#x27;s name.</span></span>
<span class="line"><span style="color:#FF79C6">private</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> string</span><span style="color:#50FA7B"> ValidateName</span><span style="color:#F8F8F2">(</span><span style="color:#FF79C6">string</span><span style="color:#FFB86C;font-style:italic"> name</span><span style="color:#F8F8F2">)</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#FF79C6"> if</span><span style="color:#F8F8F2"> (</span><span style="color:#FF79C6">string</span><span style="color:#F8F8F2">.</span><span style="color:#50FA7B">IsNullOrEmpty</span><span style="color:#F8F8F2">(name))</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#FF79C6"> throw</span><span style="color:#FF79C6"> new</span><span style="color:#8BE9FD;font-style:italic"> Exception</span><span style="color:#F8F8F2">(</span><span style="color:#E9F284">&quot;</span><span style="color:#F1FA8C">Names must not be empty</span><span style="color:#E9F284">&quot;</span><span style="color:#F8F8F2">);</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#FF79C6"> return</span><span style="color:#F8F8F2"> name;</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="send-messages">Send messages<a href="#send-messages" class="hash-link" aria-label="Direct link to Send messages" title="Direct link to Send messages" translate="no">​</a></h2>
<p>We define a reducer <code>SendMessage</code>, which clients will call to send messages. It will validate the message&#x27;s text, then insert a new <code>Message</code> record using <code>Message.Insert</code>, with the <code>Sender</code> identity and <code>Time</code> timestamp taken from the <code>ReducerContext</code>.</p>
<p>In <code>server/Lib.cs</code>, add to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#F8F8F2">[</span><span style="color:#8BE9FD;font-style:italic">Reducer</span><span style="color:#F8F8F2">]</span></span>
<span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> void</span><span style="color:#50FA7B"> SendMessage</span><span style="color:#F8F8F2">(</span><span style="color:#8BE9FD;font-style:italic">ReducerContext</span><span style="color:#FFB86C;font-style:italic"> ctx</span><span style="color:#F8F8F2">, </span><span style="color:#FF79C6">string</span><span style="color:#FFB86C;font-style:italic"> text</span><span style="color:#F8F8F2">)</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#F8F8F2"> text </span><span style="color:#FF79C6">=</span><span style="color:#50FA7B"> ValidateMessage</span><span style="color:#F8F8F2">(text);</span></span>
<span class="line"><span style="color:#F8F8F2"> Log.</span><span style="color:#50FA7B">Info</span><span style="color:#F8F8F2">(text);</span></span>
<span class="line"><span style="color:#F8F8F2"> ctx.Db.message.</span><span style="color:#50FA7B">Insert</span><span style="color:#F8F8F2">(</span></span>
<span class="line"><span style="color:#FF79C6"> new</span><span style="color:#8BE9FD;font-style:italic"> Message</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#F8F8F2"> Sender </span><span style="color:#FF79C6">=</span><span style="color:#F8F8F2"> ctx.Sender,</span></span>
<span class="line"><span style="color:#F8F8F2"> Text </span><span style="color:#FF79C6">=</span><span style="color:#F8F8F2"> text,</span></span>
<span class="line"><span style="color:#F8F8F2"> Sent </span><span style="color:#FF79C6">=</span><span style="color:#F8F8F2"> ctx.Timestamp,</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#F8F8F2"> );</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<p>We&#x27;ll want to validate messages&#x27; texts in much the same way we validate users&#x27; chosen names. As above, we&#x27;ll do the bare minimum, rejecting only empty messages.</p>
<p>In <code>server/Lib.cs</code>, add to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#6272A4">/// Takes a message&#x27;s text and checks if it&#x27;s acceptable to send.</span></span>
