qr code generator asp net c# Lesson 1: Using COM Objects in C#

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Lesson 1: Using COM Objects
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Property Get Lastname() As String mLastName = Firstname End Property Property Let Lastname(Value As String) mLastName = Value End Property
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Perform the following steps to execute the Regsvr32 command to ensure that Person.dll (or whatever COM DLL you are using) is registered: 1. Open a new Command window or the Run dialog box (which you can access by choosing Start and then Run). 2. Execute Regsvr32 Person.dll. Now that the DLL has been registered, you have two ways to import it:
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Visual Studio 2005 TlbImp.exe
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Strictly speaking, you can also use some of the services available in the System.Runtime.InteropServices namespace, but doing so is cumbersome and error prone. For the sake of this lesson, we ll focus on the two approaches just listed.
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Using Visual Studio 2005 to Import a Type
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With one minor exception, importing a COM library is virtually indistinguishable from importing any other type. The exception is that most of the DLLs you reference will be located on the COM tab of the Add Reference dialog box, as illustrated in Figure 13-1.
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Figure 13-1
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COM tab of the Add Reference dialog box
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13
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Interoperation
All registered COM components will be visible here, so simply do the following to finalize the process: 1. Create a new blank solution named COMDemos. 2. Create a new Visual Basic 2005 or C# 2005 project, and name it TypeDemoVB or TypeDemoCS, depending on the language you use. 3. Expand the Project list in Solution Explorer, and right-click the References node. 4. Choose the Add Reference option, and select the COM tab. (With most configurations, this will be the second tab of the dialog box, located next to the .NET tab.) 5. Find the component you want to register, and click OK. After you have followed the steps just shown, the component should be registered. If the component has been registered correctly, it will be visible on the COM tab.
Using TlbImp.exe to Import a Type
Using the Type Library Importer utility (TlbImp.exe) is a little more intricate but still quite straightforward. To import a library using the TlbImp.exe, do the following: 1. Open the Visual Studio 2005 command prompt. 2. Navigate to the location of the DLL you want to import. 3. Type tlbimp <dllname>.dll This will import the DLL and create a .NET assembly with its original name. For example, Person.dll will be imported as Person.dll, MyObject will be imported as MyObject.dll, and so forth. 4. If you want a name to be used other than the original DLL name, type tlbimp <dllname>.dll /out:<DesiredName>.dll. Now add a reference to the assembly name you chose just as you would for any other .NET assembly. The important thing to remember is that TlbImp.exe is creating a new assembly for you from the COM library. So now you have a brand new .NET assembly, and it will be visible only under the .NET tab of the Add Reference dialog box. Although it would be almost fair to say that either of the preceding approaches is all that you need to know about the subject, that wouldn t be entirely true. One area in particular might give you a lot of headaches, mainly because of the inherent differences between Visual Basic 2005 and C#. C# doesn t support optional parameters whereas Visual Basic 2005 does. So what s the problem COM components don t
Lesson 1: Using COM Objects
support parameter overloading, so for each value in a parameter list, you ve got to pass in something, even if it does nothing. Moreover, COM parameters are always passed by reference, which means that you can t pass in a null value. In Visual Basic 2005, this isn t really an issue because many of these parameters are optional and you can just leave them out (or include them) as you see fit. C# doesn t support this, though, so you have to create object variables (remember, they can t be null) and then pass them in. This approach is problematic for the following reasons:
It leads to unnecessary and confusing code. In many instances, it leads to code that s virtually unreadable. (Imagine a 15item parameter list, for instance.)
To address this problem, a new feature of the Type class has been provided: Type.Missing. Examine the following code samples:
' VB Imports Microsoft.Office.Core Imports Microsoft.Office.Interop.Excel ' Must have Office installed for this demo Dim NewExcelApp As New Microsoft.Office.Interop.Excel.Application 'This works fine NewExcelApp.Worksheets.Add() // C# using Microsoft.Office.Core using Microsoft.Office.Interop.Excel; // Must have Office installed for this demo Application NewExcelApp = new Application(); // This will not comipile. NewExcelApp.Worksheets.Add();
Instead of creating dummy object variables, the Type.Missing field can be used. That field can be passed in with the C# code and the application will work as expected. In a pure sense, this approach is unnecessary in Visual Basic 2005 because of optional parameter support; however, many consider the use of optional parameters problematic and avoid them. (Optional parameters are included in this book for consistency.)
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