Access Cookbook, 2nd Edition [Electronic resources] نسخه متنی

Recipe 17.1 Call a .NET Component from Access

17.1.1 Problem

Access makes it easy to call code inside a component built using
Visual Basic 6.0 or another COM-based programming language (see Chapter 12). By default, however, Access
can't normally call code in a .NET component. Is
there some way that Access can call a .NET component created using Visual
Basic .NET, Visual C# .NET, or another .NET language?

17.1.2 Solution

By default, .NET components can't be called by
Access and other COM programs for at
least two reasons. First, .NET components are not installed into the
registry. In order to automate a component, however, certain registry
entries must be present. Second, .NET components
aren't COM components; they aren't
structured to look and behave like a COM component and they have
separate and distinct type systems.

Fortunately, the Microsoft .NET SDK includes a utility, RegAsm.exe, that you can use to create a
COM-callable wrapper for a .NET component. RegAsm.exe also registers
the .NET component so that it can be called from a COM program such
as Access.

One nice feature of .NET is that it only takes a single line of code
to determine the current Windows user name. Trying to do this from
Access requires at the very least a cumbersome Windows API call.

Follow these steps to create a simple .NET component,
UserNameVB, that contains a single class named
UserName with a single method,
GetUserName, which returns the current user name:

1. 
   Start Visual Studio .NET.

   
   

2. 
   Create a new VB .NET Class Library project named
   UserNameVB.

   
   

3. 
   Delete the initial Class1.vb file from the project.

   
   

4. 
   Select Project Add Class... to add a new class file to the
   project named UserName.cls.

   
   

5. 
   Add a method named GetUserName to the class that
   returns the Environment.UserName property. The complete code for the
   UserName class should look like the following:

   
   

   Public Class UserName
   Public Function GetUserName( ) As String
   Return Environment.UserName
   End Function
   End Class

6. 
   Compile the project by selecting Build Build Solution. If
   all goes well, the status bar will display "Build
   succeeded."

   
   

At this point you could easily create a .NET Windows Form or Web Form
application that calls the .NET component. To make it callable from
Access, however, you need to use the RegAsm utility to create a
COM-callable wrapper component that will call
UserNameVB on your behalf.
RegAsm also takes care of making the necessary
registry entries as well so that Access and other COM programs can
see your component.

Follow these steps to make the UserNameVB component callable from
Access:

1. 
   From the Microsoft Visual Studio .NET 2003 Start menu, select Visual
   Studio .NET Tools Visual Studio .NET Command Prompt to
   create a Visual Studio .NET command prompt.

   
   

Do not use the Command Prompt menu entry found under Accessories. This command prompt will not have the needed path settings that allow you to run the .NET command line tools.

2. 
   Navigate to the folder containing the compiled assembly by using the
   CD command. By default, the assembly should be found in the following
   location:

   
   

   C:\Documents and Settings\<yourusername>\My Documents\Visual Studio 
   ProjectsUserNameVB\bin

3. 
   Use the .NET registration assembly utility (RegAsm.exe) to register
   the UserNameVB.dll by entering the following into the command prompt
   window:

   
   

   regasm UserNameVB.dll /tlb:UserNameVB.tlb /codebase

   
   RegAsm will display a warning about this being an
   unsigned assembly but you can safely ignore the warning.

   
   

Now you are ready to create the Access application that will call the
UserNameVB component. Follow these steps to create an Access form
that calls the .NET component:

1. 
   Create a new Access form named frmGetUserName.

   
   

2. 
   Add a command button to the form named
   cmdGetUserName and a label named
   lblUserName.

   
   

3. 
   From the VBA IDE, select Tools References. At the
   References dialog, select the UserNameVB component (see Figure 17-1).

   
   

Figure 17-1. Setting a reference to the UserNameVB component from the Tools References dialog

4. 
   Attach the following code to the Click event of the cmdGetUserName
   command button to instantiate the UserNameVB.UserName class and call
   its GetUserName method:

   
   

   Private Sub cmdGetUserName_Click( )
   Dim objUN As UserNameVB.UserName
   Set objUN = New UserNameVB.UserName
   lblUserName.Caption = objUN.GetUserName( )
   End Sub

5. 
   Load and run the form, clicking on the cmdGetUserName command button
   to return the current user name as show in Figure 17-2.

   
   

Figure 17-2. This form calls a .NET component to determine the current Windows user name

Note: The steps for the solution are virtually identical if using another .NET programming language such as C#. The only differences would be in the type of project (you would choose to create a Visual C# .NET class library) and the source code of the component. In addition to the VB version of the component, you can find a C# version of the component, named UserNameCS, in this chapter's sample code.

17.1.3 Discussion

17.1.3.1 An alternate solution

There is an alternate technique for creating a .NET
component that can be called from
Access and other COM programs that requires a bit less work than the
solution presented here. This solution, however, only works with
Visual Basic .NET.

The basic difference with this version of the solution is to create a
special type of class library, called a COM Class, that automatically
enables it to be called from a COM application. Here are the steps:

1. 
   Follow Steps 1-3 of the solution.

   
   

2. 
   Select Project Add Add New Item.... At the Add
   New Item dialog box, select the COM Class template and name the file
   UserName.cls.

   
   

3. 
   Follow Steps 5-6 of the solution.

   
   

4. 
   Skip Steps 7-9 of the solution. They are no longer necessary.

   
   

5. 
   Follow the remaining steps of the solution.

   
   

Note: this version of the solution will only work with a Visual Basic .NET project.

17.1.3.2 Not all .NET components are callable

Not all .NET
components can be called from Access and other COM programs. The main
limitation is that you can't instantiate any objects
for classes containing parameterized constructors. A
constructor
is code that executes when an instance of a class is created.
Constructors are similar in concept to the
Class_Initialize event handler within a
Visual Basic 6 class. .NET, however, allows you to create
constructors that can accept parameters, so-called
parameterized
constructors. COM, however, has no way to call
a class containing a parameterized constructor. If you attempt to
create an object from a .NET class that contains a parameterized
constructor, you will get a runtime error. A workaround for this
issue is presented in topic 17.2.

Another limitation of calling .NET components from Access is that you
won't be able to access any properties, methods, or
events marked as static (also know as shared). A static member of a
.NET class is a member that applies across all instances of a class.
Static members cannot be called from Access or other COM programs.

17.1.4 See Also

Microsoft Office and .NET Interoperability (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnofftalk/html/office11012001.asp).