MSDN Magazine - October 2007 - (Page 34)

it will more frequently be the case that the type is difﬁcult to discern at a glance. Because of type inference, types need not be explicit,and with the new language constructs supporting LINQ, it can be quite obscure. The following variable is of type IEnumerable(Of T) as well,but it’s much less obvious here,and there is no place to call Go Figure 5 a For Loop To Deﬁnition to navigate to its type: Dim customers = From cust In _ (From c In db.Customers Select c) _ Where cust.FirstName = “Danae” _ Select cust.LastName, cust.Address For Each c In customers Enabling and Disabling Type Inference One of the major implications of type inference support is that some code compiled in a previous version of Visual Basic may not compile in a project where type inference is enabled. This is why upgraded projects will have type inference turned off by default.The main area where such build errors could occur is in code that uses late binding. Using earlier versions of Visual Basic, late binding occurs when Option Strict is turned off, and a variable with no explicit type is initialized to an expression. The variable is given type Object, and any member or method calls on that variable will be bound at run time: ‘ Late Binding with Visual Basic 2005 Dim x = 5 x = New List(Of Integer) x.Sort() x = New Queue(Of Integer) x.Enqueue(6) This issue motivated the introduction of the Go To Type Deﬁnition command in Visual Studio 2008.You can also call the command from the context menu, so you can ﬁnd the deﬁnition of a variable’s type even if only the variable identiﬁer exists in code.You can execute the Go To Type Deﬁnition command on the deﬁnition of a variable or a reference. In the previous code snippet, you could simply call Go To Type Deﬁnition on the customers variable in the deﬁnition or in the For loop, and you would be brought to the Object Browser with IEnumerable(Of T) selected (see Figure 5). Were the type explicitly deﬁned in code, you would instead navigate to that location. With type inference, such calls would result in compiler errors. Instead of giving the variable type Object, the compiler gives the variable a type to match the expression to which it is initialized. Late binding is still possible with type inference enabled,but Object must be given explicitly as the type: ‘ Late Binding with Visual Basic 2008 Dim x As Object = 5 x = New List(Of Integer) x.Sort() x = New Queue(Of Integer) x.Enqueue(6) There are other times you’ll want to turn type inference off as well.Fortunately there is a compiler option not unlike Option Strict or Option Explicit, called Option Infer, that lets you turn type inference on or off. It’s placed at the beginning of a code ﬁle like all compiler options in Visual Basic: Option Infer On Option Infer Off ‘ This enables type inference for the file ‘ This disables type inference, so declarations ‘ with no type specified will use Object If Option Infer is not speciﬁed for a particular code ﬁle, the project-level setting is used. The project-level default setting for Option Infer can be set on the Compile tab of the Project Designer. The Project Designer can be accessed from the Project menu or My Project node of Solution Explorer. Option Infer is On by default in new Visual Basic 2008 projects and Off by default for upgraded projects in order to avoid breaking pre-existing code. File-level settings override this option for speciﬁc ﬁles. While there are plenty of advantages to using type inference, as you will see when you begin using LINQ and other technologies that beneﬁt from it, you have the ability to specify when it will be invoked and when it will not, providing you with maximum ﬂexibility. Send your questions and comments for Bill to instinct@microsoft.com. Bill Horst is a Software Design Engineer in Test with the Microsoft Visual Basic Team. He has done much of the testing for LINQ Queries and also tests Visual Basic debugging. 34 msdnmagazine Basic Instincts http://www.steema.com http://www.steema.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - October 2007

Cover Contents Toolbox CLR Inside Out Basic Instincts Data Points Cutting Edge Pooled Threads WPF Threads Parallel Linq Parallel Performance Mobile Apps Test Run Foundations Windows with C++ Netting C++ .NET Matters { End Bracket } Net Nuptials

MSDN Magazine - October 2007

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