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    <p>Okay, this is just to put a bit more flesh on Michael's answer (he's welcome to add it in if he wants to, in which case I'll remove this one).</p> <p>Looking at the original PIA for Word.Application, there are three types involved (ignoring the events):</p> <pre><code>[ComImport, TypeLibType(...), Guid("..."), DefaultMember("Name")] public interface _Application { ... } [ComImport, Guid("..."), CoClass(typeof(ApplicationClass))] public interface Application : _Application { } [ComImport, ClassInterface(...), ComSourceInterfaces("..."), Guid("..."), TypeLibType((short) 2), DefaultMember("Name")] public class ApplicationClass : _Application, Application { } </code></pre> <p>There are two interfaces for reasons that Eric Lippert talks about in <a href="https://stackoverflow.com/questions/1051464/excel-interop-worksheet-or-worksheet/1051546#1051546">another answer</a>. And there, as you said, is the <code>CoClass</code> - both in terms of the class itself and the attribute on the <code>Application</code> interface.</p> <p>Now if we use PIA linking in C# 4, <em>some</em> of this is embedded in the resulting binary... but not all of it. An application which <em>just</em> creates an instance of <code>Application</code> ends up with these types:</p> <pre><code>[ComImport, TypeIdentifier, Guid("..."), CompilerGenerated] public interface _Application [ComImport, Guid("..."), CompilerGenerated, TypeIdentifier] public interface Application : _Application </code></pre> <p>No <code>ApplicationClass</code> - presumably because that will be loaded dynamically from the <em>real</em> COM type at execution time.</p> <p>Another interesting thing is the difference in the code between the linked version and the non-linked version. If you decompile the line</p> <pre><code>Word.Application application = new Word.Application(); </code></pre> <p>in the <em>referenced</em> version it ends up as:</p> <pre><code>Application application = new ApplicationClass(); </code></pre> <p>whereas in the <em>linked</em> version it ends up as</p> <pre><code>Application application = (Application) Activator.CreateInstance(Type.GetTypeFromCLSID(new Guid("..."))); </code></pre> <p>So it looks like the "real" PIA needs the <code>CoClass</code> attribute, but the linked version doesn't because there <em>isn't</em> a <code>CoClass</code> the compiler can actually reference. It has to do it dynamically.</p> <p>I might try to fake up a COM interface using this information and see if I can get the compiler to link it...</p>
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