For IAssembly.GetTypeDefinition, treat ns==null the same as ns==string.Empty.

ICSharpCode.NRefactory.CSharp/TypeSystem/CSharpAssembly.cs

@@ -154,7 +154,7 @@ namespace ICSharpCode.NRefactory.CSharp.TypeSystem
 		
 		public ITypeDefinition GetTypeDefinition(string ns, string name, int typeParameterCount)
 		{
-			var key = new FullNameAndTypeParameterCount(ns, name, typeParameterCount);
+			var key = new FullNameAndTypeParameterCount(ns ?? string.Empty, name, typeParameterCount);
 			DefaultResolvedTypeDefinition def;
 			if (GetTypes().TryGetValue(key, out def))
 				return def;

ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultUnresolvedAssembly.cs

@@ -105,7 +105,7 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 		
 		public IUnresolvedTypeDefinition GetTypeDefinition(string ns, string name, int typeParameterCount)
 		{
-			var key = new FullNameAndTypeParameterCount(ns, name, typeParameterCount);
+			var key = new FullNameAndTypeParameterCount(ns ?? string.Empty, name, typeParameterCount);
 			IUnresolvedTypeDefinition td;
 			if (typeDefinitions.TryGetValue(key, out td))
 				return td;

ICSharpCode.NRefactory/TypeSystem/Implementation/FullNameAndTypeParameterCount.cs

@@ -30,6 +30,10 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 		
 		public FullNameAndTypeParameterCount(string nameSpace, string name, int typeParameterCount)
 		{
+			if (nameSpace == null)
+				throw new ArgumentNullException("nameSpace");
+			if (name == null)
+				throw new ArgumentNullException("name");
 			this.Namespace = nameSpace;
 			this.Name = name;
 			this.TypeParameterCount = typeParameterCount;