diff --git a/src/WireMock.Net/Http/PortUtil.cs b/src/WireMock.Net/Http/PortUtil.cs
index 519a9902..d98a2e30 100644
--- a/src/WireMock.Net/Http/PortUtil.cs
+++ b/src/WireMock.Net/Http/PortUtil.cs
@@ -9,6 +9,7 @@ namespace WireMock.Http
     /// </summary>
     public static class PortUtil
     {
+        private static readonly IPEndPoint DefaultLoopbackEndpoint = new IPEndPoint(IPAddress.Loopback, port: 0);
         private static readonly Regex UrlDetailsRegex = new Regex(@"^(?<proto>\w+)://[^/]+?(?<port>\d+)?/", RegexOptions.Compiled);
 
         /// <summary>
@@ -17,17 +18,10 @@ namespace WireMock.Http
         /// <remarks>see http://stackoverflow.com/questions/138043/find-the-next-tcp-port-in-net.</remarks>
         public static int FindFreeTcpPort()
         {
-            TcpListener tcpListener = null;
-            try
+            using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
             {
-                tcpListener = new TcpListener(IPAddress.Loopback, 0);
-                tcpListener.Start();
-
-                return ((IPEndPoint)tcpListener.LocalEndpoint).Port;
-            }
-            finally
-            {
-                tcpListener?.Stop();
+                socket.Bind(DefaultLoopbackEndpoint);
+                return ((IPEndPoint)socket.LocalEndPoint).Port;
             }
         }
 
diff --git a/src/WireMock.Net/Owin/WireMockMiddleware.cs b/src/WireMock.Net/Owin/WireMockMiddleware.cs
index b065090e..dbfc24c6 100644
--- a/src/WireMock.Net/Owin/WireMockMiddleware.cs
+++ b/src/WireMock.Net/Owin/WireMockMiddleware.cs
@@ -58,7 +58,7 @@ namespace WireMock.Owin
                     // Set start
                     if (!_options.Scenarios.ContainsKey(mapping.Scenario) && mapping.IsStartState)
                     {
-                        _options.Scenarios.Add(mapping.Scenario, null);
+                        _options.Scenarios.TryAdd(mapping.Scenario, null);
                     }
                 }
 
diff --git a/src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs b/src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs
index 54b84d1b..694ec065 100644
--- a/src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs
+++ b/src/WireMock.Net/Owin/WireMockMiddlewareOptions.cs
@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Concurrent;
-using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using WireMock.Logging;
 using WireMock.Matchers;
@@ -23,7 +22,9 @@ namespace WireMock.Owin
 
         public bool AllowPartialMapping { get; set; }
 
-        public IDictionary<Guid, Mapping> Mappings { get; } = new ConcurrentDictionary<Guid, Mapping>();
+        public ConcurrentDictionary<Guid, Mapping> Mappings { get; } = new ConcurrentDictionary<Guid, Mapping>(); // Checked
+
+        public ConcurrentDictionary<string, object> Scenarios { get; } = new ConcurrentDictionary<string, object>(); // Checked
 
         public ObservableCollection<LogEntry> LogEntries { get; } = new ConcurentObservableCollection<LogEntry>();
 
@@ -31,8 +32,6 @@ namespace WireMock.Owin
 
         public int? MaxRequestLogCount { get; set; }
 
-        public IDictionary<string, object> Scenarios { get; } = new ConcurrentDictionary<string, object>();
-
 #if !NETSTANDARD
         public Action<IAppBuilder> PreWireMockMiddlewareInit { get; set; }
 
diff --git a/src/WireMock.Net/ResponseMessage.cs b/src/WireMock.Net/ResponseMessage.cs
index ccdce00d..79551b67 100644
--- a/src/WireMock.Net/ResponseMessage.cs
+++ b/src/WireMock.Net/ResponseMessage.cs
@@ -1,5 +1,4 @@
-using System.Collections.Concurrent;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using WireMock.Util;
@@ -8,14 +7,14 @@ using WireMock.Validation;
 namespace WireMock
 {
     /// <summary>
-    /// The response.
+    /// The ResponseMessage.
     /// </summary>
     public class ResponseMessage
     {
         /// <summary>
         /// Gets the headers.
         /// </summary>
-        public IDictionary<string, WireMockList<string>> Headers { get; set; } = new ConcurrentDictionary<string, WireMockList<string>>();
+        public IDictionary<string, WireMockList<string>> Headers { get; set; } = new Dictionary<string, WireMockList<string>>();
 
