When you automate an application such as a Microsoft Office application, the calls to the properties and methods of the Office application's objects must be connected in some way to those objects. The process of connecting property and method calls to the objects that implement those properties and methods is commonly called binding. In Visual Basic .NET, the two types of binding that are available are early binding and late binding. The type of binding you choose can affect many aspects of your program, including performance, flexibility, and maintainability.

This step-by-step article explains and compares early and late binding for Visual Basic .NET Automation clients and provides code samples that demonstrate both types of binding.

Early Binding

With early binding, Visual Basic .NET uses type information that is available about the Office application in question to bind directly to the methods or properties that it needs to use. The compiler can perform type and syntax checks to ensure that the correct number and type of parameters are passed to the method or property, and that the returned value will be of the expected type. Because less work is required at run time to make a call to a property or method, early binding is sometimes faster; however, although early binding may be faster, performance differences when compared to late binding are often negligible.

Early binding does have the minor disadvantage that it can introduce possible version compatibility issues. For example, suppose that an Automation server such as Microsoft Excel 2002 introduces a new method or property that was unavailable in Excel 2000, or makes a change to an existing property or method. These changes may alter the binary layout of the object and cause problems with a Visual Basic .NET application that uses the Excel 2002 type information to automate Excel 2000. To avoid this problem with early binding, it is generally recommended that you use the type information for the earliest version of the Office application that you wish to support when you develop and test your Automation client.

The following steps demonstrate how to build an Automation client that uses early binding. Note that, as the steps illustrate, early binding requires you to reference the type library for the Automation client.

Create an Automation Client That Uses Early Binding

1. Start Microsoft Visual Studio .NET. On the File menu, click New and then click Project. Select Windows Application from the Visual Basic .NET Projects types. Form1 is created by default.
2. Add a reference to the Microsoft Excel Object Library. To do this, follow these steps:
   1. On the Project menu, click Add Reference.
   2. On the COM tab, locate theMicrosoft Excel Object Library, and then click Select.
      
      Note Office 2003 includes Primary Interop Assemblies (PIAs). Office XP does not include PIAs, but they can be downloaded. For additional information about Office XP PIAs, click the article number below to view the article in the Microsoft Knowledge Base:
      328912  (http://kbalertz.com/Feedback.aspx?kbNumber=328912/EN-US/ ) INFO: Microsoft Office XP PIAs Are Available for Download
   3. Click OK in the Add References dialog box to accept your selections.
3. On the View menu, select Toolbox to display the Toolbox, and add a button to Form1.
4. Double-click Button1. The code window for the Form appears.
5. In the code window, replace the following code

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
   
   End Sub
   						

   with:

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
   	Dim objApp As Excel.Application
           Dim objBook As Excel._Workbook
           Dim objBooks As Excel.Workbooks
           Dim objSheets As Excel.Sheets
           Dim objSheet As Excel._Worksheet
           Dim objRange As Excel.Range
   
           ' Instantiate Excel and start a new workbook.
           objApp = New Excel.Application()
           objBooks = objApp.Workbooks
           objBook = objBooks.Add
           objSheets = objBook.Worksheets
           objSheet = objSheets.Item(1)
   
           objRange = objSheet.Range("A1")
   
           'Set the range value.
           objRange.Value(Excel.XlRangeValueDataType.xlRangeValueDefault) = "Hello, World!"
   
   
           'Return control of Excel to the user.
           objApp.Visible = True
           objApp.UserControl = True
       End Sub
   					

6. Add the following directive at the top of code window:

   	Imports Excel = Microsoft.Office.Interop.Excel
    
   					

Late Binding

In contrast to early binding, late binding waits until run time to bind property and method calls to their objects. To do this, the target object must implement a special COM interface: IDispatch. The IDispatch::GetIDsOfNames method allows Visual Basic .NET to interrogate an object about what methods and properties it supports, and the IDispatch::Invoke method then allows Visual Basic .NET to call those methods and properties. Late binding in this fashion has the advantage of removing some of the version dependencies that are inherent with early binding. However, late binding has the disadvantages of removing compile-time checks on the integrity of automation code, as well as not providing Intellisense features that can provide clues to correct calls to methods and properties.

To use late binding in Visual Basic .NET, change the declarations of the objects in your automation code from their early bound types, such as Excel.Application to Object.

Create an Automation Client That Uses Late Binding

1. Start Microsoft Visual Studio .NET. On the File menu, click New and then click Project. Select Windows Application from the Visual Basic .NET Projects types. Form1 is created by default.
2. On the View menu, select Toolbox to display the Toolbox, and add a button to Form1.
3. Double-click Button1. The code window for the Form appears.
4. In the code window, replace the following code

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
   
   End Sub
   						

   with:

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
       Dim objApp As Object
       Dim objBook As Object
       Dim objBooks As Object
       Dim objSheets As Object
       Dim objSheet As Object
       Dim range As Object
   
       ' Instantiate Excel and start a new workbook.
      objApp = CreateObject("Excel.Application")
       objBooks = objApp.Workbooks
       objBook = objBooks.Add
       objSheets = objBook.Worksheets
       objSheet = objSheets.Item(1)
   
       range = objSheet.Range("A1")
   
       'Set the range value.
       range.Value = "Hello, World!"
   
       'Return control of Excel to the user.
       objApp.Visible = True
       objApp.UserControl = True
   End Sub
   					

For more information, visit the following Microsoft Web site: For additional information about binding, click the article numbers below to view the articles in the Microsoft Knowledge Base: