remove subscriptions race

src/Microsoft.ApplicationInsights.AspNetCore/Implementation/ApplicationInsightsInitializer.cs

@@ -2,6 +2,7 @@ namespace Microsoft.ApplicationInsights.AspNetCore
 {
     using System;
     using System.Collections.Generic;
+    using System.Collections.Concurrent;
     using System.Diagnostics;
     using Extensions;
     using Microsoft.ApplicationInsights.AspNetCore.DiagnosticListeners;
@@ -13,8 +14,9 @@ namespace Microsoft.ApplicationInsights.AspNetCore
     /// </summary>
     internal class ApplicationInsightsInitializer : IObserver<DiagnosticListener>, IDisposable
     {
-        private readonly List<IDisposable> subscriptions;
+        private readonly ConcurrentBag<IDisposable> subscriptions;
         private readonly IEnumerable<IApplicationInsightDiagnosticListener> diagnosticListeners;
+        private volatile bool shuttingDown = false;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ApplicationInsightsInitializer"/> class.
@@ -26,7 +28,7 @@ public ApplicationInsightsInitializer(
             IServiceProvider serviceProvider)
         {
             this.diagnosticListeners = diagnosticListeners;
-            this.subscriptions = new List<IDisposable>();
+            this.subscriptions = new ConcurrentBag<IDisposable>();
 
             // Add default logger factory for debug mode only if enabled and instrumentation key not set
             if (options.Value.EnableDebugLogger && string.IsNullOrEmpty(options.Value.InstrumentationKey))
@@ -48,6 +50,11 @@ public void Start()
         /// <inheritdoc />
         void IObserver<DiagnosticListener>.OnNext(DiagnosticListener value)
         {
+            if (shuttingDown)
+            {
+                return;
+            }
+
             foreach (var applicationInsightDiagnosticListener in this.diagnosticListeners)
             {
                 if (applicationInsightDiagnosticListener.ListenerName == value.Name)
@@ -75,13 +82,16 @@ public void Dispose()
 
         protected virtual void Dispose(bool disposing)
         {
-            if (disposing)
+            if (!disposing)
             {
-                foreach (var subscription in this.subscriptions)
-                {
-                    subscription.Dispose();
-                }
+                return;
+            }
+
+            shuttingDown = true;
+            foreach (var subscription in this.subscriptions)
+            {
+                subscription.Dispose();
             }
         }
     }
-}
+}