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POHow to "override" extension methods in .NET?
 

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  1. POHow to "override" extension methods in .NET?
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    <p>So I have an object hierarchy to produce ui controls in asp.net mvc and try to achive a fluent api. I make some dummy class to focus on the current problem.<br> So here is the "wrong" codebase:</p> <pre><code>public abstract class HtmlElement { /* ... */ } public abstract class UIElement : HtmlElement { /* ... */ } public abstract class ButtonBase : UIElement { /* ... */ } public class LinkButton : ButtonBase { /* ... */ } public class ActionButton : ButtonBase { /* ... */ } public static class HtmlElementExtensions { public static T Id&lt;T&gt;(this T item, string id) where T : HtmlElement { /* set the id */ return item; } } public static class ButtonBaseExtensions { public static T Id&lt;T&gt;(this T item, string id) where T : ButtonBase { /* set the id and do some button specific stuff*/ return item; } } </code></pre> <p>When I try to call the Id on a LinkButton the compiler says there is ambiguous call: </p> <pre><code>LinkButton lb = new LinkButton().Id("asd"); </code></pre> <p>I really thought that the compiler choose the closest match in this case so if I have a Script class that inherits from HtmlElement than HtmlExtensions Id method called, and for a LinkButton (because of the restrictions) the ButtonBase method will be called. I have a solution, but I'm not sure there is a better one.<br> I delete the Id method from ButtonBaseExtensions and modified the HtmlElementExtensions Id method as the following manner:</p> <pre><code>public static T Id&lt;T&gt;(this T item, string id) where T : HtmlElement { if (item is ButtonBase) { /* do some button specific stuff*/ } /* set the id */ return item; } </code></pre> <p>This way every class descendant from the ButtonBase is working. I not really like my solution because it mix the HtmlElement logic with the ButtonBase logic. Any idea/suggestion for a better solution? I thought I put them in different namespace, but just for a second. I should have to use both namespace, so don't solve the problem. </p> <p>Do you think it's worth to mention on the msdn forums as an idea that the compiler should watch the restrictions on generic extension methods?</p> <p>In the meantime I make some more research and start a thread on msdn forums: <a href="http://social.msdn.microsoft.com/Forums/en-US/clr/thread/c9ad51c1-0c5a-4eb6-a530-89eaa628c07f" rel="nofollow">link</a><br> I tried some non-generic extension methodds: </p> <pre><code> public class BaseClass { /*...*/ } public class InheritedClass : BaseClass { /*...*/ } public static class BaseClassExtensions { public static void SomeMethod(this BaseClass item, string someParameter) { Console.WriteLine(string.Format("BaseClassExtensions.SomeMethod called wtih parameter: {0}", someParameter)); } } public static class InheritedClassExtensions { public static void SomeMethod(this InheritedClass item, string someParameter) { Console.WriteLine(string.Format("InheritedClassExtensions.SomeMethod called wtih parameter: {0}", someParameter)); } } </code></pre> <p>And if I instantiate these:</p> <pre><code>BaseClass bc = new BaseClass(); InheritedClass ic = new InheritedClass(); BaseClass ic_as_bc = new InheritedClass(); bc.SomeMethod("bc"); ic.SomeMethod("ic"); ic_as_bc.SomeMethod("ic_as_bc"); </code></pre> <p>Produced this output:</p> <pre><code>BaseClassExtensions.SomeMethod called wtih parameter: bc InheritedClassExtensions.SomeMethod called wtih parameter: ic BaseClassExtensions.SomeMethod called wtih parameter: ic_as_bc </code></pre> <p><a href="http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/3886276-generic-extension-method-type-restriction-should-b" rel="nofollow">You can vote it for now</a></p> <p>Thanks,<br> Péter</p>
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    1. COAn extension method is **not** a substitute for a virtual method, nor can it be made virtual. It is quite unclear why you need one, given that you declared these classes yourself. Just add a virtual method to one of the base classes and override it, where necessary, in a derived class.
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    2. COBecause of the fluent api I have to use extension methods. The generics inheritance only works with 2 level inheritence. I wrote an example with the non-genric extension method where the compiler use the parameter restriction to determine the right method to call. I just wish that the compiler also evaluate generic parameter restrictions. I can't write "public static ButtonBase Id(this ButtonBase item, string id)" instead of "public static T Id<T>(this T item, string id) where T : ButtonBase" because the return type would be ButtonBase and it break the method chain of the fluent api.
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