Index: Core/Common/src/Core.Common.Base/Observable.cs
===================================================================
diff -u -r1e4d2363eb853577139c402364c4462dad821046 -r94ef875f26bff005b1d53880c92f350b373a31ec
--- Core/Common/src/Core.Common.Base/Observable.cs	(.../Observable.cs)	(revision 1e4d2363eb853577139c402364c4462dad821046)
+++ Core/Common/src/Core.Common.Base/Observable.cs	(.../Observable.cs)	(revision 94ef875f26bff005b1d53880c92f350b373a31ec)
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 
 namespace Core.Common.Base
 {
@@ -35,8 +36,15 @@
         /// </summary>
         public void NotifyObservers()
         {
-            foreach (var observer in observers)
+            // Iterate through a copy of the list of observers; an update of one observer might result in detaching another observer (which will result in a "list modified" exception over here otherwise)
+            foreach (var observer in observers.ToList())
             {
+                // Ensure the observer is still part of the original list of observers
+                if (!observers.Contains(observer))
+                {
+                    continue;
+                }
+
                 observer.UpdateObserver();
             }
         }
Index: Core/Common/src/Core.Common.Base/Project.cs
===================================================================
diff -u -r04f06b5762b758295a52944dd232394a462cf72c -r94ef875f26bff005b1d53880c92f350b373a31ec
--- Core/Common/src/Core.Common.Base/Project.cs	(.../Project.cs)	(revision 04f06b5762b758295a52944dd232394a462cf72c)
+++ Core/Common/src/Core.Common.Base/Project.cs	(.../Project.cs)	(revision 94ef875f26bff005b1d53880c92f350b373a31ec)
@@ -1,13 +1,11 @@
-using System.Collections.Generic;
-using System.Linq;
 using Core.Common.Utils.Collections.Generic;
 
 namespace Core.Common.Base
 {
     /// <summary>
     /// Container of all data and tasks.
     /// </summary>
-    public class Project : IObservable
+    public class Project : Observable
     {
         /// <summary>
         /// Creates instance of the Project.
@@ -39,29 +37,5 @@
         /// The items in the project.
         /// </summary>
         public IEventedList<object> Items { get; private set; }
-
-        # region IObservable
-
-        private readonly IList<IObserver> observers = new List<IObserver>();
-
-        public void Attach(IObserver observer)
-        {
-            observers.Add(observer);
-        }
-
-        public void Detach(IObserver observer)
-        {
-            observers.Remove(observer);
-        }
-
-        public void NotifyObservers()
-        {
-            foreach (var observer in observers.ToList()) // Copy the list of observers; an update of one observer might result in detaching another observer (which will result in a "list modified" exception over here otherwise)
-            {
-                observer.UpdateObserver();
-            }
-        }
-
-        # endregion
     }
 }
\ No newline at end of file
Index: Core/Common/test/Core.Common.Base.Tests/Core.Common.Base.Tests.csproj
===================================================================
diff -u -ree60be046a1b4e4151bb567e9a9c3ab0876d3b55 -r94ef875f26bff005b1d53880c92f350b373a31ec
--- Core/Common/test/Core.Common.Base.Tests/Core.Common.Base.Tests.csproj	(.../Core.Common.Base.Tests.csproj)	(revision ee60be046a1b4e4151bb567e9a9c3ab0876d3b55)
+++ Core/Common/test/Core.Common.Base.Tests/Core.Common.Base.Tests.csproj	(.../Core.Common.Base.Tests.csproj)	(revision 94ef875f26bff005b1d53880c92f350b373a31ec)
@@ -123,6 +123,7 @@
     <Compile Include="Controls\Swf\TreeViewControls\TreeViewNodePresenterBaseTest.cs" />
     <Compile Include="Controls\Swf\TreeViewControls\TreeViewControllerTest.cs" />
     <Compile Include="Controls\Swf\TreeViewControls\TreeViewTest.cs" />
+    <Compile Include="ObservableTest.cs" />
     <Compile Include="Properties\Resources.Designer.cs">
       <AutoGen>True</AutoGen>
       <DesignTime>True</DesignTime>
Index: Core/Common/test/Core.Common.Base.Tests/ObservableTest.cs
===================================================================
diff -u
--- Core/Common/test/Core.Common.Base.Tests/ObservableTest.cs	(revision 0)
+++ Core/Common/test/Core.Common.Base.Tests/ObservableTest.cs	(revision 94ef875f26bff005b1d53880c92f350b373a31ec)
@@ -0,0 +1,51 @@
+using System;
+using NUnit.Framework;
+using Rhino.Mocks;
+
+namespace Core.Common.Base.Tests
+{
+    [TestFixture]
+    public class ObservableTest
+    {
+        [Test]
+        public void NotifyObservers_MultipleObserversDetachingOrAttachingOthers_NoUpdatesForAttachedAndDetachedObservers()
+        {
+            // Setup
+            var mocks = new MockRepository();
+            var testObservable = new TestObservable();
+
+            var observer1 = mocks.Stub<IObserver>();
+            var observer2 = mocks.Stub<IObserver>();
+            var observer3 = mocks.Stub<IObserver>();
+            var observer4 = mocks.Stub<IObserver>();
+            var observer5 = mocks.Stub<IObserver>();
+            var observer6 = mocks.Stub<IObserver>();
+
+            testObservable.Attach(observer1);
+            testObservable.Attach(observer2);
+            testObservable.Attach(observer3);
+            testObservable.Attach(observer4);
+            testObservable.Attach(observer6);
+
+            observer1.Expect(o => o.UpdateObserver()).Repeat.Once();
+            observer2.Expect(o => o.UpdateObserver()).Do((Action) (() => testObservable.Detach(observer3))).Repeat.Once();
+            observer3.Expect(o => o.UpdateObserver()).Repeat.Never(); // A detached observer should no longer be updated
+            observer4.Expect(o => o.UpdateObserver()).Do((Action)(() => testObservable.Attach(observer5))).Repeat.Once();
+            observer5.Expect(o => o.UpdateObserver()).Repeat.Never(); // An attached observer should not be updated too
+            observer6.Expect(o => o.UpdateObserver()).Repeat.Once();
+
+            mocks.ReplayAll();
+
+            // Call
+            testObservable.NotifyObservers();
+
+            // Assert
+            mocks.VerifyAll();
+        }
+
+        private class TestObservable : Observable
+        {
+            
+        }
+    }
+}