Refactor mailbox interface and factories

src/Proto.Actor/Mailbox/BatchingMailbox.cs

@@ -21,7 +21,10 @@ public class BatchingMailbox : IMailbox
     private IDispatcher _dispatcher = null!;
     private IMessageInvoker _invoker = null!;
 
-    private int _status = MailboxStatus.Idle;
+    private const int Idle = 0;
+    private const int Busy = 1;
+
+    private int _status = Idle;
     private bool _suspended;
 
     public BatchingMailbox(int batchSize)
@@ -99,7 +102,7 @@ private async Task RunAsync()
             _invoker.EscalateFailure(x, currentMessage);
         }
 
-        Interlocked.Exchange(ref _status, MailboxStatus.Idle);
+        Interlocked.Exchange(ref _status, Idle);
 
         if (_systemMessages.HasMessages || (_userMessages.HasMessages && !_suspended))
         {
@@ -109,7 +112,7 @@ private async Task RunAsync()
 
     private void Schedule()
     {
-        if (Interlocked.CompareExchange(ref _status, MailboxStatus.Busy, MailboxStatus.Idle) == MailboxStatus.Idle)
+        if (Interlocked.CompareExchange(ref _status, Busy, Idle) == Idle)
         {
             _dispatcher.Schedule(RunAsync);
         }

src/Proto.Actor/Mailbox/BoundedMailbox.cs

@@ -0,0 +1,21 @@
+// -----------------------------------------------------------------------
+// <copyright file="BoundedMailbox.cs" company="Asynkron AB">
+//      Copyright (C) 2015-2025 Asynkron AB All rights reserved
+// </copyright>
+// -----------------------------------------------------------------------
+
+using System.Threading.Channels;
+
+namespace Proto.Mailbox;
+
+/// <summary>
+/// Factory helpers for creating bounded mailboxes.
+/// </summary>
+public static class BoundedMailbox
+{
+    public static IMailbox Create(int size, params IMailboxStatistics[] stats) =>
+        new DefaultMailbox(new LockingUnboundedMailboxQueue(4), new BoundedMailboxQueue(size), stats);
+
+    public static IMailbox Create(int size, BoundedChannelFullMode fullMode, params IMailboxStatistics[] stats) =>
+        new DefaultMailbox(new LockingUnboundedMailboxQueue(4), new BoundedMailboxQueue(size, fullMode), stats);
+}

src/Proto.Actor/Mailbox/Mailbox.cs → src/Proto.Actor/Mailbox/DefaultMailbox.cs

@@ -6,55 +6,23 @@
 
 using System;
 using System.Threading;
-using System.Threading.Channels;
 using System.Threading.Tasks;
 
 namespace Proto.Mailbox;
 
-internal static class MailboxStatus
-{
-    public const int Idle = 0;
-    public const int Busy = 1;
-}
-
-public interface IMailbox
-{
-    int UserMessageCount { get; }
-
-    void PostUserMessage(object msg);
-
-    void PostSystemMessage(object msg);
-
-    void RegisterHandlers(IMessageInvoker invoker, IDispatcher dispatcher);
-
-    void Start();
-}
-
-public static class BoundedMailbox
-{
-    public static IMailbox Create(int size, params IMailboxStatistics[] stats) =>
-        new DefaultMailbox(new LockingUnboundedMailboxQueue(4), new BoundedMailboxQueue(size), stats);
-
-    public static IMailbox Create(int size, BoundedChannelFullMode fullMode, params IMailboxStatistics[] stats) =>
-        new DefaultMailbox(new LockingUnboundedMailboxQueue(4), new BoundedMailboxQueue(size, fullMode), stats);
-}
-
-public static class UnboundedMailbox
-{
-    public static IMailbox Create(params IMailboxStatistics[] stats) =>
-        new DefaultMailbox(new LockingUnboundedMailboxQueue(4), new UnboundedMailboxQueue(), stats);
-}
-
 public sealed class DefaultMailbox : IMailbox, IThreadPoolWorkItem
 
