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POC# generics problem - newing up the generic type with parameters in the constructor
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  1. POC# generics problem - newing up the generic type with parameters in the constructor
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    copied!<p>I am trying to create a generic class which new's up an instance of the generic type. As follows:</p> <pre><code>public class HomepageCarousel&lt;T&gt; : List&lt;T&gt; where T: IHomepageCarouselItem, new() { private List&lt;T&gt; GetInitialCarouselData() { List&lt;T&gt; carouselItems = new List&lt;T&gt;(); if (jewellerHomepages != null) { foreach (PageData pageData in jewellerHomepages) { T item = new T(pageData); // this line wont compile carouselItems.Add(item); } } return carouselItems; } } </code></pre> <p>But I get the following error:</p> <blockquote> <p><strong><em>cannot provide arguments when creating an instance of a variable type</em></strong></p> </blockquote> <p>I found the following related question which is very close to what I need: <a href="https://stackoverflow.com/questions/840261/c-generic-new-constructor-problem/1681770#1681770">Passing arguments to C# generic new() of templated type</a></p> <p>However, I can't used Jared's suggested answer as I am calling the method within the Generic class, not outside of it, so I can't specify the concrete class.</p> <p>Is there a way around this?</p> <p>I have tried the following based on the other question, but it doesn't work as I don't know the concrete type of T to specify. As it is called from inside the generic class, not outside:</p> <pre><code>public class HomepageCarousel&lt;T&gt; : List&lt;T&gt; where T: IHomepageCarouselItem, new() { private List&lt;T&gt; LoadCarouselItems() { if (IsCarouselConfigued) { return GetConfiguredCarouselData(); } // ****** I don't know the concrete class for the following line, // so how can it be instansiated correctly? return GetInitialCarouselData(l =&gt; new T(l)); } private List&lt;T&gt; GetInitialCarouselData(Func&lt;PageData, T&gt; del) { List&lt;T&gt; carouselItems = new List&lt;T&gt;(); if (jewellerHomepages != null) { foreach (PageData pageData in jewellerHomepages) { T item = del(pageData); carouselItems.Add(item); } } return carouselItems; } } </code></pre> <p>********EDIT : ADDED POSSIBLE SOLUTIONS**</p> <p>So I have tested 2 possible solutions:</p> <p>First is exactly as explained below by <a href="https://stackoverflow.com/users/22656/jon-skeet">Jon Skeet</a>. This definitely works but means having an obscure lambda in the constructor. I am not very comfortable with this as it means users need to know the correct lambda that is expected. After all, they could pass a lambda which doesn't new up the type, but does something entirely unexpected</p> <p>Secondly, I went down the Factory method route; I added a Create method to the common interface:</p> <pre><code>IJewellerHomepageCarouselItem Create(PageData pageData); </code></pre> <p>Then provided an implementation in each Concrete class:</p> <pre><code>public IJewellerHomepageCarouselItem Create(PageData pageData) { return new JewellerHomepageCarouselItem(pageData, null); } </code></pre> <p>And used a two step initialisation syntax:</p> <pre><code>T carouselItem = new T(); T homepageMgmtCarouselItem = (T) carouselItem.Create(jewellerPage); </code></pre> <p>Would love to hear some feedback on the merit of each of these approaches.</p>
 

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