When accessing a pad-service from the SD service container, the SD.MainThread.VerifyAccess() call is now performed on every GetService() call, not just on the first call.

src/Main/Base/Project/ICSharpCode.SharpDevelop.addin

@@ -172,7 +172,6 @@
 		     title    = "${res:MainWindow.Windows.OutputWindow}"
 		     icon     = "PadIcons.Output"
 		     class    = "ICSharpCode.SharpDevelop.Gui.CompilerMessageView"
-		     serviceInterface = "ICSharpCode.SharpDevelop.Workbench.IOutputPad"
 		     defaultPosition = "Bottom" />
 		
 		<Pad id       = "PropertyPad"

src/Main/Base/Project/Services/SD.cs

@@ -44,7 +44,8 @@ namespace ICSharpCode.SharpDevelop
 		/// </summary>
 		public static void InitializeForUnitTests()
 		{
-			var container = new SharpDevelopServiceContainer(ServiceSingleton.FallbackServiceProvider);
+			var container = new SharpDevelopServiceContainer();
+			container.AddFallbackProvider(ServiceSingleton.FallbackServiceProvider);
 			container.AddService(typeof(IPropertyService), new PropertyServiceImpl());
 			container.AddService(typeof(IAddInTree), new AddInTreeImpl(null));
 			ServiceSingleton.ServiceProvider = container;

src/Main/Base/Project/Util/SharpDevelopServiceContainer.cs

@@ -2,6 +2,7 @@
 // This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
 
 using System;
+using System.Collections.Concurrent;
 using System.ComponentModel.Design;
 using System.Linq;
 using System.Collections.Generic;
@@ -14,7 +15,7 @@ namespace ICSharpCode.SharpDevelop
 	/// </summary>
 	sealed class SharpDevelopServiceContainer : IServiceProvider, IServiceContainer, IDisposable
 	{
-		readonly IServiceProvider parentProvider;
+		readonly ConcurrentStack<IServiceProvider> fallbackProviders = new ConcurrentStack<IServiceProvider>();
 		readonly Dictionary<Type, object> services = new Dictionary<Type, object>();
 		readonly List<Type> servicesToDispose = new List<Type>();
 		readonly Dictionary<Type, object> taskCompletionSources = new Dictionary<Type, object>(); // object = TaskCompletionSource<T> for various T
@@ -25,9 +26,9 @@ namespace ICSharpCode.SharpDevelop
 			services.Add(typeof(IServiceContainer), this);
 		}
 		
-		public SharpDevelopServiceContainer(IServiceProvider parentProvider) : this()
+		public void AddFallbackProvider(IServiceProvider provider)
 		{
-			this.parentProvider = parentProvider;
+			this.fallbackProviders.Push(provider);
 		}
 		
 		public object GetService(Type serviceType)
@@ -50,8 +51,12 @@ namespace ICSharpCode.SharpDevelop
 			}
 			if (instance != null)
 				return instance;
-			else
-				return parentProvider != null ? parentProvider.GetService(serviceType) : null;
+			foreach (var fallbackProvider in fallbackProviders) {
+				instance = fallbackProvider.GetService(serviceType);
+				if (instance != null)
+					return instance;
+			}
+			return null;
 		}
 		
 		public void Dispose()

src/Main/Base/Project/Workbench/IOutputPad.cs

@@ -3,6 +3,7 @@
 
 using System;
 using ICSharpCode.AvalonEdit.Highlighting;
+using ICSharpCode.Core;
 
 namespace ICSharpCode.SharpDevelop.Workbench
 {
@@ -10,6 +11,8 @@ namespace ICSharpCode.SharpDevelop.Workbench
 	/// The 'Output' pad.
 	/// Allows showing a text log to the user.
 	/// </summary>
+	/// <remarks>This service is thread-safe.</remarks>
+	[SDService("SD.OutputPad")]
 	public interface IOutputPad
 	{
 		/// <summary>
@@ -39,6 +42,10 @@ namespace ICSharpCode.SharpDevelop.Workbench
 		IOutputCategory BuildCategory { get; }
 	}
 	
