Workaround of Delegate.Combine issues with contra-variance by introducing custom delegate lists for CollectionChanged event of models.

src/Main/Base/Project/Dom/IModelCollection.cs

@@ -15,6 +15,49 @@ namespace ICSharpCode.SharpDevelop.Dom
 	/// </remarks>
 	public delegate void ModelCollectionChangedEventHandler<in T>(IReadOnlyCollection<T> removedItems, IReadOnlyCollection<T> addedItems);
 	
+	public class ModelCollectionChangedEvent<T>
+	{
+		List<ModelCollectionChangedEventHandler<T>> _handlers = new List<ModelCollectionChangedEventHandler<T>>();
+		
+//		public static ModelCollectionChangedEvent<T> operator+(ModelCollectionChangedEvent<T> eventObject, ModelCollectionChangedEventHandler<T> handler)
+//		{
+//			eventObject._handlers.Add(handler);
+//			return eventObject;
+//		}
+		
+		public void AddHandler(ModelCollectionChangedEventHandler<T> handler)
+		{
+			_handlers.Add(handler);
+		}
+		
+//		public static ModelCollectionChangedEvent<T> operator-(ModelCollectionChangedEvent<T> eventObject, ModelCollectionChangedEventHandler<T> handler)
+//		{
+//			eventObject._handlers.Remove(handler);
+//			return eventObject;
+//		}
+		
+		public void RemoveHandler(ModelCollectionChangedEventHandler<T> handler)
+		{
+			_handlers.Remove(handler);
+		}
+		
+		public void Fire(IReadOnlyCollection<T> removedItems, IReadOnlyCollection<T> addedItems)
+		{
+			foreach (var handler in _handlers) {
+				if (handler != null) {
+					handler(removedItems, addedItems);
+				}
+			}
+		}
+		
+		public bool ContainsHandlers
+		{
+			get {
+				return _handlers.Count > 0;
+			}
+		}
+	}
+	
 	/// <summary>
 	/// A read-only collection that provides change notifications.
 	/// </summary>

src/Main/Base/Project/Dom/ModelCollectionLinq.cs

@@ -70,6 +70,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 		
 		sealed class SelectManyModelCollection<TSource, TCollection, TResult> : IModelCollection<TResult>
 		{
+			readonly ModelCollectionChangedEvent<TResult> collectionChangedEvent;
 			readonly IModelCollection<TSource> source;
 			readonly Func<TSource, IModelCollection<TCollection>> collectionSelector;
 			readonly Func<TSource, TCollection, TResult> resultSelector;
@@ -80,10 +81,9 @@ namespace ICSharpCode.SharpDevelop.Dom
 				this.source = source;
 				this.collectionSelector = collectionSelector;
 				this.resultSelector = resultSelector;
+				collectionChangedEvent = new ModelCollectionChangedEvent<TResult>();
 			}
 			
-			ModelCollectionChangedEventHandler<TResult> collectionChanged;
-			
 			public event ModelCollectionChangedEventHandler<TResult> CollectionChanged {
 				add {
 					if (value == null)
@@ -97,13 +97,13 @@ namespace ICSharpCode.SharpDevelop.Dom
 							inputCollections.Add(inputCollection);
 						}
 					}
-					collectionChanged += value;
+					collectionChangedEvent.AddHandler(value);
 				}
 				remove {
-					if (collectionChanged == null)
+					if (!collectionChangedEvent.ContainsHandlers)
 						return;
-					collectionChanged -= value;
-					if (collectionChanged == null) {
+					collectionChangedEvent.RemoveHandler(value);
+					if (!collectionChangedEvent.ContainsHandlers) {
 						source.CollectionChanged -= OnSourceCollectionChanged;
 						foreach (var inputCollection in inputCollections) {
 							inputCollection.UnregisterEvent();
@@ -115,8 +115,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 			
 			void OnCollectionChanged(IReadOnlyCollection<TResult> removedItems, IReadOnlyCollection<TResult> addedItems)
 			{
-				if (collectionChanged != null)
-					collectionChanged(removedItems, addedItems);
+				collectionChangedEvent.Fire(removedItems, addedItems);
 			}
 			
 			void OnSourceCollectionChanged(IReadOnlyCollection<TSource> removedItems, IReadOnlyCollection<TSource> addedItems)