<span class="line"><span style="color:#FF79C6">private</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> string</span><span style="color:#50FA7B"> ValidateMessage</span><span style="color:#F8F8F2">(</span><span style="color:#FF79C6">string</span><span style="color:#FFB86C;font-style:italic"> text</span><span style="color:#F8F8F2">)</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#FF79C6"> if</span><span style="color:#F8F8F2"> (</span><span style="color:#FF79C6">string</span><span style="color:#F8F8F2">.</span><span style="color:#50FA7B">IsNullOrEmpty</span><span style="color:#F8F8F2">(text))</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#FF79C6"> throw</span><span style="color:#FF79C6"> new</span><span style="color:#8BE9FD;font-style:italic"> ArgumentException</span><span style="color:#F8F8F2">(</span><span style="color:#E9F284">&quot;</span><span style="color:#F1FA8C">Messages must not be empty</span><span style="color:#E9F284">&quot;</span><span style="color:#F8F8F2">);</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#FF79C6"> return</span><span style="color:#F8F8F2"> text;</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<p>You could extend the validation in <code>ValidateMessage</code> in similar ways to <code>ValidateName</code>, or add additional checks to <code>SendMessage</code>, like:</p>
<ul>
<li>Rejecting messages from senders who haven&#x27;t set their names.</li>
<li>Rate-limiting users so they can&#x27;t send new messages too quickly.</li>
</ul>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="set-users-online-status">Set users&#x27; online status<a href="#set-users-online-status" class="hash-link" aria-label="Direct link to Set users&#x27; online status" title="Direct link to Set users&#x27; online status" translate="no">​</a></h2>
<p>In C# modules, you can register for <code>Connect</code> and <code>Disconnect</code> events by using a special <code>ReducerKind</code>. We&#x27;ll use the <code>Connect</code> event to create a <code>User</code> record for the client if it doesn&#x27;t yet exist, and to set its online status.</p>
<p>We&#x27;ll use <code>reducerContext.Db.User.Identity.Find</code> to look up a <code>User</code> row for <code>ctx.Sender</code>, if one exists. If we find one, we&#x27;ll use <code>reducerContext.Db.User.Identity.Update</code> to overwrite it with a row that has <code>Online: true</code>. If not, we&#x27;ll use <code>User.Insert</code> to insert a new row for our new user. All three of these methods are generated by the <code>[SpacetimeDB.Table]</code> attribute, with rows and behavior based on the row attributes. <code>User.Identity.Find</code> returns a nullable <code>User</code>, because the unique constraint from the <code>[PrimaryKey]</code> attribute means there will be either zero or one matching rows. <code>Insert</code> will throw an exception if the insert violates this constraint; if we want to overwrite a <code>User</code> row, we need to do so explicitly using <code>User.Identity.Update</code>.</p>
<p>In <code>server/Lib.cs</code>, add the definition of the connect reducer to the <code>Module</code> class:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#F8F8F2">[</span><span style="color:#8BE9FD;font-style:italic">Reducer</span><span style="color:#F8F8F2">(ReducerKind.ClientConnected)]</span></span>
<span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> void</span><span style="color:#50FA7B"> ClientConnected</span><span style="color:#F8F8F2">(</span><span style="color:#8BE9FD;font-style:italic">ReducerContext</span><span style="color:#FFB86C;font-style:italic"> ctx</span><span style="color:#F8F8F2">)</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#F8F8F2"> Log.</span><span style="color:#50FA7B">Info</span><span style="color:#F8F8F2">(</span><span style="color:#E9F284">$&quot;</span><span style="color:#F1FA8C">Connect </span><span style="color:#FF79C6">{</span><span style="color:#F8F8F2">ctx</span><span style="color:#F1FA8C">.</span><span style="color:#F8F8F2">Sender</span><span style="color:#FF79C6">}</span><span style="color:#E9F284">&quot;</span><span style="color:#F8F8F2">);</span></span>
<span class="line"></span>
