fix rare race condition with asteroid cache

On a very rare occasion, a race would result in a collection was
modified exception would be thrown when traversing the collection
of asteroids in the CacheManager.  This adds a lock to block when
accessed from multiple threads.

Asteroids.Standard/Managers/CacheManager.cs

@@ -1,9 +1,8 @@
-using System.Collections.Generic;
-using System.Collections.ObjectModel;
+using Asteroids.Standard.Components;
+using Asteroids.Standard.Screen;
+using System.Collections.Generic;
 using System.Drawing;
 using System.Linq;
-using Asteroids.Standard.Components;
-using Asteroids.Standard.Screen;
 
 namespace Asteroids.Standard.Managers
 {
@@ -23,15 +22,16 @@ public CacheManager(ScoreManager score, Ship? ship, AsteroidBelt belt, IList<Bul
             Score = score;
             Ship = ship;
             Belt = belt;
-            _bullets = bullets;
+            _bullets = bullets.ToList();
 
             _bulletLock = new object();
             _explosionLock = new object();
+            _asteroidsLock = new object();
 
             _explosions = new List<Explosion>();
             _bulletsInFlight = new List<CachedObject<Bullet>>();
             _bulletsAvailable = new List<CachedObject<Bullet>>();
-            Asteroids = new List<CachedObject<Asteroid>>();
+            _asteroids = new List<CachedObject<Asteroid>>();
 
             Repopulate();
         }
@@ -43,11 +43,13 @@ public CacheManager(ScoreManager score, Ship? ship, AsteroidBelt belt, IList<Bul
         //Read-only
         private readonly object _bulletLock;
         private readonly object _explosionLock;
+        private readonly object _asteroidsLock;
 
-        private readonly IList<Explosion> _explosions;
-        private readonly IList<Bullet> _bullets;
-        private readonly IList<CachedObject<Bullet>> _bulletsInFlight;
-        private readonly IList<CachedObject<Bullet>> _bulletsAvailable;
+        private readonly List<Explosion> _explosions;
+        private readonly List<Bullet> _bullets;
+        private readonly List<CachedObject<Bullet>> _bulletsInFlight;
+        private readonly List<CachedObject<Bullet>> _bulletsAvailable;
+        private readonly List<CachedObject<Asteroid>> _asteroids;
 
         public ScoreManager Score { get; }
 
@@ -66,11 +68,6 @@ public CacheManager(ScoreManager score, Ship? ship, AsteroidBelt belt, IList<Bul
         /// </summary>
         public AsteroidBelt Belt { get; private set; }
 
-        /// <summary>
-        /// Collection of cached <see cref="Asteroid"/>s with parameters cached for optimization. 
-        /// </summary>
-        public IList<CachedObject<Asteroid>> Asteroids { get; private set; }
-
         /// <summary>
         /// Collection of points associated with the <see cref="Saucer"/>, if present.
         /// </summary>
@@ -155,10 +152,28 @@ public void UpdateBelt(AsteroidBelt belt)
         {
             Belt = belt;
 
-            Asteroids = Belt
+            var list = Belt
                 .GetAsteroids()
                 .Select(a => new CachedObject<Asteroid>(a))
                 .ToList();
+
+            lock (_asteroidsLock)
+            {
+                _asteroids.Clear();
+                _asteroids.AddRange(list);
+            }
+        }
+
+        /// <summary>
+        /// Returns the current collection of cached Asteroids in a thread-safe manor.
+        /// </summary>
+        /// <returns>Disconnected list of cached Asteroids.</returns>
+        public IList<CachedObject<Asteroid>> GetAsteroids()
+        {
+            lock (_asteroidsLock)
+            {
+                return _asteroids.ToList();
+            }
         }
 
         /// <summary>
@@ -167,8 +182,11 @@ public void UpdateBelt(AsteroidBelt belt)
         /// </summary>
         public void AddAsteroid(Asteroid asteroid)
         {
-            Asteroids.Add(new CachedObject<Asteroid>(asteroid));
-            Belt.SetAsteroids(Asteroids.Select(c => c.ScreenObject).ToList());
+            lock (_asteroidsLock)
+            {
+                _asteroids.Add(new CachedObject<Asteroid>(asteroid));
+                Belt.SetAsteroids(_asteroids.Select(c => c.ScreenObject).ToList());
+            }
         }
 
         /// <summary>
@@ -177,8 +195,11 @@ public void AddAsteroid(Asteroid asteroid)
         /// </summary>
         public void RemoveAsteroid(int index)
         {
-            Asteroids.RemoveAt(index);
-            Belt.SetAsteroids(Asteroids.Select(c => c.ScreenObject).ToList());
+            lock (_asteroidsLock)
+            {
+                _asteroids.RemoveAt(index);
+                Belt.SetAsteroids(_asteroids.Select(c => c.ScreenObject).ToList());
+            }
         }
 
         /// <summary>

Asteroids.Standard/Managers/CollisionManager.cs

@@ -43,7 +43,7 @@ public CollisionManager(CacheManager cache)
         public bool AsteroidBeltCollision(IList<Point> pointsToCheck)
         {
             //Get a copy for collection adds/removes
-            var asteroids = _cache.Asteroids.ToList();
+            var asteroids = _cache.GetAsteroids();
             var score = 0;
 
             //Go through each but break on first hit
@@ -131,7 +131,7 @@ public bool IsCenterSafe()
         {
             bool safe = true;
 
-            foreach (var asteroid in _cache.Asteroids)
+            foreach (var asteroid in _cache.GetAsteroids())
             {
                 var separation = asteroid
                     .Location
@@ -222,7 +222,7 @@ public void MoveBullets()
         public void MoveAsteroids()
         {
             //Move does not use locks
-            foreach (var asteroid in _cache.Asteroids)
+            foreach (var asteroid in _cache.GetAsteroids())
                 asteroid.ScreenObject.Move();
         }
 

Asteroids.Standard/Managers/DrawingManager.cs

@@ -156,7 +156,7 @@ private void DrawBullets()
         private void DrawBelt()
         {
             var asteroids = _cache
-                 .Asteroids
+                 .GetAsteroids()
                  .Where(a => a.ScreenObject.Size != Asteroid.AsteroidSize.Dne);
 
             foreach (var asteroid in asteroids)

Asteroids.Standard/Screen/TitleScreen.cs

@@ -112,7 +112,7 @@ public void DrawScreen()
             // Draw the asteroid belt
             _cache.Repopulate();
 
-            foreach (var asteroid in _cache.Asteroids)
+            foreach (var asteroid in _cache.GetAsteroids())
             {
                 asteroid.ScreenObject.Move();
                 _canvas.LoadPolygon(asteroid.PolygonPoints, DrawColor.White);