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POHow could an extension method be attached to a generic class when the type argument is IEnumerable<T>?
 

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  1. POHow could an extension method be attached to a generic class when the type argument is IEnumerable<T>?
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    <p>As a hobby project (and to immerse myself more deeply in generics/extension methods), I'm writing a parameter checking library!</p> <p>I have a model called Argument that describes a parameter and looks like this:</p> <pre><code>public class Argument&lt;T&gt; { internal Argument(string name, T value) { Name = name; Value = value; } public string Name { get; private set; } public T Value { get; private set; } } </code></pre> <p>When validation for a parameter begins, an instance of this object is created, and individual validations are performed by invoking extension methods (that contain the actual logic) that hang off of it.</p> <p>One such extension method verifies that a collection contains at least one item, and currently looks like this:</p> <pre><code>public static Argument&lt;IEnumerable&lt;T&gt;&gt; HasItems&lt;T&gt;(this Argument&lt;IEnumerable&lt;T&gt;&gt; argument) { if (!argument.Value.Any()) throw Error.Generic(argument.Name, "Collection contains no items."); return argument; } </code></pre> <p>But it doesn't appear to work. If I were, say, to write this unit test:</p> <pre><code>[TestMethod] public void TestMethod1() { var argument = new List&lt;int&gt;() { 1, 2, 6, 3, -1, 5, 0 }; Validate.Argument("argument", argument) .IsNotNull() .HasItems() .All(v =&gt; v.IsGreaterThan(0)); } </code></pre> <p>HasItems doesn't show up in Intellisense, <em>and</em> I get this compile error:</p> <blockquote> <p><code>'Validation.Argument&lt;System.Collections.Generic.List&lt;int&gt;&gt;'</code> does not contain a definition for 'HasItems' and no extension method 'HasItems' accepting a first argument of type <code>'Validation.Argument&lt;System.Collections.Generic.List&lt;int&gt;&gt;'</code> could be found (are you missing a using directive or an assembly reference?)</p> </blockquote> <p>And if I try passing the value directly into the extension method, like so:</p> <pre><code>CollectionTypeExtensions.HasItems(Validate.Argument("argument", argument)); </code></pre> <p>I get this:</p> <blockquote> <p>The best overloaded method match for <code>'Validation.CollectionTypeExtensions.HasItems&lt;int&gt;(Validation.Argument&lt;System.Collections.Generic.IEnumerable&lt;int&gt;&gt;)'</code> has some invalid arguments</p> </blockquote> <p>Based on my research, what I'd need for this to work is called "variance," and applies to interfaces and delegates, but not to classes (ie: all classes are invariant.)</p> <p>That said, it might work another way. One that comes to mind is to rewrite it to go straight to T, like so:</p> <pre><code>public static Argument&lt;T&gt; HasItems&lt;T, TElement&gt;(this Argument&lt;T&gt; argument) where T : IEnumerable&lt;TElement&gt; { if (!argument.Value.Any()) throw Error.Generic(argument.Name, "Collection contains no items."); return argument; } </code></pre> <p>..But that doesn't work either because it requires TElement to be specified explicitly when the method is invoked. I could also fall back to using the non-generic IEnumerable interface in the type constraint, but then I'd have to either find a way to coerce the IEnumerable into IEnumerable (which would require knowing what T is in that context), or duplicating the functionality of Any() in order to test for the existence of any items, <em>and</em> there's another extension method (All) that that would be very, very messy for, so I'd rather avoid it.</p> <p>So ultimately, I guess my question is: how do I get my extension method to attach properly?</p>
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