{"id":4552,"date":"2019-05-10T21:59:28","date_gmt":"2019-05-11T01:59:28","guid":{"rendered":"https:\/\/ikriv.com\/blog\/?p=4552"},"modified":"2019-05-10T22:08:26","modified_gmt":"2019-05-11T02:08:26","slug":"c-how-to-shoot-yourself-in-the-foot-with-a-generic-extension-method","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=4552","title":{"rendered":"C#: How to shoot yourself in the foot with a generic extension method"},"content":{"rendered":"\n

TL;DR<\/strong>: wrapping existing method DoSomething(MyData data)<\/code>, with an identically named generic extension method DoSomething<T>(T data)<\/code> is a bad idea, since it erases compile-time type checking. DoSomething()<\/code> may now be called with any type, and it may be hard to tell whether you are calling the original or the wrapper.<\/p>\n\n\n\n

The following is a true story, even though some details have been changed to protect the innocent. Suppose we have a messaging library along these lines:<\/p>\n\n\n\n

interface<\/span> IMessage<\/span>\n{\n    Type<\/span> Type { get<\/span>; }\n    object<\/span> Data { get<\/span>; }\n}\n\nclass<\/span> Message<\/span> : IMessage<\/span>\n{\n    public<\/span> Type<\/span> Type { get<\/span>; set<\/span>; }\n    public<\/span> object<\/span> Data { get<\/span>; set<\/span>; }\n}\n\nclass<\/span> Sender<\/span>\n{\n    public<\/span> void<\/span> Send(Message<\/span> message)\n    {\n        \/\/ ...do actual sending here...<\/span>\n        Console<\/span>.WriteLine($"Send: Type=<\/span>{message.Type.Name}, Data=<\/span>{message.Data}"<\/span>);\n    }\n}\n<\/pre>\n\n\n\n
Expressions like sender.Send(new Message {Type = typeof(int), Data = 42})<\/code> quickly become an annoyance, so we create an extension method to simplify them:<\/p>\n\n\n\n
static<\/span> class<\/span> SenderExtensions<\/span>\n{\n    public<\/span> static<\/span> void<\/span> Send<T<\/span>>(this<\/span> Sender<\/span> sender, T<\/span> data)\n    {\n        sender.Send(new<\/span> Message<\/span> {Type = typeof<\/span>(T<\/span>), Data = data});\n    }\n}\n<\/pre>\n\n\n\n
Now we can simply write sender.Send(42)<\/code>and it works as expected. But… Consider this code:<\/p>\n\n\n\n
var<\/span> message = new<\/span> Message<\/span> {Type = typeof<\/span>(int<\/span>), Data = 42};\nsender.Send(message); \/\/ prints Send: Type=Int32, Data=42<\/span>\n\nvar<\/span> iMessage = (IMessage<\/span>) message;\nsender.Send(iMessage); \/\/ oops! Type=IMessage, Data=ShootFoot1.Message<\/span>\n<\/pre>\n\n\n\n
We cast our message to IMessage<\/code> type, but the Send()<\/code> method expects parameter of type Message<\/code>. This by itself is probably a design error, but let’s ignore it. \u00a1No problemo! says the compiler. Passing IMessage<\/code> compiles, but it calls the extension method instead of the regular one, and wraps our message in another Message<\/code> object.<\/p>\n\n\n\n
The same would have happened if we had a generic overload of Send()<\/code>, but extension method is especially evil, since it is invisilbe. You cannot guess its existence by looking at either the call site or the definition of the Sender<\/code> class. I personally marveled at the wrapped message in debugger for at least 10 minutes before I realized what’s going on. Obviously, the real program was much more complex and had many additional details that caused distraction.<\/p>\n\n\n\n
Moral of the story: if you create a generic extension method like this, give it a distinct name. E.g. if the extension method were called SendObject()<\/code>, then Send(iMessage)<\/code>would simply have not compiled.
<\/p>\n\n\n\n
Complete code:<\/strong>
The original, buggy version<\/a>
Fixed version with renamed extension method<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
TL;DR: wrapping existing method DoSomething(MyData data), with an identically named generic extension method DoSomething<T>(T data) is a bad idea, since it erases compile-time type checking. DoSomething() may now be called […]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"Layout":"","footnotes":""},"categories":[3,4],"tags":[],"class_list":["entry","author-ikriv","post-4552","post","type-post","status-publish","format-standard","category-dotnet","category-hack"],"_links":{"self":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4552","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4552"}],"version-history":[{"count":18,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4552\/revisions"}],"predecessor-version":[{"id":4570,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4552\/revisions\/4570"}],"wp:attachment":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4552"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4552"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4552"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}