diff --git a/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs b/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs
index 9d4b360635..7e0a89d8f8 100644
--- a/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs
@@ -107,7 +107,7 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		/// <summary>
 		/// Gets the effective interface set of this type parameter.
 		/// </summary>
-		IList<IType> EffectiveInterfaceSet { get; }
+		ICollection<IType> EffectiveInterfaceSet { get; }
 		
 		/// <summary>
 		/// Gets if the type parameter has the 'new()' constraint.
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractResolvedTypeParameter.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractResolvedTypeParameter.cs
index 8b12e0e920..cede6c7a8b 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractResolvedTypeParameter.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractResolvedTypeParameter.cs
@@ -94,48 +94,73 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 		
 		public IType EffectiveBaseClass {
 			get {
-				if (effectiveBaseClass == null)
-					effectiveBaseClass = CalculateEffectiveBaseClass();
+				if (effectiveBaseClass == null) {
+					// protect against cyclic type parameters
+					using (var busyLock = BusyManager.Enter(this)) {
+						if (!busyLock.Success)
+							return SpecialType.UnknownType; // don't cache this error
+						effectiveBaseClass = CalculateEffectiveBaseClass();
+					}
+				}
 				return effectiveBaseClass;
 			}
 		}
 		
 		IType CalculateEffectiveBaseClass()
 		{
-			// protect against cyclic type parameters
-			using (var busyLock = BusyManager.Enter(this)) {
-				if (!busyLock.Success)
-					return SpecialType.UnknownType;
-				
-				if (HasValueTypeConstraint)
-					return this.Compilation.FindType(KnownTypeCode.ValueType);
-				
-				List<IType> classTypeConstraints = new List<IType>();
-				foreach (IType constraint in this.DirectBaseTypes) {
-					if (constraint.Kind == TypeKind.Class) {
-						classTypeConstraints.Add(constraint);
-					} else if (constraint.Kind == TypeKind.TypeParameter) {
-						IType baseClass = ((ITypeParameter)constraint).EffectiveBaseClass;
-						if (baseClass.Kind == TypeKind.Class)
-							classTypeConstraints.Add(baseClass);
-					}
-				}
-				if (classTypeConstraints.Count == 0)
-					return this.Compilation.FindType(KnownTypeCode.Object);
-				// Find the derived-most type in the resulting set:
-				IType result = classTypeConstraints[0];
-				for (int i = 1; i < classTypeConstraints.Count; i++) {
-					if (classTypeConstraints[i].GetDefinition().IsDerivedFrom(result.GetDefinition()))
-						result = classTypeConstraints[i];
+			if (HasValueTypeConstraint)
+				return this.Compilation.FindType(KnownTypeCode.ValueType);
+			
+			List<IType> classTypeConstraints = new List<IType>();
+			foreach (IType constraint in this.DirectBaseTypes) {
+				if (constraint.Kind == TypeKind.Class) {
+					classTypeConstraints.Add(constraint);
+				} else if (constraint.Kind == TypeKind.TypeParameter) {
+					IType baseClass = ((ITypeParameter)constraint).EffectiveBaseClass;
+					if (baseClass.Kind == TypeKind.Class)
+						classTypeConstraints.Add(baseClass);
 				}
-				return result;
 			}
+			if (classTypeConstraints.Count == 0)
+				return this.Compilation.FindType(KnownTypeCode.Object);
+			// Find the derived-most type in the resulting set:
+			IType result = classTypeConstraints[0];
+			for (int i = 1; i < classTypeConstraints.Count; i++) {
+				if (classTypeConstraints[i].GetDefinition().IsDerivedFrom(result.GetDefinition()))
+					result = classTypeConstraints[i];
+			}
+			return result;
 		}
 		
-		public IList<IType> EffectiveInterfaceSet {
+		ICollection<IType> effectiveInterfaceSet;
+		
+		public ICollection<IType> EffectiveInterfaceSet {
 			get {
-				throw new NotImplementedException();
+				var result = LazyInit.VolatileRead(ref effectiveInterfaceSet);
+				if (result != null) {
+					return result;
+				} else {
+					// protect against cyclic type parameters
+					using (var busyLock = BusyManager.Enter(this)) {
+						if (!busyLock.Success)
+							return EmptyList<IType>.Instance; // don't cache this error
+						return LazyInit.GetOrSet(ref effectiveInterfaceSet, CalculateEffectiveInterfaceSet());
+					}
+				}
+			}
+		}
+		
+		ICollection<IType> CalculateEffectiveInterfaceSet()
+		{
+			HashSet<IType> result = new HashSet<IType>();
+			foreach (IType constraint in this.DirectBaseTypes) {
+				if (constraint.Kind == TypeKind.Interface) {
+					result.Add(constraint);
+				} else if (constraint.Kind == TypeKind.TypeParameter) {
+					result.UnionWith(((ITypeParameter)constraint).EffectiveInterfaceSet);
+				}
 			}
+			return result;
 		}
 		
 		public abstract bool HasDefaultConstructorConstraint { get; }
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/DummyTypeParameter.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/DummyTypeParameter.cs
index 22e7fc4fc8..f0ec26d0e1 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/DummyTypeParameter.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/DummyTypeParameter.cs
@@ -189,7 +189,7 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 			get { return SpecialType.UnknownType; }
 		}
 		
-		IList<IType> ITypeParameter.EffectiveInterfaceSet {
+		ICollection<IType> ITypeParameter.EffectiveInterfaceSet {
 			get { return EmptyList<IType>.Instance; }
 		}