+	/// <summary>
+	/// Represents a category in the <see cref="IOutputPad"/>.
+	/// </summary>
+	/// <remarks>This interface is thread-safe.</remarks>
 	public interface IOutputCategory
 	{
 		/// <summary>

src/Main/SharpDevelop/Sda/CallHelper.cs

@@ -38,7 +38,8 @@ namespace ICSharpCode.SharpDevelop.Sda
 		public void InitSharpDevelopCore(SharpDevelopHost.CallbackHelper callback, StartupSettings properties)
 		{
 			// Initialize the most important services:
-			var container = new SharpDevelopServiceContainer(ServiceSingleton.FallbackServiceProvider);
+			var container = new SharpDevelopServiceContainer();
+			container.AddFallbackProvider(ServiceSingleton.FallbackServiceProvider);
 			container.AddService(typeof(IMessageService), new SDMessageService());
 			container.AddService(typeof(ILoggingService), new log4netLoggingService());
 			ServiceSingleton.ServiceProvider = container;

src/Main/SharpDevelop/SharpDevelop.csproj

@@ -212,6 +212,7 @@
     <Compile Include="Workbench\FullScreenEnabledWindow.cs" />
     <Compile Include="Workbench\IWorkbenchLayout.cs" />
     <Compile Include="Workbench\LayoutConfiguration.cs" />
+    <Compile Include="Workbench\PadServiceProvider.cs" />
     <Compile Include="Workbench\RecentOpen.cs" />
     <Compile Include="Workbench\SDStatusBar.cs" />
     <Compile Include="Workbench\ShutdownService.cs" />

src/Main/SharpDevelop/Workbench/PadServiceProvider.cs

@@ -0,0 +1,38 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.Collections.Generic;
+
+namespace ICSharpCode.SharpDevelop.Workbench
+{
+	/// <summary>
+	/// A service provider that provides the pad services (as declared by the 'serviceInterface' attribute in the &lt;Pad&gt; codon).
+	/// </summary>
+	/// <remarks>
+	/// We register this as a fallback provider for the SD.Services container instead of directly registering the pads (using ServiceCreationCallback)
+	/// so that the thread-safety check in the .PadContent property accessor runs on every access to the service.
+	/// </remarks>
+	class PadServiceProvider : IServiceProvider
+	{
+		readonly List<PadDescriptor> pads = new List<PadDescriptor>();
+		
+		public PadServiceProvider(IEnumerable<PadDescriptor> descriptors)
+		{
+			foreach (var descriptor in descriptors) {
+				if (descriptor.ServiceInterface != null) {
+					pads.Add(descriptor);
+				}
+			}
+		}
+		
+		public object GetService(Type serviceType)
+		{
+			foreach (var pad in pads) {
+				if (serviceType == pad.ServiceInterface)
+					return pad.PadContent;
+			}
+			return null;
+		}
+	}
+}

src/Main/SharpDevelop/Workbench/WorkbenchStartup.cs

@@ -47,7 +47,6 @@ namespace ICSharpCode.SharpDevelop.Workbench
 			
 			UILanguageService.ValidateLanguage();
 			
-			SD.GetService<IOutputPad>(); // HACK: eagerly load output pad because pad services cannnot be instanciated from background threads
 			TaskService.Initialize();
 			Project.CustomToolsService.Initialize();
 			
@@ -55,6 +54,9 @@ namespace ICSharpCode.SharpDevelop.Workbench
 			workbench.SetMemento(SD.PropertyService.NestedProperties(workbenchMemento));
 			workbench.WorkbenchLayout = layout;
 			
+			// HACK: eagerly load output pad because pad services cannnot be instanciated from background threads
+			SD.Services.AddService(typeof(IOutputPad), CompilerMessageView.Instance);
+			
 			var dlgMsgService = SD.MessageService as IDialogMessageService;
 			if (dlgMsgService != null) {
 				dlgMsgService.DialogSynchronizeInvoke = SD.MainThread.SynchronizingObject;

src/Main/SharpDevelop/Workbench/WpfWorkbench.cs

@@ -105,10 +105,10 @@ namespace ICSharpCode.SharpDevelop.Workbench
 		{
 			UpdateFlowDirection();
 			
-			foreach (PadDescriptor content in AddInTree.BuildItems<PadDescriptor>(viewContentPath, this, false)) {
-				if (content != null) {
-					ShowPad(content);
-				}
+			var padDescriptors = AddInTree.BuildItems<PadDescriptor>(viewContentPath, this, false);
+			((SharpDevelopServiceContainer)SD.Services).AddFallbackProvider(new PadServiceProvider(padDescriptors));
+			foreach (PadDescriptor content in padDescriptors) {
+				ShowPad(content);
 			}
 			
 			mainMenu.ItemsSource = MenuService.CreateMenuItems(this, this, mainMenuPath, activationMethod: "MainMenu", immediatelyExpandMenuBuildersForShortcuts: true);
@@ -432,9 +432,6 @@ namespace ICSharpCode.SharpDevelop.Workbench
 				throw new ArgumentException("Pad is already loaded");
 			
 			padDescriptorCollection.Add(content);
-			if (content.ServiceInterface != null) {
-				SD.Services.AddService(content.ServiceInterface, delegate { return content.PadContent; });
-			}
 			
 			if (WorkbenchLayout != null) {
 				WorkbenchLayout.ShowPad(content);