         /// <summary>
         /// Gets or sets the status code.
diff --git a/src/WireMock.Net/Server/FluentMockServer.Admin.cs b/src/WireMock.Net/Server/FluentMockServer.Admin.cs
index 124215ed..40cd60b7 100644
--- a/src/WireMock.Net/Server/FluentMockServer.Admin.cs
+++ b/src/WireMock.Net/Server/FluentMockServer.Admin.cs
@@ -210,7 +210,7 @@ namespace WireMock.Server
             if (settings.SaveMapping)
             {
                 var mapping = ToMapping(requestMessage, responseMessage, settings.BlackListedHeaders ?? new string[] { });
-                _options.Mappings.Add(mapping.Guid, mapping);
+                _options.Mappings.TryAdd(mapping.Guid, mapping);
 
                 if (settings.SaveMappingToFile)
                 {
@@ -577,7 +577,7 @@ namespace WireMock.Server
         #region Scenarios
         private ResponseMessage ScenariosGet(RequestMessage requestMessage)
         {
-            var scenarios = Scenarios.Select(s => new
+            var scenarios = Scenarios.ToArray().Select(s => new
             {
                 Name = s.Key,
                 Started = s.Value != null,
diff --git a/src/WireMock.Net/Server/FluentMockServer.cs b/src/WireMock.Net/Server/FluentMockServer.cs
index 7804a434..e941dcd9 100644
--- a/src/WireMock.Net/Server/FluentMockServer.cs
+++ b/src/WireMock.Net/Server/FluentMockServer.cs
@@ -56,7 +56,7 @@ namespace WireMock.Server
         /// Gets the scenarios.
         /// </summary>
         [PublicAPI]
-        public IDictionary<string, object> Scenarios => new ConcurrentDictionary<string, object>(_options.Scenarios);
+        public ConcurrentDictionary<string, object> Scenarios => new ConcurrentDictionary<string, object>(_options.Scenarios); // Checked
 
         #region Start/Stop
         /// <summary>
@@ -292,9 +292,9 @@ namespace WireMock.Server
         [PublicAPI]
         public void ResetMappings()
         {
-            foreach (var nonAdmin in _options.Mappings.Where(m => !m.Value.IsAdminInterface))
+            foreach (var nonAdmin in _options.Mappings.ToArray().Where(m => !m.Value.IsAdminInterface))
             {
-                _options.Mappings.Remove(nonAdmin);
+                _options.Mappings.TryRemove(nonAdmin.Key, out _);
             }
         }
 
@@ -308,7 +308,7 @@ namespace WireMock.Server
             // Check a mapping exists with the same GUID, if so, remove it.
             if (_options.Mappings.ContainsKey(guid))
             {
-                return _options.Mappings.Remove(guid);
+                return _options.Mappings.TryRemove(guid, out _);
             }
 
             return false;
@@ -317,7 +317,7 @@ namespace WireMock.Server
         private bool DeleteMapping(string path)
         {
             // Check a mapping exists with the same path, if so, remove it.
-            var mapping = _options.Mappings.FirstOrDefault(entry => string.Equals(entry.Value.Path, path, StringComparison.OrdinalIgnoreCase));
+            var mapping = _options.Mappings.ToArray().FirstOrDefault(entry => string.Equals(entry.Value.Path, path, StringComparison.OrdinalIgnoreCase));
             return DeleteMapping(mapping.Key);
         }
 
@@ -415,7 +415,7 @@ namespace WireMock.Server
             }
             else
             {
-                _options.Mappings.Add(mapping.Guid, mapping);
+                _options.Mappings.TryAdd(mapping.Guid, mapping);
             }
         }
     }