<span class="line"><span style="color:#FF79C6"> if</span><span style="color:#F8F8F2"> (ctx.Db.user.Identity.</span><span style="color:#50FA7B">Find</span><span style="color:#F8F8F2">(ctx.Sender) </span><span style="color:#FF79C6">is</span><span style="color:#8BE9FD;font-style:italic"> User</span><span style="color:#F8F8F2"> user)</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#6272A4"> // If this is a returning user, i.e., we already have a `User` with this `Identity`,</span></span>
<span class="line"><span style="color:#6272A4"> // set `Online: true`, but leave `Name` and `Identity` unchanged.</span></span>
<span class="line"><span style="color:#F8F8F2"> user.Online </span><span style="color:#FF79C6">=</span><span style="color:#BD93F9"> true</span><span style="color:#F8F8F2">;</span></span>
<span class="line"><span style="color:#F8F8F2"> ctx.Db.user.Identity.</span><span style="color:#50FA7B">Update</span><span style="color:#F8F8F2">(user);</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#FF79C6"> else</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#6272A4"> // If this is a new user, create a `User` object for the `Identity`,</span></span>
<span class="line"><span style="color:#6272A4"> // which is online, but hasn&#x27;t set a name.</span></span>
<span class="line"><span style="color:#F8F8F2"> ctx.Db.user.</span><span style="color:#50FA7B">Insert</span><span style="color:#F8F8F2">(</span></span>
<span class="line"><span style="color:#FF79C6"> new</span><span style="color:#8BE9FD;font-style:italic"> User</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#F8F8F2"> Name </span><span style="color:#FF79C6">=</span><span style="color:#BD93F9"> null</span><span style="color:#F8F8F2">,</span></span>
<span class="line"><span style="color:#F8F8F2"> Identity </span><span style="color:#FF79C6">=</span><span style="color:#F8F8F2"> ctx.Sender,</span></span>
<span class="line"><span style="color:#F8F8F2"> Online </span><span style="color:#FF79C6">=</span><span style="color:#BD93F9"> true</span><span style="color:#F8F8F2">,</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#F8F8F2"> );</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<p>Similarly, whenever a client disconnects, the database will execute the <code>OnDisconnect</code> event if it&#x27;s registered with <code>ReducerKind.ClientDisconnected</code>. We&#x27;ll use it to un-set the <code>Online</code> status of the <code>User</code> for the disconnected client.</p>
<p>Add the following code after the <code>OnConnect</code> handler:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#F8F8F2">[</span><span style="color:#8BE9FD;font-style:italic">Reducer</span><span style="color:#F8F8F2">(ReducerKind.ClientDisconnected)]</span></span>
<span class="line"><span style="color:#FF79C6">public</span><span style="color:#FF79C6"> static</span><span style="color:#FF79C6"> void</span><span style="color:#50FA7B"> ClientDisconnected</span><span style="color:#F8F8F2">(</span><span style="color:#8BE9FD;font-style:italic">ReducerContext</span><span style="color:#FFB86C;font-style:italic"> ctx</span><span style="color:#F8F8F2">)</span></span>
<span class="line"><span style="color:#F8F8F2">{</span></span>
<span class="line"><span style="color:#FF79C6"> if</span><span style="color:#F8F8F2"> (ctx.Db.user.Identity.</span><span style="color:#50FA7B">Find</span><span style="color:#F8F8F2">(ctx.Sender) </span><span style="color:#FF79C6">is</span><span style="color:#8BE9FD;font-style:italic"> User</span><span style="color:#F8F8F2"> user)</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#6272A4"> // This user should exist, so set `Online: false`.</span></span>
<span class="line"><span style="color:#F8F8F2"> user.Online </span><span style="color:#FF79C6">=</span><span style="color:#BD93F9"> false</span><span style="color:#F8F8F2">;</span></span>
<span class="line"><span style="color:#F8F8F2"> ctx.Db.user.Identity.</span><span style="color:#50FA7B">Update</span><span style="color:#F8F8F2">(user);</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#FF79C6"> else</span></span>
<span class="line"><span style="color:#F8F8F2"> {</span></span>
<span class="line"><span style="color:#6272A4"> // User does not exist, log warning</span></span>
