Early and Late Binding
The Visual Basic compiler performs a process called binding when an object is assigned to an object variable. An object is early bound when it is assigned to a variable declared to be of a specific object type. Early bound objects allow the compiler to allocate memory and perform other optimizations before an application executes. For example, the following code fragment declares a variable to be of type :
' Create a variable to hold a new object. Dim FS As System.IO.FileStream ' Assign a new object to the variable. FS = New System.IO.FileStream("C:\tmp.txt", _ System.IO.FileMode.Open)
Because FileStream is a specific object type, the instance assigned to FS is early bound.
By contrast, an object is late bound when it is assigned to a variable declared to be of type Object. Objects of this type can hold references to any object, but lack many of the advantages of early-bound objects. For example, the following code fragment declares an object variable to hold an object returned by the CreateObject function:
' To use this example, you must have Microsoft Excel installed on your computer. ' Compile with Option Strict Off to allow late binding. Sub TestLateBinding() Dim xlApp As Object Dim xlBook As Object Dim xlSheet As Object xlApp = CreateObject("Excel.Application") ' Late bind an instance of an Excel workbook. xlBook = xlApp.Workbooks.Add ' Late bind an instance of an Excel worksheet. xlSheet = xlBook.Worksheets(1) xlSheet.Activate() ' Show the application. xlSheet.Application.Visible = True ' Place some text in the second row of the sheet. xlSheet.Cells(2, 2) = "This is column B row 2" End Sub
Advantages of Early Binding
You should use early-bound objects whenever possible, because they allow the compiler to make important optimizations that yield more efficient applications. Early-bound objects are significantly faster than late-bound objects and make your code easier to read and maintain by stating exactly what kind of objects are being used. Another advantage to early binding is that it enables useful features such as automatic code completion and Dynamic Help because the Visual Studio integrated development environment (IDE) can determine exactly what type of object you are working with as you edit the code. Early binding reduces the number and severity of run-time errors because it allows the compiler to report errors when a program is compiled.
Late binding can only be used to access type members that are declared as Public. Accessing members declared as Friend or Protected Friend results in a run-time error.