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copied!<p>This is because as of yet, C# does not support <a href="http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#C.23" rel="nofollow noreferrer">Covariance</a>.</p> <blockquote> <p>More formally, in C# v2.0 if T is a subtype of U, then T[] is a subtype of U[], but G is not a subtype of G (where G is any generic type). In type-theory terminology, we describe this behavior by saying that C# array types are “covariant” and generic types are “invariant”.</p> </blockquote> <p><em>Reference: <a href="http://blogs.msdn.com/rmbyers/archive/2005/02/16/375079.aspx" rel="nofollow noreferrer">http://blogs.msdn.com/rmbyers/archive/2005/02/16/375079.aspx</a></em></p> <p>If you have the following method :</p> <pre><code>public static void AllDoSomething(List<StuffBase> items) { items.ForEach(i => i.DoSomething()); } var items = new List<Stuff2>(); x.AllDoSomething(items); //Does not compile </code></pre> <p>Where as if you use the generic type constraint, it will.</p> <p>For more information about Covariance and Contravariance], check out <a href="http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx" rel="nofollow noreferrer">Eric Lippert's series of posts</a>.</p> <hr> <p>Other posts worth reading : </p> <ul> <li><a href="http://www.pabich.eu/blog/archive/2008/02/12/c-generics---parameter-variance-its-constraints-and-how-it.aspx" rel="nofollow noreferrer">http://www.pabich.eu/blog/archive/2008/02/12/c-generics---parameter-variance-its-constraints-and-how-it.aspx</a></li> <li><a href="http://blogs.msdn.com/rmbyers/archive/2006/06/01/613690.aspx" rel="nofollow noreferrer">http://blogs.msdn.com/rmbyers/archive/2006/06/01/613690.aspx</a></li> <li><a href="http://msdn.microsoft.com/en-us/library/ms228359(VS.80).aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/ms228359(VS.80).aspx</a></li> <li><a href="http://www.csharp411.com/convert-between-generic-ienumerablet/" rel="nofollow noreferrer">http://www.csharp411.com/convert-between-generic-ienumerablet/</a></li> <li><a href="http://research.microsoft.com/apps/pubs/default.aspx?id=64042" rel="nofollow noreferrer">http://research.microsoft.com/apps/pubs/default.aspx?id=64042</a></li> <li><a href="https://stackoverflow.com/questions/1169215/why-cant-listparent-listchild">Why can't List<parent> = List<child>?</a></li> </ul>

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