MSDN Magazine - January 2009 - (Page 27)

Figure 11 ' Here's how we fetch an arbitrary property from a COM object, ' identified by its MEMBID. Dim val As Object Dim varResult As New [VARIANT] Dim dispParams As New ComTypes.DISPPARAMS With {.cArgs = 0, .cNamedArgs = 0} Dim dispatchType = If((funcDesc.invkind And _ ComTypes.INVOKEKIND.INVOKE_PROPERTYGET) 0, 2US, 1US) idisp.Invoke(funcDesc.memid, IID_NULL, 0, dispatchType, dispParams, _ varResult, IntPtr.Zero, 0) val = varResult.GetObject() If varResult.vt = VarEnum.VT_PTR AndAlso varResult.ptr IntPtr.Zero _ Then Marshal.Release(varResult.ptr) End If ITypeInfo.Invoke(com,12) directly; you first must call QueryInterface to get com’s SpVoice interface, and then call ITypeInfo.Invoke on that. In the end, it’s easier to use IDispatch.Invoke. Discussion Now you’ve seen how to reflect on COM objects through ITypeInfo. This is useful for COM classes that lack interop types. And you’ve seen how to use IDispatch.Invoke to retrieve values from COM, stored in a VARIANT structure. I did wonder about creating a full wrapper around ITypeInfo and TYPEDESC, one that inherits from System.Type. With this, users could use the same code for reflection on COM types as .NET types. But in the end, at least for my project, this kind of wrapper was too much work for negligible gain. For more on what reflection can do for you, see “Dodge Common Performance Pitfalls to Craft Speedy Applications” at msdn.microsoft.com/magazine/cc163759, and “CLR Inside Out: Reflections on Reflection” at msdn.microsoft.com/magazine/cc163408. Sincere thanks to Eric Lippert, Sonja Keserovic, and Calvin Hsia for their help with this column. LUCIAN WISCHIK is the Visual Basic Specification Lead. Since joining the Visual Basic compiler team he has worked on new features relating to type inference, lambdas, and generic covariance. He has also worked on the Robotics SDK and concurrency, and has published several academic papers on the subject. Lucian holds a PhD in concurrency theory from the University of Cambridge. 1 for a function-invoke. If you were invoking a function that took arguments, then you’d also set up the dispParams structure. Finally, the result comes back in varResult. As before, you can just call GetObject on it to convert the VARIANT into a .NET object. Note the call to Marshal.Release. It’s a universal pattern in COM that if a function hands someone a pointer, it first calls AddRef on it, and it’s the caller’s responsibility to call Release on it. This makes me even happier that .NET has garbage collection. Incidentally, I could have used ITypeInfo.Invoke instead of IDispatch.Invoke. But it gets a little confusing. Suppose you have a variable, “com,” which points to the IUnknown interface of a COM object. And suppose that com’s ITypeInfo is a SpeechLib.SpVoice that happens to have a property with member-id 12. You can’t call NCache TM Distributed Object Cache for .NET Boost .NET application scalability in server farms! NCache is an extremely fast & scalable in-memory distributed cache for .NET. With an API very similar to ASP.NET Cache, NCache synchronizes the cache across multiple servers to let you scale out. Enterprise Distributed Data Cache Scale with Replicated, Partitioned, Client Cache topologies 100% up time with dynamic cache clustering Use in ASP.NET, SOA & Grid Computing Distributed ASP.NET Sessions Eliminate session storage bottlenecks Use fast & scalable in-memory session storage Incorporate without any code change Ph: +1-925-236-3830 Download FREE Trial www.alachisoft.com January 2009 27 http://msdn.microsoft.com/magazine/cc163759 http://msdn.microsoft.com/magazine/cc163759 http://msdn.microsoft.com/magazine/cc163408 http://www.alachisoft.com http://www.alachisoft.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2009

Toolbox CLR Inside Out Basic Instincts Cutting Edge Test Run First Look� Geneva Framework Silverlight Windows Mobile Service Station Security Briefs Extreme ASP.NET Foundations .NET Matters { End Bracket }

MSDN Magazine - January 2009

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