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POWeird compilation error when indirectly refer to an assembly that declares a generic extension method with type restriction
 

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  1. POWeird compilation error when indirectly refer to an assembly that declares a generic extension method with type restriction
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    <p>Well, it's clear for me that the title of my question is too complicated. I just tried to make it as specific as possible. So, I'll try to explain the problem better.</p> <h2>Problem context</h2> <p>Let's assume we have three .NET projects in a solution. The main project is a simple console application <strong>ApplicationAssembly</strong>. This project has a reference to another managed assembly library <strong>DirectlyReferencedLibrary</strong>. At the same time DirectlyReferencedLibrary refers to <strong>IndirectlyUsedLibrary</strong>.</p> <p>So, the project usages looks like that: <strong>ApplicationAssembly --> DirectlyReferencedLibrary --> IndirectlyUsedLibrary</strong>.</p> <p>Notice that ApplicationAssembly doesn't use directly any type declared IndirectlyUsedLibrary. Let's also assume that all types declared in these assemblies reside in the same namespace.</p> <p>This solution compiles and works fine.</p> <h2>Weird problem</h2> <p>The problem occurs when I have together the following conditions:</p> <ol> <li>the <strong>ApplicationAssembly</strong> project has usages of LINQ expressions. For example, if there is the invocation of Select() on any object of enumerable type.</li> <li>The <strong>DirectlyReferencedLibrary</strong> declares a class which has a generic extension method with a type restriction. The type restriction says that the generic type must be a descendant of a class from the <strong>IndirectlyUsedLibrary</strong>.</li> </ol> <p>Here is the example of a such class.</p> <pre><code>using System; namespace Test { public static class UnUsedType { // It's a generic extension method with a type restriction. public static void Magic&lt;T&gt;(this T @this) // It's a type restriction that uses a type from the IndirectlyUsedLibrary. where T : ProblemType { Console.WriteLine("I do nothing actually."); } } } </code></pre> <p>When I try to compile this project, I get the following error:</p> <blockquote> <p>Error <strong>The type 'Test.ProblemType' is defined in an assembly that is not referenced. You must add a reference to assembly 'IndirectlyUsedLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. C:\Projects\Test\ApplicationAssembly\Program.cs</strong> 22 13 ApplicationAssembly</p> </blockquote> <h2>Question</h2> <p>Can anyone help me to understand why is it so?</p> <h2>P.S.</h2> <p>I've made a tiny solution for investigation. <em>If you are so kind to help me, you will be able to take <strong><a href="http://www.mediafire.com/?ca697za1zs4dhad" rel="nofollow" title="here">an archived solution here</a></em></strong></p> <h2>P.P.S.</h2> <p>Sorry for my poor English.</p> <h2>UPD1</h2> <p>Another strange thing is that different invocations of the LINQ method may or may not produce the compile time error:</p> <pre><code>// Ok. Let's do some work using LINQ we love so much! var strings = new[] { "aaa", "bbb", "ccc" }; Func&lt;string, object&gt; converter = item =&gt; (object) item; // The following line makes problems. var asObjects1 = strings.Select(converter); // Everything is OK if we use the following line: var asObjects2 = Enumerable.Select(strings, converter); </code></pre>
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    1. USIgor Soloydenko
    1. USIgor Soloydenko
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    1. COWhy don't you add a reference to `IndirectlyUsedLibrary` in `ApplicationAssembly` project
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    2. COIt's simple. I don't want to do it. There is no sense in it in my real project. I don't know how to express it polite in English... Don't try to change the definition of problem. I want to understand, why the compiler asks me for that reference.
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      1. USIgor Soloydenko
 

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