Index: Core/Plugins/src/Core.Plugins.ProjectExplorer/Commands/ToggleProjectExplorerCommand.cs
===================================================================
diff -u -r78b15d3290c831ef1d7070aa1ccb48f57a15961f -re5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132
--- Core/Plugins/src/Core.Plugins.ProjectExplorer/Commands/ToggleProjectExplorerCommand.cs	(.../ToggleProjectExplorerCommand.cs)	(revision 78b15d3290c831ef1d7070aa1ccb48f57a15961f)
+++ Core/Plugins/src/Core.Plugins.ProjectExplorer/Commands/ToggleProjectExplorerCommand.cs	(.../ToggleProjectExplorerCommand.cs)	(revision e5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132)
@@ -19,6 +19,7 @@
 // Stichting Deltares and remain full property of Stichting Deltares at all times. 
 // All rights reserved.
 
+using System;
 using Core.Common.Controls.Commands;
 
 namespace Core.Plugins.ProjectExplorer.Commands
@@ -35,8 +36,13 @@
         /// </summary>
         /// <param name="viewController">The <see cref="ProjectExplorerViewController"/> to use for
         /// querying and modifying the project explorer's state.</param>
+        /// <exception cref="ArgumentNullException">Throw when <paramref name="viewController"/> is <c>null</c>.</exception>
         public ToggleProjectExplorerCommand(ProjectExplorerViewController viewController)
         {
+            if (viewController == null)
+            {
+                throw new ArgumentNullException("viewController");
+            }
             this.viewController = viewController;
         }
 
Index: Core/Plugins/src/Core.Plugins.ProjectExplorer/ProjectExplorerViewController.cs
===================================================================
diff -u -r78b15d3290c831ef1d7070aa1ccb48f57a15961f -re5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132
--- Core/Plugins/src/Core.Plugins.ProjectExplorer/ProjectExplorerViewController.cs	(.../ProjectExplorerViewController.cs)	(revision 78b15d3290c831ef1d7070aa1ccb48f57a15961f)
+++ Core/Plugins/src/Core.Plugins.ProjectExplorer/ProjectExplorerViewController.cs	(.../ProjectExplorerViewController.cs)	(revision e5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132)
@@ -66,7 +66,7 @@
                 projectExplorer = new ProjectExplorer(applicationSelection, viewCommands, treeNodeInfos);
                 projectExplorer.TreeView.TreeViewController.NodeUpdated += (s, e) => documentViewController.UpdateToolTips();
 
-                toolViewController.ToolWindowViews.Add(projectExplorer, ViewLocation.Left | ViewLocation.Top);
+                toolViewController.OpenToolView(projectExplorer);
 
