Start on unit testing WindowManager, but there's not a lot I can do since Window is so hard to mock

Stylet/WindowManager.cs

@@ -35,7 +35,7 @@ namespace Stylet
             var view = viewManager.LocateViewForModel(viewModel);
             var window = view as Window;
             if (window == null)
-                throw new Exception(String.Format("Tried to show {0} as a window, but it isn't a Window", view.GetType().Name));
+                throw new Exception(String.Format("Tried to show {0} as a window, but it isn't a Window", view == null ? "(null)" : view.GetType().Name));
 
             viewManager.BindViewToModel(window, viewModel);
 

StyletUnitTests/StyletUnitTests.csproj

@@ -71,6 +71,7 @@
     <Compile Include="StyletIoC\StyletIoCGetSingleKeyedTests.cs" />
     <Compile Include="StyletIoC\StyletIoCGetSingleTests.cs" />
     <Compile Include="StyletIoC\StyletIoCUnboundGenericTests.cs" />
+    <Compile Include="WindowManagerTests.cs" />
   </ItemGroup>
   <ItemGroup>
     <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />

StyletUnitTests/WindowManagerTests.cs

@@ -0,0 +1,47 @@
+using Moq;
+using NUnit.Framework;
+using Stylet;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace StyletUnitTests
+{
+    [TestFixture, RequiresSTA]
+    public class WindowManagerTests
+    {
+        private Mock<IViewManager> viewManager;
+        private WindowManager windowManager;
+
+        [SetUp]
+        public void SetUp()
+        {
+            this.viewManager = new Mock<IViewManager>();
+            this.windowManager = new WindowManager();
+
+            IoC.GetInstance = (service, key) => this.viewManager.Object;
+        }
+
+        [Test]
+        public void ShowWindowAsksViewManagerForView()
+        {
+            var model = new object();
+            this.viewManager.Setup(x => x.LocateViewForModel(model)).Verifiable();
+            // Don't care if this throws - that's OK
+            try { this.windowManager.ShowWindow(model);  }
+            catch (Exception) { }
+            this.viewManager.VerifyAll();
+        }
+
+        [Test]
+        public void ShowWindowThrowsIfViewIsntAWindow()
+        {
+            var model = new object();
+            this.viewManager.Setup(x => x.LocateViewForModel(model)).Returns(new UIElement());
+            Assert.Throws<Exception>(() => this.windowManager.ShowWindow(model));
+        }
+    }
+}