Add IsKnownType() extension method.

ICSharpCode.NRefactory/TypeSystem/ExtensionMethods.cs

@@ -92,7 +92,7 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		}
 		#endregion
 		
-		#region IsOpen / IsUnbound
+		#region IsOpen / IsUnbound / IsKnownType
 		sealed class TypeClassificationVisitor : TypeVisitor
 		{
 			internal bool isOpen;
@@ -140,6 +140,16 @@ namespace ICSharpCode.NRefactory.TypeSystem
 				throw new ArgumentNullException("type");
 			return type is ITypeDefinition && type.TypeParameterCount > 0;
 		}
+		
+		/// <summary>
+		/// Gets whether the type is the specified known type.
+		/// For generic known types, this returns true any parameterization of the type (and also for the definition itself).
+		/// </summary>
+		public static bool IsKnownType(this IType type, KnownTypeCode knownType)
+		{
+			var def = type.GetDefinition();
+			return def != null && def.KnownTypeCode == knownType;
+		}
 		#endregion
 		
 		#region Import