Index: Core/Common/src/Core.Common.Base/IO/FileImporterBase.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Core/Common/src/Core.Common.Base/IO/FileImporterBase.cs	(.../FileImporterBase.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Core/Common/src/Core.Common.Base/IO/FileImporterBase.cs	(.../FileImporterBase.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -13,6 +13,13 @@
     /// <seealso cref="Core.Common.Base.IO.IFileImporter" />
     public abstract class FileImporterBase : IFileImporter
     {
+        /// <summary>
+        /// Indicates if a cancel request has been made. When true, no changes should be
+        /// made to the data model unless the importer is already in progress of changing
+        /// the data model.
+        /// </summary>
+        protected bool ImportIsCancelled;
+
         public abstract string Name { get; }
         public abstract string Category { get; }
         public abstract Bitmap Image { get; }
@@ -22,10 +29,18 @@
 
         public abstract bool Import(object targetItem, string filePath);
 
-        public abstract void Cancel();
+        public void Cancel()
+        {
+            ImportIsCancelled = true;
+        }
 
         public void DoPostImportUpdates(object targetItem)
         {
+            if (ImportIsCancelled)
+            {
+                return;
+            }
+
             var observableTarget = targetItem as IObservable;
             if (observableTarget != null)
             {
Index: Core/Common/test/Core.Common.Base.Test/IO/FileImporterBaseTest.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Core/Common/test/Core.Common.Base.Test/IO/FileImporterBaseTest.cs	(.../FileImporterBaseTest.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Core/Common/test/Core.Common.Base.Test/IO/FileImporterBaseTest.cs	(.../FileImporterBaseTest.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -65,6 +65,30 @@
             mocks.VerifyAll(); // Assert NotifyObservers is called
         }
 
+        [Test]
+        public void DoPostImportUpdates_ImportCancelled_NoNotifyObserversCalled()
+        {
+            // Setup
+            var mocks = new MockRepository();
+            var observableInstance = mocks.StrictMock<IObservable>();
+
+            var observableTarget = mocks.StrictMock<IObservable>();
+            mocks.ReplayAll();
+
+            var simpleImporter = new SimpleFileImporter();
+            simpleImporter.GetAffectedNonTargetObservableInstancesOverride = new[]
+            {
+                observableInstance
+            };
+            simpleImporter.Cancel();
+
+            // Call
+            simpleImporter.DoPostImportUpdates(observableTarget);
+
+            // Assert
+            mocks.VerifyAll(); // Assert no NotifyObservers were called
+        }
+
         private class SimpleFileImporter : FileImporterBase
         {
             public override string Name
@@ -119,11 +143,6 @@
             {
                 throw new NotImplementedException();
             }
-
-            public override void Cancel()
-            {
-                throw new NotImplementedException();
-            }
         }
     }
 }
\ No newline at end of file
Index: Core/Common/test/Core.Common.Base.Test/Service/FileImportActivityTest.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Core/Common/test/Core.Common.Base.Test/Service/FileImportActivityTest.cs	(.../FileImportActivityTest.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Core/Common/test/Core.Common.Base.Test/Service/FileImportActivityTest.cs	(.../FileImportActivityTest.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -213,11 +213,6 @@
 
                 return true;
             }
-
-            public override void Cancel()
-            {
-
-            }
         }
     }
 }
Index: Ringtoets/HydraRing/src/Ringtoets.HydraRing.Plugin/HydraulicBoundaryLocationsImporter.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Ringtoets/HydraRing/src/Ringtoets.HydraRing.Plugin/HydraulicBoundaryLocationsImporter.cs	(.../HydraulicBoundaryLocationsImporter.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Ringtoets/HydraRing/src/Ringtoets.HydraRing.Plugin/HydraulicBoundaryLocationsImporter.cs	(.../HydraulicBoundaryLocationsImporter.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -43,7 +43,6 @@
     public class HydraulicBoundaryLocationsImporter : FileImporterBase
     {
         private readonly ILog log = LogManager.GetLogger(typeof(HydraulicBoundaryLocationsImporter));
-        private bool shouldCancel;
 
         /// <summary>
         /// Gets the version of the used Hydraulic Boundary Database.
@@ -86,7 +85,7 @@
 
         /// <summary>
         /// Gets the <see cref="Type"/> of the item supported by the <see cref="HydraulicBoundaryLocationsImporter"/>.
-        /// </summary
+        /// </summary>
         public override Type SupportedItemType
         {
             get
@@ -142,28 +141,20 @@
 
             if (!importResult.CriticalErrorOccurred)
             {
-                if (!shouldCancel)
+                if (!ImportIsCancelled)
                 {
                     AddImportedDataToModel(targetItem, importResult);
                     log.Info(ApplicationResources.HydraulicBoundaryLocationsImporter_Import_Import_successful);
                     return true;
                 }
 
                 log.Info(ApplicationResources.HydraulicBoundaryLocationsImporter_Import_cancelled);
-                shouldCancel = false;
+                ImportIsCancelled = false;
             }
 