                 if (OnOpenView != null)
                 {
@@ -108,7 +108,7 @@
         {
             if (IsViewActive())
             {
-                toolViewController.ToolWindowViews.Remove(projectExplorer); // Disposes the view.
+                toolViewController.CloseToolView(projectExplorer); // Disposes the view.
                 projectExplorer = null;
             }
         }
Index: Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/Commands/ToggleProjectExplorerCommandTest.cs
===================================================================
diff -u
--- Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/Commands/ToggleProjectExplorerCommandTest.cs	(revision 0)
+++ Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/Commands/ToggleProjectExplorerCommandTest.cs	(revision e5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132)
@@ -0,0 +1,123 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Core.Common.Controls.Commands;
+using Core.Common.Controls.TreeView;
+using Core.Common.Gui;
+using Core.Common.Gui.Commands;
+using Core.Common.Gui.Forms.ViewManager;
+using Core.Common.Gui.Selection;
+using Core.Plugins.ProjectExplorer.Commands;
+using NUnit.Framework;
+using Rhino.Mocks;
+
+namespace Core.Plugins.ProjectExplorer.Test.Commands
+{
+    [TestFixture]
+    public class ToggleProjectExplorerCommandTest
+    {
+        [Test]
+        public void Constructor_WithoutController_ThrowsArgumentNullException()
+        {
+            // Call
+            TestDelegate test = () => new ToggleProjectExplorerCommand(null);
+
+            // Assert
+            Assert.Throws<ArgumentNullException>(test);
+        }
+
+        [Test]
+        public void Constructor_WithController_CreatesNewICommand()
+        {
+            // Setup
+            var mocks = new MockRepository();
+
+            var documentViewController = mocks.StrictMock<IDocumentViewController>();
+            var viewCommands = mocks.StrictMock<IViewCommands>();
+            var applicationSelection = mocks.StrictMock<IApplicationSelection>();
+            var toolViewController = mocks.StrictMock<IToolViewController>();
+
+            mocks.ReplayAll();
+
+            var treeNodeInfos = Enumerable.Empty<TreeNodeInfo>();
+
+            var explorerViewController = new ProjectExplorerViewController(documentViewController, viewCommands, applicationSelection, toolViewController, treeNodeInfos);
+
+            // Call
+            var command = new ToggleProjectExplorerCommand(explorerViewController);
+
+            // Assert
+            Assert.IsInstanceOf<ICommand>(command);
+            Assert.IsTrue(command.Enabled);
+            mocks.VerifyAll();
+        }
+
+        [Test]
+        [TestCase(true)]
+        [TestCase(false)]
+        public void Checked_Always_CallsViewControllerIsViewActive(bool isViewOpen)
+        {
+            // Setup
+            var mocks = new MockRepository();
+
+            var documentViewController = mocks.StrictMock<IDocumentViewController>();
+            var viewCommands = mocks.StrictMock<IViewCommands>();
+            var applicationSelection = mocks.StrictMock<IApplicationSelection>();
+            var toolViewController = mocks.StrictMock<IToolViewController>();
+            toolViewController.Expect(tvc => tvc.IsToolWindowOpen<ProjectExplorer>()).Return(isViewOpen);
+
+            mocks.ReplayAll();
+
+            var treeNodeInfos = Enumerable.Empty<TreeNodeInfo>();
+
+            var explorerViewController = new ProjectExplorerViewController(documentViewController, viewCommands, applicationSelection, toolViewController, treeNodeInfos);
+
+            var command = new ToggleProjectExplorerCommand(explorerViewController);
+
+            // Call
+            var result = command.Checked;
+
+            // Assert
+            Assert.AreEqual(isViewOpen, result);
+            mocks.VerifyAll();
+        }
+
+        [Test]
+        [TestCase(true)]
+        [TestCase(false)]
+        public void Execute_Always_CallsViewControllerToggleView(bool isViewOpen)
+        {
+            // Setup
+            var mocks = new MockRepository();
+
+            var documentViewController = mocks.StrictMock<IDocumentViewController>();
+            var viewCommands = mocks.StrictMock<IViewCommands>();
+            var applicationSelection = mocks.StrictMock<IApplicationSelection>();
+            var toolViewController = mocks.StrictMock<IToolViewController>();
+            toolViewController.Expect(tvc => tvc.IsToolWindowOpen<ProjectExplorer>()).Return(isViewOpen).Repeat.Twice();
+            if (isViewOpen)
+            {
+                toolViewController.Expect(tvc => tvc.CloseToolView(Arg<ProjectExplorer>.Matches(x => true)));
+            }
+            else
+            {
+                applicationSelection.Expect(a => a.SelectionChanged += null).IgnoreArguments();
+                toolViewController.Expect(tvc => tvc.OpenToolView(Arg<ProjectExplorer>.Matches(x => true)));
+            }
+
+            mocks.ReplayAll();
+
+            var treeNodeInfos = Enumerable.Empty<TreeNodeInfo>();
+
+            var explorerViewController = new ProjectExplorerViewController(documentViewController, viewCommands, applicationSelection, toolViewController, treeNodeInfos);
+
+            var command = new ToggleProjectExplorerCommand(explorerViewController);
+
+            // Call
+            command.Execute();
+
+            // Assert
+            mocks.VerifyAll();
+        }
+    }
+}
\ No newline at end of file
Index: Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/Core.Plugins.ProjectExplorer.Test.csproj
===================================================================
diff -u -r78b15d3290c831ef1d7070aa1ccb48f57a15961f -re5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132
--- Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/Core.Plugins.ProjectExplorer.Test.csproj	(.../Core.Plugins.ProjectExplorer.Test.csproj)	(revision 78b15d3290c831ef1d7070aa1ccb48f57a15961f)
+++ Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/Core.Plugins.ProjectExplorer.Test.csproj	(.../Core.Plugins.ProjectExplorer.Test.csproj)	(revision e5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132)
@@ -95,6 +95,7 @@
     <Reference Include="WindowsBase" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="Commands\ToggleProjectExplorerCommandTest.cs" />
     <Compile Include="RibbonTest.cs" />
     <Compile Include="TreeNodeInfos\ProjectTreeNodeInfoTest.cs" />
     <Compile Include="ProjectExplorerGuiPluginTest.cs" />
Index: Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/ProjectExplorerGuiPluginTest.cs
===================================================================
diff -u -r83e8ea6f186faad530933f5620faa91f1920d1e1 -re5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132
--- Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/ProjectExplorerGuiPluginTest.cs	(.../ProjectExplorerGuiPluginTest.cs)	(revision 83e8ea6f186faad530933f5620faa91f1920d1e1)
+++ Core/Plugins/test/Core.Plugins.ProjectExplorer.Test/ProjectExplorerGuiPluginTest.cs	(.../ProjectExplorerGuiPluginTest.cs)	(revision e5b51f8c239f5bb4f4a3dce0399dbdd26ecc4132)
@@ -24,35 +24,25 @@
         {
             // Setup
             var mocks = new MockRepository();
-            var applicationCore = new ApplicationCore();
-            var guiStub = mocks.Stub<IGui>();
-            var otherGuiPlugin = mocks.Stub<GuiPlugin>();
+            var gui = mocks.StrictMock<IGui>();
+            var otherGuiPlugin = mocks.StrictMock<GuiPlugin>();
 
