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POWhy can't a list of an interface type accept instances of an inheriting interface?
 

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  1. POWhy can't a list of an interface type accept instances of an inheriting interface?
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    <p>Given the following types:</p> <pre><code>public interface IPrimary{ void doBattle(); } // an ISecondary "is" an IPrimary public interface ISecondary : IPrimary { } // An implementation of ISecondary is also an IPrimary: internal class SecondaryImpl : ISecondary { // Required, since this is an IPrimary public void doBattle(){ } } </code></pre> <p>Why can I not do this? </p> <pre><code>List&lt;IPrimary&gt; list = new List&lt;ISecondary&gt;(); </code></pre> <p>This results in the following compilation error:</p> <blockquote> <p>Argument type 'System.Collections.Generic.List' is not assignable to parameter type 'System.Collections.Generic.List'</p> </blockquote> <p>I understand the error, and I realize there are workarounds. I just do not see any clear reason why this direct conversion is disallowed. The values contained in a list of <code>ISecondary</code>, should after all, be (by extension) values of type of <code>IPrimary</code>.Why then are <code>List&lt;IPrimary&gt;</code> and <code>List&lt;ISecondary&gt;</code> being interpreted as unrelated types?</p> <p><strong>Can anyone explain clearly the reasoning for C# being designed this way?</strong></p> <p><em>A slightly extended example:</em> I came across the issue when trying to do something similar to the following:</p> <pre><code>internal class Program { private static void Main(string[] args) { // Instance of ISecondary, and by extention, IPrimary: var mySecondaryInstance = new SecondaryImpl(); // This works as expected: AcceptImpl(mySecondaryInstance); // List of instances of ISecondary, which are also, // by extention, instances of IPrimary: var myListOfSecondaries = new List&lt;ISecondary&gt; {mySecondaryInstance}; // This, however, does not work (results in a compilation error): AcceptList(myListOfSecondaries); } // Note: IPrimary parameter: public static void AcceptImpl(IPrimary instance){ } // Note: List of type IPrimary: public static void AcceptList(List&lt;IPrimary&gt; list){ } } </code></pre>
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    1. COYou want to look at "Generics Covariance in C#"
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      1. USKarthik T
    2. CO[Covariance and contravariance](http://msdn.microsoft.com/en-gb/library/ee207183.aspx). And `List<IPrimary> list = new List<ISecondary>();` makes no sense to actually do, even if you could.
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      1. USGrant Thomas
    3. CO@GrantThomas Ok, maybe a bit simplified there; but what about `someListOfTypePrimary.AddRange(someProvider.getListOftypeSecondary());`? Is there some reason this should not occur?
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