diff --git a/Ragon.Core/Lobby/Abstract/ILobby.cs b/Ragon.Core/Lobby/Abstract/ILobby.cs
index 112b499..7425560 100644
--- a/Ragon.Core/Lobby/Abstract/ILobby.cs
+++ b/Ragon.Core/Lobby/Abstract/ILobby.cs
@@ -1,11 +1,12 @@
-using Ragon.Core.Game;
-
-namespace Ragon.Core.Lobby;
-
-public interface ILobby
-{
-  public bool FindRoomById(string roomId, out Room room);
-  public bool FindRoomByMap(string map, out Room room);
-  public void Persist(Room room);
-  public void Remove(Room room);
+using System.Diagnostics.CodeAnalysis;
+using Ragon.Core.Game;
+
+namespace Ragon.Core.Lobby;
+
+public interface ILobby
+{
+  public bool FindRoomById(string roomId, [MaybeNullWhen(false)] out Room room);
+  public bool FindRoomByMap(string map, [MaybeNullWhen(false)] out Room room);
+  public void Persist(Room room);
+  public void Remove(Room room);
 }
\ No newline at end of file
diff --git a/Ragon.Core/Lobby/LobbyInMemory.cs b/Ragon.Core/Lobby/LobbyInMemory.cs
index 676eabe..61c596f 100644
--- a/Ragon.Core/Lobby/LobbyInMemory.cs
+++ b/Ragon.Core/Lobby/LobbyInMemory.cs
@@ -1,56 +1,57 @@
-using System.Collections.Generic;
-using NLog;
-using Ragon.Core.Game;
-
-namespace Ragon.Core.Lobby;
-
-public class LobbyInMemory: ILobby
-{
-  private readonly List<Room> _rooms = new();
-  private readonly Logger _logger = LogManager.GetCurrentClassLogger();
-  
-  public bool FindRoomById(string roomId, out Room room)
-  {
-    foreach (var existRoom in _rooms)
-    {
-      var info = existRoom.Info;
-      if (existRoom.Id == roomId && info.Min < info.Max)
-      {
-        room = existRoom;
-        return true;
-      }
-    }
-
-    room = null;
-    return false;
-  }
-
-  public bool FindRoomByMap(string map, out Room room)
-  {
-    foreach (var existRoom in _rooms)
-    {
-      var info = existRoom.Info;
-      if (info.Map == map && existRoom.Players.Count < info.Max)
-      {
-        room = existRoom;
-        return true;
-      }
-    }
-
-    room = null;
-    return false;
-  }
-
-  public void Persist(Room room)
-  {
-    _rooms.Add(room);
-
-    foreach (var r in _rooms)
-      _logger.Trace($"{r.Id} {r.Info}");
-  }
-
-  public void Remove(Room room)
-  {
-    _rooms.Remove(room);
-  }
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using NLog;
+using Ragon.Core.Game;
+
+namespace Ragon.Core.Lobby;
+
+public class LobbyInMemory : ILobby
+{
+  private readonly List<Room> _rooms = new();
+  private readonly Logger _logger = LogManager.GetCurrentClassLogger();
+
+  public bool FindRoomById(string roomId, [MaybeNullWhen(false)] out Room room)
+  {
+    foreach (var existRoom in _rooms)
+    {
+      var info = existRoom.Info;
+      if (existRoom.Id == roomId && info.Min < info.Max)
+      {
+        room = existRoom;
+        return true;
+      }
+    }
+
+    room = default;
+    return false;
+  }
+
+  public bool FindRoomByMap(string map, [MaybeNullWhen(false)] out Room room)
+  {
+    foreach (var existRoom in _rooms)
+    {
+      var info = existRoom.Info;
+      if (info.Map == map && existRoom.Players.Count < info.Max)
+      {
+        room = existRoom;
+        return true;
+      }
+    }
+
+    room = default;
+    return false;
+  }
+
+  public void Persist(Room room)
+  {
+    _rooms.Add(room);
+
+    foreach (var r in _rooms)
+      _logger.Trace($"{r.Id} {r.Info}");
+  }
+
+  public void Remove(Room room)
+  {
+    _rooms.Remove(room);
+  }
 }
\ No newline at end of file