<span class="line"><span style="color:#F8F8F2"> Log.</span><span style="color:#50FA7B">Warn</span><span style="color:#F8F8F2">(</span><span style="color:#E9F284">&quot;</span><span style="color:#F1FA8C">Warning: No user found for disconnected client.</span><span style="color:#E9F284">&quot;</span><span style="color:#F8F8F2">);</span></span>
<span class="line"><span style="color:#F8F8F2"> }</span></span>
<span class="line"><span style="color:#F8F8F2">}</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="start-the-server">Start the Server<a href="#start-the-server" class="hash-link" aria-label="Direct link to Start the Server" title="Direct link to Start the Server" translate="no">​</a></h2>
<p>If you haven&#x27;t already started the SpacetimeDB server, run the <code>spacetime start</code> command in a <em>separate</em> terminal and leave it running while you continue following along.</p>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="publish-the-module">Publish the module<a href="#publish-the-module" class="hash-link" aria-label="Direct link to Publish the module" title="Direct link to Publish the module" translate="no">​</a></h2>
<p>And that&#x27;s all of our module code! We&#x27;ll run <code>spacetime publish</code> to compile our module and publish it on SpacetimeDB. <code>spacetime publish</code> takes an optional name which will map to the database&#x27;s unique address. Clients can connect either by name or by address, but names are much more pleasant. In this example, we&#x27;ll be using <code>quickstart-chat</code>. Feel free to come up with a unique name, and in the CLI commands, replace where we&#x27;ve written <code>quickstart-chat</code> with the name you chose.</p>
<p>From the <code>quickstart-chat</code> directory, run:</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">spacetime</span><span style="color:#F1FA8C"> publish</span><span style="color:#BD93F9"> --project-path</span><span style="color:#F1FA8C"> server</span><span style="color:#F1FA8C"> quickstart-chat</span></span></code></pre>
<p>Note: If the WebAssembly optimizer <code>wasm-opt</code> is installed, <code>spacetime publish</code> will automatically optimize the Web Assembly output of the published module. Instruction for installing the <code>wasm-opt</code> binary can be found in <a href="https://docs.rs/wasm-opt/latest/wasm_opt/" target="_blank" rel="noopener noreferrer">Rust&#x27;s wasm-opt documentation</a>.</p>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="call-reducers">Call Reducers<a href="#call-reducers" class="hash-link" aria-label="Direct link to Call Reducers" title="Direct link to Call Reducers" translate="no">​</a></h2>
<p>You can use the CLI (command line interface) to run reducers. The arguments to the reducer are passed in JSON format.</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">spacetime</span><span style="color:#F1FA8C"> call</span><span style="color:#F1FA8C"> quickstart-chat</span><span style="color:#F1FA8C"> SendMessage</span><span style="color:#E9F284"> &quot;</span><span style="color:#F1FA8C">Hello, World!</span><span style="color:#E9F284">&quot;</span></span></code></pre>
<p>Once we&#x27;ve called our <code>SendMessage</code> reducer, we can check to make sure it ran by running the <code>logs</code> command.</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">spacetime</span><span style="color:#F1FA8C"> logs</span><span style="color:#F1FA8C"> quickstart-chat</span></span></code></pre>
<p>You should now see the output that your module printed in the database.</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">info:</span><span style="color:#F1FA8C"> Hello,</span><span style="color:#F1FA8C"> World!</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="sql-queries">SQL Queries<a href="#sql-queries" class="hash-link" aria-label="Direct link to SQL Queries" title="Direct link to SQL Queries" translate="no">​</a></h2>
<p>SpacetimeDB supports a subset of the SQL syntax so that you can easily query the data of your database. We can run a query using the <code>sql</code> command.</p>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B">spacetime</span><span style="color:#F1FA8C"> sql</span><span style="color:#F1FA8C"> quickstart-chat</span><span style="color:#E9F284"> &quot;</span><span style="color:#F1FA8C">SELECT * FROM message</span><span style="color:#E9F284">&quot;</span></span></code></pre>