src/Main/Base/Project/Dom/SimpleModelCollection.cs

@@ -16,6 +16,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 	/// </summary>
 	public class SimpleModelCollection<T> : IMutableModelCollection<T>
 	{
+		readonly ModelCollectionChangedEvent<T> collectionChangedEvent;
 		readonly List<T> list;
 		List<T> addedItems;
 		List<T> removedItems;
@@ -23,11 +24,13 @@ namespace ICSharpCode.SharpDevelop.Dom
 		public SimpleModelCollection()
 		{
 			this.list = new List<T>();
+			collectionChangedEvent = new ModelCollectionChangedEvent<T>();
 		}
 		
 		public SimpleModelCollection(IEnumerable<T> items)
 		{
 			this.list = new List<T>(items);
+			collectionChangedEvent = new ModelCollectionChangedEvent<T>();
 		}
 		
 		protected void CheckReentrancy()
@@ -41,7 +44,15 @@ namespace ICSharpCode.SharpDevelop.Dom
 		}
 		
 		#region CollectionChanged / BatchUpdate()
-		public event ModelCollectionChangedEventHandler<T> CollectionChanged;
+		public event ModelCollectionChangedEventHandler<T> CollectionChanged
+		{
+			add {
+				collectionChangedEvent.AddHandler(value);
+			}
+			remove {
+				collectionChangedEvent.RemoveHandler(value);
+			}
+		}
 		
 		bool isWithinBatchOperation;
 		bool isRaisingEvent;
@@ -64,9 +75,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 		
 		protected virtual void OnCollectionChanged(IReadOnlyCollection<T> removedItems, IReadOnlyCollection<T> addedItems)
 		{
-			var handler = CollectionChanged;
-			if (handler != null)
-				handler(removedItems, addedItems);
+			collectionChangedEvent.Fire(removedItems, addedItems);
 		}
 		
 		public virtual IDisposable BatchUpdate()

src/Main/Base/Project/Src/Project/MSBuildConfigurationOrPlatformNameCollection.cs

@@ -16,9 +16,8 @@ namespace ICSharpCode.SharpDevelop.Project
 	/// </summary>
 	class MSBuildConfigurationOrPlatformNameCollection : IConfigurationOrPlatformNameCollection
 	{
-		public event ModelCollectionChangedEventHandler<string> CollectionChanged;
-		
 		volatile IReadOnlyList<string> listSnapshot = EmptyList<string>.Instance;
+		readonly ModelCollectionChangedEvent<string> collectionChangedEvent;
 		readonly MSBuildBasedProject project;
 		readonly bool isPlatform;
 		
@@ -26,6 +25,7 @@ namespace ICSharpCode.SharpDevelop.Project
 		{
 			this.project = project;
 			this.isPlatform = isPlatform;
+			collectionChangedEvent = new ModelCollectionChangedEvent<string>();
 		}
 		
 		internal void SetContents(IEnumerable<string> updatedItems)
@@ -33,13 +33,21 @@ namespace ICSharpCode.SharpDevelop.Project
 			this.listSnapshot = updatedItems.ToArray();
 		}
 		
+		public event ModelCollectionChangedEventHandler<string> CollectionChanged
+		{
+			add {
+				collectionChangedEvent.AddHandler(value);
+			}
+			remove {
+				collectionChangedEvent.RemoveHandler(value);
+			}
+		}
+		
 		internal void OnCollectionChanged(IReadOnlyCollection<string> oldItems, IReadOnlyCollection<string> newItems)
 		{
 			if (oldItems.SequenceEqual(newItems))
 				return;
-			var eh = CollectionChanged;
-			if (eh != null)
-				eh(oldItems, newItems);
+			collectionChangedEvent.Fire(oldItems, newItems);
 		}
 		
 		#region IReadOnlyCollection implementation

src/Main/SharpDevelop/Dom/NestedTypeDefinitionModelCollection.cs

@@ -11,15 +11,25 @@ namespace ICSharpCode.SharpDevelop.Dom
 {
 	sealed class NestedTypeDefinitionModelCollection : IModelCollection<ITypeDefinitionModel>
 	{
+		readonly ModelCollectionChangedEvent<ITypeDefinitionModel> collectionChangedEvent;
 		readonly IEntityModelContext context;
 		List<TypeDefinitionModel> list = new List<TypeDefinitionModel>();
 		
 		public NestedTypeDefinitionModelCollection(IEntityModelContext context)
 		{
 			this.context = context;
+			collectionChangedEvent = new ModelCollectionChangedEvent<ITypeDefinitionModel>();
 		}
 		