             return false;
         }
 
-        /// <summary>
-        /// This method cancels an import.
-        /// </summary>
-        public override void Cancel()
-        {
-            shouldCancel = true;
-        }
-
         private ReadResult<HydraulicBoundaryLocation> ReadHydraulicBoundaryLocations(string path)
         {
             NotifyProgress(ApplicationResources.HydraulicBoundaryLocationsImporter_ReadHydraulicBoundaryLocations, 1, 1);
@@ -196,7 +187,7 @@
             var locations = new Collection<HydraulicBoundaryLocation>();
             while (hydraulicBoundarySqLiteDatabaseReader.HasNext)
             {
-                if (shouldCancel)
+                if (ImportIsCancelled)
                 {
                     return new ReadResult<HydraulicBoundaryLocation>(false);
                 }
Index: Ringtoets/Integration/src/Ringtoets.Integration.Plugin/FileImporters/ReferenceLineImporter.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Ringtoets/Integration/src/Ringtoets.Integration.Plugin/FileImporters/ReferenceLineImporter.cs	(.../ReferenceLineImporter.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Ringtoets/Integration/src/Ringtoets.Integration.Plugin/FileImporters/ReferenceLineImporter.cs	(.../ReferenceLineImporter.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -114,6 +114,11 @@
                 }
             }
 
+            if (ImportIsCancelled)
+            {
+                return false;
+            }
+
             ReferenceLine importedReferenceLine;
             try
             {
@@ -130,12 +135,15 @@
                 return false;
             }
 
+            if (ImportIsCancelled)
+            {
+                return false;
+            }
+
             AddReferenceLineToDataModel(importTarget.Parent, importedReferenceLine, clearReferenceLineDependentData);
             return true;
         }
 
-        public override void Cancel() {}
-
         protected override IEnumerable<IObservable> GetAffectedNonTargetObservableInstances()
         {
             return changedObservables;
Index: Ringtoets/Integration/test/Ringtoets.Integration.Plugin.Test/FileImporters/ReferenceLineImporterTest.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Ringtoets/Integration/test/Ringtoets.Integration.Plugin.Test/FileImporters/ReferenceLineImporterTest.cs	(.../ReferenceLineImporterTest.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Ringtoets/Integration/test/Ringtoets.Integration.Plugin.Test/FileImporters/ReferenceLineImporterTest.cs	(.../ReferenceLineImporterTest.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -332,7 +332,90 @@
             mocks.VerifyAll(); // Expect NotifyObservers on cleared calculations and context
         }
 
-        // TODO: Cancel
+        [Test]
+        public void Import_CancellingImport_ReturnFalseAndNoChanges()
+        {
+            // Setup
+            var originalReferenceLine = new ReferenceLine();
+
+            var mocks = new MockRepository();
+            var assessmentSection = mocks.Stub<AssessmentSectionBase>();
+            assessmentSection.ReferenceLine = originalReferenceLine;
+            mocks.ReplayAll();
+
+            var path = TestHelper.GetTestDataPath(TestDataPath.Ringtoets.Common.IO, "traject_10-2.shp");
+
+            var referenceLineContext = new ReferenceLineContext(assessmentSection);
+
+            var importer = new ReferenceLineImporter();
+
+            DialogBoxHandler = (name, wnd) =>
+            {
+                importer.Cancel();
+
+                var messageBoxTester = new MessageBoxTester(wnd);
+                messageBoxTester.ClickOk();
+            };
+
+            // Call
+            bool importSuccesful = importer.Import(referenceLineContext, path);
+
+            // Assert
+            Assert.IsFalse(importSuccesful);
+            Assert.AreSame(originalReferenceLine, assessmentSection.ReferenceLine);
+            mocks.VerifyAll();
+        }
+
+        [Test]
+        public void DoPostImportUpdates_CancellingImport_DoNotNotifyObservers()
+        {
+            // Setup
+            var originalReferenceLine = new ReferenceLine();
+
+            var mocks = new MockRepository();
+            var calculation1 = mocks.StrictMock<ICalculationItem>();
+
+            var failureMechanism1 = mocks.Stub<IFailureMechanism>();
+            failureMechanism1.Stub(fm => fm.CalculationItems).Return(new[]
+            {
+                calculation1
+            });
+
+            var assessmentSection = mocks.Stub<AssessmentSectionBase>();
+            assessmentSection.ReferenceLine = originalReferenceLine;
+            assessmentSection.Stub(a => a.GetFailureMechanisms()).Return(new[]
+            {
+                failureMechanism1
+            });
+
+            var contextObserver = mocks.StrictMock<IObserver>();
+            mocks.ReplayAll();
+
+            var path = TestHelper.GetTestDataPath(TestDataPath.Ringtoets.Common.IO, "traject_10-2.shp");
+
+            var referenceLineContext = new ReferenceLineContext(assessmentSection);
+            referenceLineContext.Attach(contextObserver);
+
+            var importer = new ReferenceLineImporter();
+
+            DialogBoxHandler = (name, wnd) =>
+            {
+                importer.Cancel();
+
+                var messageBoxTester = new MessageBoxTester(wnd);
+                messageBoxTester.ClickOk();
+            };
+
+            // Precondition
+            Assert.IsFalse(importer.Import(referenceLineContext, path));
+
+            // Call
+            importer.DoPostImportUpdates(referenceLineContext);
+
+            // Assert
+            mocks.VerifyAll(); // Expect no NotifyObserver calls
+        }
+
         // TODO: Progress reporting
         // TODO: Instance reuse
     }
Index: Ringtoets/Piping/src/Ringtoets.Piping.Plugin/FileImporter/PipingSoilProfilesImporter.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Ringtoets/Piping/src/Ringtoets.Piping.Plugin/FileImporter/PipingSoilProfilesImporter.cs	(.../PipingSoilProfilesImporter.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Plugin/FileImporter/PipingSoilProfilesImporter.cs	(.../PipingSoilProfilesImporter.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -42,7 +42,6 @@
     public class PipingSoilProfilesImporter : FileImporterBase
     {
         private readonly ILog log = LogManager.GetLogger(typeof(PipingSoilProfilesImporter));
-        private bool shouldCancel;
 
