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POHow does the C# compiler detect COM types?
 

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  1. POHow does the C# compiler detect COM types?
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    <p><strong>EDIT:</strong> I've written the results up as a <a href="http://codeblog.jonskeet.uk/2009/07/07/faking-com-to-fool-the-c-compiler.aspx" rel="noreferrer">blog post</a>.</p> <hr> <p>The C# compiler treats COM types somewhat magically. For instance, this statement looks normal...</p> <pre><code>Word.Application app = new Word.Application(); </code></pre> <p>... until you realise that <code>Application</code> is an interface. Calling a constructor on an interface? Yoiks! This actually gets translated into a call to <a href="http://msdn.microsoft.com/en-us/library/system.type.gettypefromclsid.aspx" rel="noreferrer"><code>Type.GetTypeFromCLSID()</code></a> and another to <a href="http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx" rel="noreferrer"><code>Activator.CreateInstance</code></a>.</p> <p>Additionally, in C# 4, you can use non-ref arguments for <code>ref</code> parameters, and the compiler just adds a local variable to pass by reference, discarding the results:</p> <pre><code>// FileName parameter is *really* a ref parameter app.ActiveDocument.SaveAs(FileName: "test.doc"); </code></pre> <p>(Yeah, there are a bunch of arguments missing. Aren't optional parameters nice? :)</p> <p>I'm trying to investigate the compiler behaviour, and I'm failing to fake the first part. I can do the second part with no problem:</p> <pre><code>using System; using System.Runtime.InteropServices; using System.Runtime.CompilerServices; [ComImport, GuidAttribute("00012345-0000-0000-0000-000000000011")] public interface Dummy { void Foo(ref int x); } class Test { static void Main() { Dummy dummy = null; dummy.Foo(10); } } </code></pre> <p>I'd like to be able to write:</p> <pre><code>Dummy dummy = new Dummy(); </code></pre> <p>though. Obviously it'll go bang at execution time, but that's okay. I'm just experimenting.</p> <p>The other attributes added by the compiler for linked COM PIAs (<code>CompilerGenerated</code> and <code>TypeIdentifier</code>) don't seem to do the trick... what's the magic sauce?</p>
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    1. USJon Skeet
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    1. COYou're way ahead of me on this stuff (most stuff), but just for clairification, it sounds like what you are after is dependency injection functionality with a more normalized syntax--would that be accurate?
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      1. USBill K
    2. COAren't optional parameters nice? IMO, No they are not nice. Microsoft is trying to fix the flaw in Office COM interfaces by adding bloat to C#.
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      1. USMehrdad Afshari
    3. CONice question (+1). I've noticed the same thing about calling a constructor on an interface. It looks horrendous in code because it'll really confuse any developers not familiar with what's going on. I always wondered what was going on behind the scenes to make it work. Looks like I'll be buying your new book ;-)
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      1. USDoctor Jones
 

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