-		public event ModelCollectionChangedEventHandler<ITypeDefinitionModel> CollectionChanged;
+		public event ModelCollectionChangedEventHandler<ITypeDefinitionModel> CollectionChanged
+		{
+			add {
+				collectionChangedEvent.AddHandler(value);
+			}
+			remove {
+				collectionChangedEvent.RemoveHandler(value);
+			}
+		}
 		
 		public IReadOnlyCollection<ITypeDefinitionModel> CreateSnapshot()
 		{
@@ -107,9 +117,9 @@ namespace ICSharpCode.SharpDevelop.Dom
 				}
 			}
 			// Raise the event if necessary:
-			if (CollectionChanged != null && (oldModels != null || newModels != null)) {
+			if (collectionChangedEvent.ContainsHandlers && (oldModels != null || newModels != null)) {
 				IReadOnlyCollection<ITypeDefinitionModel> emptyList = EmptyList<ITypeDefinitionModel>.Instance;
-				CollectionChanged(oldModels ?? emptyList, newModels ?? emptyList);
+				collectionChangedEvent.Fire(oldModels ?? emptyList, newModels ?? emptyList);
 			}
 		}
 	}

src/Main/SharpDevelop/Dom/TopLevelTypeDefinitionModelCollection.cs

@@ -16,6 +16,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 	/// </summary>
 	sealed class TopLevelTypeDefinitionModelCollection : ITypeDefinitionModelCollection
 	{
+		readonly ModelCollectionChangedEvent<ITypeDefinitionModel> collectionChangedEvent;
 		readonly IEntityModelContext context;
 		Dictionary<TopLevelTypeName, TypeDefinitionModel> dict = new Dictionary<TopLevelTypeName, TypeDefinitionModel>();
 		
@@ -24,9 +25,18 @@ namespace ICSharpCode.SharpDevelop.Dom
 			if (context == null)
 				throw new ArgumentNullException("context");
 			this.context = context;
+			collectionChangedEvent = new ModelCollectionChangedEvent<ITypeDefinitionModel>();
 		}
 		
-		public event ModelCollectionChangedEventHandler<ITypeDefinitionModel> CollectionChanged;
+		public event ModelCollectionChangedEventHandler<ITypeDefinitionModel> CollectionChanged
+		{
+			add {
+				collectionChangedEvent.AddHandler(value);
+			}
+			remove {
+				collectionChangedEvent.RemoveHandler(value);
+			}
+		}
 		
 		public IReadOnlyCollection<ITypeDefinitionModel> CreateSnapshot()
 		{
@@ -122,9 +132,9 @@ namespace ICSharpCode.SharpDevelop.Dom
 				}
 			}
 			// Raise the event if necessary:
-			if (CollectionChanged != null && (oldModels != null || newModels != null)) {
+			if (collectionChangedEvent.ContainsHandlers && (oldModels != null || newModels != null)) {
 				IReadOnlyCollection<ITypeDefinitionModel> emptyList = EmptyList<ITypeDefinitionModel>.Instance;
-				CollectionChanged(oldModels ?? emptyList, newModels ?? emptyList);
+				collectionChangedEvent.Fire(oldModels ?? emptyList, newModels ?? emptyList);
 			}
 		}
 		

src/Main/SharpDevelop/Dom/TypeDefinitionModel.cs

@@ -105,12 +105,14 @@ namespace ICSharpCode.SharpDevelop.Dom
 		#region Members collection
 		sealed class MemberCollection : IModelCollection<MemberModel>
 		{
+			readonly ModelCollectionChangedEvent<MemberModel> collectionChangedEvent;
 			readonly TypeDefinitionModel parent;
 			List<List<MemberModel>> lists = new List<List<MemberModel>>();
 			
 			public MemberCollection(TypeDefinitionModel parent)
 			{
 				this.parent = parent;
+				collectionChangedEvent = new ModelCollectionChangedEvent<MemberModel>();
 			}
 			
 			public void InsertPart(int partIndex, IUnresolvedTypeDefinition newPart)
@@ -120,16 +122,14 @@ namespace ICSharpCode.SharpDevelop.Dom
 					newItems.Add(new MemberModel(parent.context, newMember) { strongParentCollectionReference = this });
 				}
 				lists.Insert(partIndex, newItems);
-				if (collectionChanged != null)
-					collectionChanged(EmptyList<MemberModel>.Instance, newItems);
+				collectionChangedEvent.Fire(EmptyList<MemberModel>.Instance, newItems);
 			}
 			
