diff --git a/.vsts-dotnet-ci.yml b/.vsts-dotnet-ci.yml
index 0cfb136fdf..59e2d324c6 100644
--- a/.vsts-dotnet-ci.yml
+++ b/.vsts-dotnet-ci.yml
@@ -11,59 +11,18 @@ resources:
     image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-mlnet-207e097-20190312152303
 
 jobs:
-- template: /build/ci/job-template.yml
-  parameters:
-    name: Centos_x64_NetCoreApp31
-    buildScript: ./build.sh
-    container: CentosContainer
-    customMatrixes:
-      Debug_Build:
-        _configuration: Debug-netcoreapp3_1
-        _config_short: DI
-        _includeBenchmarkData: false
-        _targetFramework: netcoreapp3.1
-      Release_Build:
-        _configuration: Release-netcoreapp3_1
-        _config_short: RI
-        _includeBenchmarkData: true
-        _targetFramework: netcoreapp3.1
-    innerLoop: true
-    pool:
-      name: Hosted Ubuntu 1604
-
-- template: /build/ci/job-template.yml
-  parameters:
-    name: Ubuntu_x64_NetCoreApp21
-    buildScript: ./build.sh
-    container: UbuntuContainer
-    innerLoop: true
-    pool:
-      name: Hosted Ubuntu 1604
-
-- template: /build/ci/job-template.yml
-  parameters:
-    name: MacOS_x64_NetCoreApp21
-    buildScript: ./build.sh
-    innerLoop: true
-    pool:
-      name: Hosted macOS
-
 - template: /build/ci/job-template.yml
   parameters:
     name: Windows_x64_NetCoreApp31
     buildScript: build.cmd
     customMatrixes:
-      Debug_Build:
-        _configuration: Debug-netcoreapp3_1
-        _config_short: DI
-        _includeBenchmarkData: false
-        _targetFramework: netcoreapp3.1
       Release_Build:
         _configuration: Release-netcoreapp3_1
         _config_short: RI
         _includeBenchmarkData: true
         _targetFramework: netcoreapp3.1
-    innerLoop: true
+    innerLoop: false
+    runSpecific: false
     vsTestConfiguration: "/Framework:.NETCoreApp,Version=v3.0"
     pool:
       name: Hosted VS2017
@@ -72,37 +31,19 @@ jobs:
   parameters:
     name: Windows_x64_NetCoreApp21
     buildScript: build.cmd
-    innerLoop: true
+    innerLoop: false
+    runSpecific: false
     vsTestConfiguration: "/Framework:.NETCoreApp,Version=v2.1"
     pool:
       name: Hosted VS2017
 
-- template: /build/ci/job-template.yml
-  parameters:
-    name: Windows_x64_NetFx461
-    buildScript: build.cmd
-    customMatrixes:
-      Debug_Build:
-        _configuration: Debug-netfx
-        _config_short: DFX
-        _includeBenchmarkData: false
-        _targetFramework: win-x64
-      Release_Build:
-        _configuration: Release-netfx
-        _config_short: RFX
-        _includeBenchmarkData: false
-        _targetFramework: win-x64
-    innerLoop: true
-    vsTestConfiguration: "/Framework:.NETCoreApp,Version=v4.0"
-    pool:
-      name: Hosted VS2017
-
 - template: /build/ci/job-template.yml
   parameters:
     name: Windows_x86_NetCoreApp21
     architecture: x86
     buildScript: build.cmd
-    innerLoop: true
+    innerLoop: false
+    runSpecific: false
     vsTestConfiguration: "/Framework:.NETCoreApp,Version=v2.1"
     pool:
       name: Hosted VS2017
diff --git a/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs b/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
index f776b08f56..4a3efeec3d 100644
--- a/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
+++ b/src/Microsoft.ML.Core/Environment/HostEnvironmentBase.cs
@@ -127,12 +127,18 @@ 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)
             {
                 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)
@@ -385,12 +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));