-            guiStub.Stub(g => g.ApplicationCommands).Return(mocks.Stub<IApplicationFeatureCommands>());
-            guiStub.Stub(g => g.ProjectCommands).Return(mocks.Stub<IProjectCommands>());
-            guiStub.Stub(g => g.ViewCommands).Return(mocks.Stub<IViewCommands>());
-            guiStub.Stub(g => g.IsToolWindowOpen<ProjectExplorer>()).Return(false);
-            guiStub.Expect(g => g.GetTreeNodeInfos()).Return(Enumerable.Empty<TreeNodeInfo>());
+            gui.Expect(g => g.ViewCommands).Return(mocks.Stub<IViewCommands>());
+            gui.Expect(g => g.GetTreeNodeInfos()).Return(Enumerable.Empty<TreeNodeInfo>());
 
-            Expect.Call(guiStub.ApplicationCore).Return(applicationCore).Repeat.Any();
+            gui.Expect(g => g.IsToolWindowOpen<ProjectExplorer>()).Return(true).Repeat.Times(3);
+            gui.Expect(g => g.CloseToolView(Arg<ProjectExplorer>.Matches(r => true)));
 
-            guiStub.Expect(g => g.SelectionChanged += Arg<EventHandler<SelectedItemChangedEventArgs>>.Is.Anything).Repeat.Any();
-            guiStub.Expect(g => g.SelectionChanged -= Arg<EventHandler<SelectedItemChangedEventArgs>>.Is.Anything).Repeat.Any();
-            guiStub.Expect(g => g.ProjectOpened += Arg<Action<Project>>.Is.Anything).Repeat.Any();
-            guiStub.Expect(g => g.ProjectOpened -= Arg<Action<Project>>.Is.Anything).Repeat.Any();
-            guiStub.Expect(g => g.ProjectClosing += Arg<Action<Project>>.Is.Anything).Repeat.Any();
-            guiStub.Expect(g => g.ProjectClosing -= Arg<Action<Project>>.Is.Anything).Repeat.Any();
-            guiStub.Expect(g => g.ToolWindowViews).Return(mocks.Stub<IViewList>()).Repeat.Any();
-            guiStub.Expect(g => g.DocumentViews).Return(mocks.Stub<IViewList>()).Repeat.Any();
+            gui.Expect(g => g.ProjectOpened += Arg<Action<Project>>.Is.Anything);
 
-            guiStub.Replay();
+            gui.Replay();
 
             using (var projectExplorerGuiPlugin = new ProjectExplorerGuiPlugin
             {
-                Gui = guiStub
+                Gui = gui
             })
             {
-                guiStub.Expect(g => g.Plugins).Return(new List<GuiPlugin>
+                gui.Expect(g => g.Plugins).Return(new List<GuiPlugin>
                 {
                     projectExplorerGuiPlugin, otherGuiPlugin
                 }).Repeat.Any();