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PART IV
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NET
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COM is based on a binary standard whereby COM rules dictate the internal layout COM components are stored in a type library COM uses GUIDs (globally unique identifiers) for identification Object lifetime in COM is managed by reference counting (how many times has the object been used) Table 21-1
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NET is based on a type standard the Common Type System (CTS) NET components have metadata embedded inside the assembly NET uses strong names A NET component s lifetime is managed by the CLR through garbage collection
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Some Differences Between COM and NET
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Figure 21-10 The Add Reference dialog box
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Figure 21-11 Adding COM references
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21: Web Services and COM
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PART IV
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Figure 21-12
Browsing for a COM component
Once the component has been added to the project, you will see it in the References list in the Solution Explorer Open the Object Browser by selecting View | Other Windows | Object Browser Figure 21-14 shows the object listing for the ADOR library msador15dll You can now use the COM object by specifying its library and class within:
ADORRecordsetClass rs = new ADORRecordsetClass();
Behind the Scenes with COM
Here s what is happening behind the scenes when you add a COM reference to your application: A copy of the component is placed in the project directory A proxy is generated for the COM component The proxy is called a Runtime Callable Wrapper (RCW), and it essentially makes the COM component look like a NET assembly
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Figure 21-13
The selected components are listed at the bottom of the Add Reference dialog box
Although Visual Studio NET does the preceding things for you, you may be asked on the Microsoft exam to prove your expertise with the command tool that allows you to manually generate the NET proxy The tool is called TtlbImpexe, and it is the Type Library Importer The importer will convert the definitions in the COM library to equivalent definitions in CLR The tool will create an assembly that can be looked at by using Ildasmexe, which is the MSIL disassembler utility The syntax for using the Type Library Importer tool is as follows:
tlbimp <name of file containing the COM type library> [options]
Table 21-2 shows some of the syntax options available for the tlbimpexe tool
EXAM TIP
Use the following list to memorize important COM details
21: Web Services and COM
PART IV
Figure 21-14
Object Browser
Option
Description
/asmversion:versionnumber /help or / /out:filename /namespace:namespace /primary /publickey:filename /verbose /unsafe /reference:filename /silent Table 21-2
Identifies the version number of the assembly Returns the command syntax and options Identifies the name of the output assembly Identifies the namespace in which to produce the assembly Produces a primary interop assembly There is a description of interop following this table Identifies the file containing the public key to use in signing the assembly Displays additional information about the library Creates interfaces without runtime security checks Identifies the name of the assembly to use in resolving references Suppresses output display
Options for the TlbImpexe tool
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Here are some interesting (and test-worthy) points about interoperating with COM and COM+ components: The TlbImpexe tool is used to generate the proxy for a COM component COM components still have to be registered in the Registry Use regsvr32exe to do this COM components used by NET applications must be registered in the Registry COM interop is a two-way service that creates a bridge between COM and the NET framework This is the way that COM components can talk to NET components and vice versa
Platform Invoke
Platform invoke allows us to get access to functionality that is in libraries or applications that use unmanaged code EXAM TIP Unmanaged code is code that runs outside of the NET environment This means that NET cannot run garbage collection on it or check the security of the code it executes in the CLR with minimal services (security, garbage collection, and so on) An example of the type of calls that may invoke unmanaged code is a call to a Win32 API, which contains many functional pieces of code that you may choose to use instead of creating your own code Within the Win32 API, you will find user32dll, gdi32dll, and kernel32dll, which will allow you access to some of the Windows platform functionality The CLR provides for platform invoke (pinvoke), which allows managed code (our NET code) to call unmanaged native platform code EXAM TIP Make a reference to the InteropServices namespace in your C# application in order to communicate with the Win32 API
Follow these steps to make a call to the Win32 API: 1 Add the namespace like this: using SystemRuntimeInteropServices; 2 Expose the API method that you want in our example, we will use the message box from the Win32 API: [DllImport("user32dll")] public static extern int MessageBoxW (int Modal, string Message, string Caption, int Options);
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