{"id":942,"date":"2011-02-09T15:30:07","date_gmt":"2011-02-09T14:30:07","guid":{"rendered":"http:\/\/blog.eweibel.net\/?p=942"},"modified":"2011-02-09T16:00:56","modified_gmt":"2011-02-09T15:00:56","slug":"enums-and-inheritance-in-net","status":"publish","type":"post","link":"https:\/\/blog.eweibel.net\/?p=942","title":{"rendered":"Enums and inheritance in .Net"},"content":{"rendered":"

In one of my current projects I had the following code (I simplified the code a bit):<\/p>\n

\n
public string ConnectionString\n{\n\tget\n\t{\n\t\tswitch(this.Importer)\n\t\t{\n\t\t\tcase Importer.SqlServer:\n\t\t\t\treturn "Server=localhost;Database=Northwind";\n\t\t\tcase Importer.SqlServerOleDb:\n\t\t\t\treturn"Provider=SQLOLEDB;Data Source=localhost;Initial Catalog=Northwind";\n\t\t\tdefault:\n\t\t\t\tthrow new NotSupportedException(\n\t\t\t\tstring.Format("Importer {0} is not supported yet.", this.Importer));\n\t\t}\n\t}\n}<\/pre>\n<\/div>\n
After running the code coverage tool (dotCover from JetBrains<\/a>) I received the following picture:<\/p>\n
\"CodeCoverage\"<\/a><\/p>\n
First idea<\/strong><\/p>\n
So, my code was clear and understandable, but obviously not fully tested (green: covered by tests, red: not covered). I asked me the question, how could I test the rest of the method.<\/p>\n
My idea was to extend the enum:<\/p>\n
\n
[TestFixture]\npublic class ConfigurationTest\n{\n        #region Test methods\n        #endregion\n\n        private enum TestImporter : Importer\n        {\n            Test\n        }\n}<\/pre>\n<\/div>\n
But the compiler and the ReSharper<\/a> weren\u2019t happy with my idea: \u201cType byte, sbyte, short, ushort, int, uint, long, or ulong expected\u201d. I was surprised at this error: Why it isn\u2019t possible to make a subclass of an enum?<\/p>\n
I found the explanation first on stackoverflow<\/a> and then in the specification of the Common Language Infrastructure (CLI, section 8.5.2)<\/a>:<\/p>\n
\n
It shall derive from System.Enum<\/p>\n<\/blockquote>\n
An enum in .Net has to derive directly from enum, it isn\u2019t possible to inherit from another enum. So, the enums in .Net don\u2019t follow the open\/closed principle<\/a> (OCP). With the OCP you don\u2019t modify a class when you want to extend it, you inherit from it and do the extensions in the subclass:<\/p>\n
\n
Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.<\/p>\n<\/blockquote>\n
Second idea<\/strong><\/p>\n
My code wasn\u2019t tested and is it really not possible to test it? Well, it is possible, but the code isn\u2019t very nice (hard cast):<\/p>\n
\n
 [Test]\n[ExpectedException(typeof(NotSupportedException))]\npublic void TestConnectionString_With_Invalid_Enum()\n{\n\t\/\/ Arrange\n\tthis.Configuration.Importer = (Importer) 100;\n\n\t\/\/ Act\n\tstring strConnectionstring = this.Configuration.ConnectionString;\n}<\/pre>\n<\/div>\n
Conclusion<\/strong><\/p>\n
I my current decision is to leave the code as it is. The reasons were the following:<\/p>\n