 {
+    private const int Idle = 0;
+    private const int Busy = 1;
+
     private readonly IMailboxStatistics[] _stats;
     private readonly IMailboxQueue _systemMessages;
     private readonly IMailboxQueue _userMailbox;
     private IDispatcher _dispatcher;
     private IMessageInvoker _invoker;
 
-    private long _status = MailboxStatus.Idle;
+    private long _status = Idle;
     private bool _suspended;
 
     public DefaultMailbox(
@@ -174,7 +142,7 @@ private static Task RunAsync(DefaultMailbox mailbox)
             return Await(mailbox, task);
         }
 
-        Interlocked.Exchange(ref mailbox._status, MailboxStatus.Idle);
+        Interlocked.Exchange(ref mailbox._status, Idle);
 
         if (mailbox._systemMessages.HasMessages || mailbox is { _suspended: false, _userMailbox.HasMessages: true })
         {
@@ -194,7 +162,7 @@ static async Task Await(DefaultMailbox self, Task task)
         {
             await task.ConfigureAwait(false);
 
-            Interlocked.Exchange(ref self._status, MailboxStatus.Idle);
+            Interlocked.Exchange(ref self._status, Idle);
 
             if (self._systemMessages.HasMessages || self is { _suspended: false, _userMailbox.HasMessages: true })
             {
@@ -300,7 +268,7 @@ static async Task Await(object msg, Task task, DefaultMailbox self)
 
     private void Schedule()
     {
-        if (Interlocked.CompareExchange(ref _status, MailboxStatus.Busy, MailboxStatus.Idle) == MailboxStatus.Idle)
+        if (Interlocked.CompareExchange(ref _status, Busy, Idle) == Idle)
         {
             if (_dispatcher == Dispatchers.DefaultDispatcher)
             {

src/Proto.Actor/Mailbox/IMailbox.cs

@@ -0,0 +1,23 @@
+// -----------------------------------------------------------------------
+// <copyright file="IMailbox.cs" company="Asynkron AB">
+//      Copyright (C) 2015-2025 Asynkron AB All rights reserved
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Proto.Mailbox;
+
+/// <summary>
+/// Defines the core operations of a mailbox used by actors.
+/// </summary>
+public interface IMailbox
+{
+    int UserMessageCount { get; }
+
+    void PostUserMessage(object msg);
+
+    void PostSystemMessage(object msg);
+
+    void RegisterHandlers(IMessageInvoker invoker, IDispatcher dispatcher);
+
+    void Start();
+}

src/Proto.Actor/Mailbox/UnboundedMailbox.cs

@@ -0,0 +1,16 @@
+// -----------------------------------------------------------------------
+// <copyright file="UnboundedMailbox.cs" company="Asynkron AB">
+//      Copyright (C) 2015-2025 Asynkron AB All rights reserved
+// </copyright>
+// -----------------------------------------------------------------------
+
+namespace Proto.Mailbox;
+
+/// <summary>
+/// Factory helpers for creating unbounded mailboxes.
+/// </summary>
+public static class UnboundedMailbox
+{
+    public static IMailbox Create(params IMailboxStatistics[] stats) =>
+        new DefaultMailbox(new LockingUnboundedMailboxQueue(4), new UnboundedMailboxQueue(), stats);
+}

tests/Proto.Actor.Tests/Mailbox/MailboxSchedulingTests.cs

@@ -127,6 +127,7 @@ public async Task GivenNonCompletedUserMessage_ShouldSetMailboxToIdleAfterComple
         msg1.TaskCompletionSource.SetResult(0);
         await Task.Delay(1000);
 
-        Assert.Equal(MailboxStatus.Idle, mailbox.Status);
+        // Mailbox becomes idle (status 0) after completing the user message
+        Assert.Equal(0, mailbox.Status);
     }
 }