<pre tabindex="0" class="codeBlockStandalone_pMzE thin-scrollbar codeBlockContainer_HZVP theme-code-block" style="--prism-color:#bfc7d5;--prism-background-color:#292d3e"><code class="codeBlockLines_ckKi"><span class="line"><span style="color:#50FA7B"> sender</span><span style="color:#FF79C6"> |</span><span style="color:#50FA7B"> sent</span><span style="color:#FF79C6"> |</span><span style="color:#50FA7B"> text</span></span>
<span class="line"><span style="color:#50FA7B">--------------------------------------------------------------------+----------------------------------+-----------------</span></span>
<span class="line"><span style="color:#50FA7B"> 0x93dda09db9a56d8fa6c024d843e805d8262191db3b4ba84c5efcd1ad451fed4e</span><span style="color:#FF79C6"> |</span><span style="color:#50FA7B"> 2025-04-08T15:47:46.935402+00:00</span><span style="color:#FF79C6"> |</span><span style="color:#50FA7B"> &quot;Hello, world!&quot;</span></span></code></pre>
<h2 class="anchor anchorWithStickyNavbar_wKCU" id="whats-next">What&#x27;s next?<a href="#whats-next" class="hash-link" aria-label="Direct link to What&#x27;s next?" title="Direct link to What&#x27;s next?" translate="no">​</a></h2>
<p>You&#x27;ve just set up your first database in SpacetimeDB! You can find the full code for this client <a href="https://github.com/clockworklabs/SpacetimeDB/tree/master/sdks/csharp/examples~/quickstart-chat/server" target="_blank" rel="noopener noreferrer">in the C# server module example</a>.</p>
<p>The next step would be to create a client that interacts with this module. You can use any of SpacetimeDB&#x27;s supported client languages to do this. Take a look at the quick start guide for your client language of choice: <a href="/sdks/rust/quickstart">Rust</a>, <a href="/sdks/c-sharp/quickstart">C#</a>, or <a href="/sdks/typescript/quickstart">TypeScript</a>.</p>
<p>If you are planning to use SpacetimeDB with the Unity game engine, you can skip right to the <a href="/unity/part-1">Unity Comprehensive Tutorial</a>.</p></div></article><nav class="docusaurus-mt-lg pagination-nav" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/modules/rust"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Rust Reference</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/modules/c-sharp"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">C# Reference</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_Ea_L thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#install-spacetimedb" class="table-of-contents__link toc-highlight">Install SpacetimeDB</a></li><li><a href="#install-net-8" class="table-of-contents__link toc-highlight">Install .NET 8</a></li><li><a href="#project-structure" class="table-of-contents__link toc-highlight">Project structure</a></li><li><a href="#declare-imports" class="table-of-contents__link toc-highlight">Declare imports</a></li><li><a href="#define-tables" class="table-of-contents__link toc-highlight">Define tables</a></li><li><a href="#set-users-names" class="table-of-contents__link toc-highlight">Set users&#39; names</a></li><li><a href="#send-messages" class="table-of-contents__link toc-highlight">Send messages</a></li><li><a href="#set-users-online-status" class="table-of-contents__link toc-highlight">Set users&#39; online status</a></li><li><a href="#start-the-server" class="table-of-contents__link toc-highlight">Start the Server</a></li><li><a href="#publish-the-module" class="table-of-contents__link toc-highlight">Publish the module</a></li><li><a href="#call-reducers" class="table-of-contents__link toc-highlight">Call Reducers</a></li><li><a href="#sql-queries" class="table-of-contents__link toc-highlight">SQL Queries</a></li><li><a href="#whats-next" class="table-of-contents__link toc-highlight">What&#39;s next?</a></li></ul></div></div></div></div></main></div></div></div><footer class="theme-layout-footer footer"><div class="container container-fluid"></div></footer></div>
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