         public override string Name
         {
@@ -87,18 +86,13 @@
 
         public override ProgressChangedDelegate ProgressChanged { protected get; set; }
 
-        public override void Cancel()
-        {
-            shouldCancel = true;
-        }
-
         public override bool Import(object targetItem, string filePath)
         {
             var importResult = ReadSoilProfiles(filePath);
 
             if (!importResult.CriticalErrorOccurred)
             {
-                if (!shouldCancel)
+                if (!ImportIsCancelled)
                 {
                     AddImportedDataToModel(targetItem, importResult);
 
@@ -144,7 +138,7 @@
             var profiles = new Collection<PipingSoilProfile>();
             while (soilProfileReader.HasNext)
             {
-                if (shouldCancel)
+                if (ImportIsCancelled)
                 {
                     return new ReadResult<PipingSoilProfile>(false);
                 }
@@ -198,7 +192,7 @@
         {
             log.Info(ApplicationResources.PipingSoilProfilesImporter_Import_Import_cancelled);
 
-            shouldCancel = false;
+            ImportIsCancelled = false;
         }
     }
 }
\ No newline at end of file
Index: Ringtoets/Piping/src/Ringtoets.Piping.Plugin/FileImporter/PipingSurfaceLinesCsvImporter.cs
===================================================================
diff -u -rdcd6469f6000957bc1604da8e92bd5ea09e43769 -r9a426b227a1f3902ae05dea3c2b0e94f4b4551cb
--- Ringtoets/Piping/src/Ringtoets.Piping.Plugin/FileImporter/PipingSurfaceLinesCsvImporter.cs	(.../PipingSurfaceLinesCsvImporter.cs)	(revision dcd6469f6000957bc1604da8e92bd5ea09e43769)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Plugin/FileImporter/PipingSurfaceLinesCsvImporter.cs	(.../PipingSurfaceLinesCsvImporter.cs)	(revision 9a426b227a1f3902ae05dea3c2b0e94f4b4551cb)
@@ -47,7 +47,6 @@
     public class PipingSurfaceLinesCsvImporter : FileImporterBase
     {
         private readonly ILog log;
-        private bool shouldCancel;
         private const string characteristicPointsFileSubExtension = ".krp";
 
         public PipingSurfaceLinesCsvImporter()
@@ -98,11 +97,6 @@
 
         public override ProgressChangedDelegate ProgressChanged { protected get; set; }
 
-        public override void Cancel()
-        {
-            shouldCancel = true;
-        }
-
         public override bool Import(object targetItem, string filePath)
         {
             var importSurfaceLinesResult = ReadPipingSurfaceLines(filePath);
@@ -111,7 +105,7 @@
                 return false;
             }
 
-            if (shouldCancel)
+            if (ImportIsCancelled)
             {
                 HandleUserCancellingImport();
                 return false;
@@ -124,7 +118,7 @@
                 return false;
             }
 
-            if (shouldCancel)
+            if (ImportIsCancelled)
             {
                 HandleUserCancellingImport();
                 return false;
@@ -172,7 +166,7 @@
 
             var readSurfaceLines = new List<RingtoetsPipingSurfaceLine>(itemCount);
             var readSurfaceLineIdentifiers = new HashSet<string>();
-            for (int i = 0; i < itemCount && !shouldCancel; i++)
+            for (int i = 0; i < itemCount && !ImportIsCancelled; i++)
             {
                 try
                 {
@@ -250,7 +244,7 @@
 
             var readCharacteristicPointsLocations = new List<CharacteristicPoints>(itemCount);
             var readCharacteristicPointsLocationIdentifiers = new HashSet<string>();
-            for (int i = 0; i < itemCount && !shouldCancel; i++)
+            for (int i = 0; i < itemCount && !ImportIsCancelled; i++)
             {
                 try
                 {
@@ -408,7 +402,7 @@
         {
             log.Info(RingtoetsPluginResources.PipingSurfaceLinesCsvImporter_Import_Import_cancelled);
 
-            shouldCancel = false;
+            ImportIsCancelled = false;
         }
     }
 }
\ No newline at end of file