 			public void RemovePart(int partIndex)
 			{
 				var oldItems = lists[partIndex];
 				lists.RemoveAt(partIndex);
-				if (collectionChanged != null)
-					collectionChanged(oldItems, EmptyList<MemberModel>.Instance);
+				collectionChangedEvent.Fire(oldItems, EmptyList<MemberModel>.Instance);
 			}
 			
 			public void UpdatePart(int partIndex, IUnresolvedTypeDefinition newPart)
@@ -158,7 +158,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 				// We might try to be clever here and find a LCS so that we only update the members that were actually changed,
 				// or we might consider moving members around (INotifyCollectionChanged supports moves)
 				// However, the easiest solution by far is to just remove + readd the whole middle portion.
-				var oldItems = collectionChanged != null ? list.GetRange(startPos, endPosOld - startPos) : null;
+				var oldItems = collectionChangedEvent.ContainsHandlers ? list.GetRange(startPos, endPosOld - startPos) : null;
 				list.RemoveRange(startPos, endPosOld - startPos);
 				var newItems = new MemberModel[endPosNew - startPos];
 				for (int i = 0; i < newItems.Length; i++) {
@@ -166,8 +166,8 @@ namespace ICSharpCode.SharpDevelop.Dom
 					newItems[i].strongParentCollectionReference = this;
 				}
 				list.InsertRange(startPos, newItems);
-				if (collectionChanged != null && (oldItems.Count > 0 || newItems.Length > 0)) {
-					collectionChanged(oldItems, newItems);
+				if (collectionChangedEvent.ContainsHandlers && (oldItems.Count > 0 || newItems.Length > 0)) {
+					collectionChangedEvent.Fire(oldItems, newItems);
 				}
 			}
 			
@@ -176,18 +176,16 @@ namespace ICSharpCode.SharpDevelop.Dom
 				return memberModel.SymbolKind == newMember.SymbolKind && memberModel.Name == newMember.Name;
 			}
 			
-			ModelCollectionChangedEventHandler<MemberModel> collectionChanged;
-			
 			public event ModelCollectionChangedEventHandler<MemberModel> CollectionChanged {
 				add {
-					collectionChanged += value;
+					collectionChangedEvent.AddHandler(value);
 					// Set strong reference to collection while there are event listeners
-					if (collectionChanged != null)
+					if (collectionChangedEvent.ContainsHandlers)
 						parent.membersStrongReference = this;
 				}
 				remove {
-					collectionChanged -= value;
-					if (collectionChanged == null)
+					collectionChangedEvent.RemoveHandler(value);
+					if (!collectionChangedEvent.ContainsHandlers)
 						parent.membersStrongReference = null;
 				}
 			}

src/Main/SharpDevelop/Project/Configuration/SolutionConfigurationOrPlatformNameCollection.cs

@@ -22,8 +22,7 @@ namespace ICSharpCode.SharpDevelop.Project
 	/// </remarks>
 	class SolutionConfigurationOrPlatformNameCollection : IConfigurationOrPlatformNameCollection
 	{
-		public event ModelCollectionChangedEventHandler<string> CollectionChanged;
-		
+		readonly ModelCollectionChangedEvent<string> collectionChangedEvent;
 		readonly List<string> list = new List<string>();
 		volatile IReadOnlyList<string> listSnapshot = EmptyList<string>.Instance;
 		readonly ISolution solution;
@@ -33,14 +32,23 @@ namespace ICSharpCode.SharpDevelop.Project
 		{
 			this.solution = solution;
 			this.isPlatform = isPlatform;
+			collectionChangedEvent = new ModelCollectionChangedEvent<string>();
 		}
 		
 		void OnCollectionChanged(IReadOnlyCollection<string> oldItems, IReadOnlyCollection<string> newItems)
 		{
 			this.listSnapshot = list.ToArray();
-			var eh = CollectionChanged;
-			if (eh != null)
-				eh(oldItems, newItems);
+			collectionChangedEvent.Fire(oldItems, newItems);
+		}
+		
+		public event ModelCollectionChangedEventHandler<string> CollectionChanged
+		{
+			add {
+				collectionChangedEvent.AddHandler(value);
+			}
+			remove {
+				collectionChangedEvent.RemoveHandler(value);
+			}
 		}
 		
 		#region IReadOnlyCollection implementation