From 28c14f979bdb9d1ab437a3b82aeff2d71237c833 Mon Sep 17 00:00:00 2001
From: Mustafa Bal <mubal@microsoft.com>
Date: Wed, 1 Apr 2020 14:37:13 -0700
Subject: [PATCH 1/2] Removing unused IHosts

---
 src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs b/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
index f776b08f56..3090a4e5a5 100644
--- a/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
+++ b/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
@@ -127,8 +127,14 @@ public HostBase(HostEnvironmentBase<TEnv> source, string shortName, string paren
                 Depth = source.Depth + 1;
             }
 
+            /// <summary>
+            /// This method registers and returns the host for the calling component. The generated host is also
+            /// added to <see cref="_children"/> and encapsulated by <see cref="WeakReference"/>. It becomes
+            /// necessary to remove these hosts when they are reclaimed by the Garbage Collector.
+            /// </summary>
             public new IHost Register(string name, int? seed = null, bool? verbose = null)
             {
+                _children.RemoveAll(r => r.TryGetTarget(out IHost _) == false);
                 Contracts.CheckNonEmpty(name, nameof(name));
                 IHost host;
                 lock (_cancelLock)

From 4c54ff61d71cb4976a6172b9e95aa9b76d98d4d1 Mon Sep 17 00:00:00 2001
From: Mustafa Bal <mubal@microsoft.com>
Date: Wed, 1 Apr 2020 19:22:33 -0700
Subject: [PATCH 2/2] Removed unused hosts in both Register methods

---
 src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs b/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
index 3090a4e5a5..4a3efeec3d 100644
--- a/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
+++ b/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
@@ -134,11 +134,11 @@ public HostBase(HostEnvironmentBase<TEnv> source, string shortName, string paren
             /// </summary>
             public new IHost Register(string name, int? seed = null, bool? verbose = null)
             {
-                _children.RemoveAll(r => r.TryGetTarget(out IHost _) == false);
                 Contracts.CheckNonEmpty(name, nameof(name));
                 IHost host;
                 lock (_cancelLock)
                 {
+                    _children.RemoveAll(r => r.TryGetTarget(out IHost _) == false);
                     Random rand = (seed.HasValue) ? RandomUtils.Create(seed.Value) : RandomUtils.Create(_rand);
                     host = RegisterCore(this, name, Master?.FullName, rand, verbose ?? Verbose);
                     if (!IsCanceled)
@@ -391,12 +391,18 @@ protected HostEnvironmentBase(HostEnvironmentBase<TEnv> source, Random rand, boo
             _children = new List<WeakReference<IHost>>();
         }
 
+        /// <summary>
+        /// This method registers and returns the host for the calling component. The generated host is also
+        /// added to <see cref="_children"/> and encapsulated by <see cref="WeakReference"/>. It becomes
+        /// necessary to remove these hosts when they are reclaimed by the Garbage Collector.
+        /// </summary>
         public IHost Register(string name, int? seed = null, bool? verbose = null)
         {
             Contracts.CheckNonEmpty(name, nameof(name));
             IHost host;
             lock (_cancelLock)
             {
+                _children.RemoveAll(r => r.TryGetTarget(out IHost _) == false);
                 Random rand = (seed.HasValue) ? RandomUtils.Create(seed.Value) : RandomUtils.Create(_rand);
                 host = RegisterCore(this, name, Master?.FullName, rand, verbose ?? Verbose);
                 _children.Add(new WeakReference<IHost>(host));