MSDN Magazine - October 2007 - (Page 31)

Type Inference in Visual Basic 2008 BILL HORST n order to support Language Integrated Query (LINQ), a suite of new technologies was added to the upcoming release of Visual Basic® 2008 that includes type inference. In earlier versions of Visual Basic, a member’s type was assumed to be Object unless a different type was explicitly declared in code, as shown in Figure 1. A drawback to declaring a variable this way is that the compiler is only able to assume that the value of the variable will be of type Object. The experience for the developer is that IntelliSense® is only able to display information for the members that are common to all Objects, though there may be other members available on the actual instance at run time. With late binding, the other members are bound at run time, which means they will not be verified in any way until it is time for the line to be executed. So, nonexistent members (such as cust.Name in Figure 1) can be referenced in code, and the problem will not be caught by the compiler. This can easily result in a MissingMemberException and generally requires you to keep track of more information on your own. However, with type inference in Visual Basic 2008, the types of method-level members are inferred based on the right-hand side of any assignments in the declaration statements. You can find several examples in Figure 2. Here, the compiler will catch the reference to the nonexistent member cust.Name shown in Figure 1, and IntelliSense information will be provided for all available members, as if the type were specified explicitly. In the For loop shown in Figure 2, the variable g is recognized as a declaration, with the type of the “procs” expression. However, there are times when type inference doesn’t happen. For local Dim statements, type inference only occurs when there is an assignment on the declaration line. So, assignments made outside the declaration of the variable will cause the compiler to assume that Object is the type. Object is also still inferred as the type for class-level members, so type inference does not apply to functions, subs, properties, class/structure fields, and so on. Some specific examples are shown in Figure 3. I Figure 1 Lack of Type Inference in Visual Basic 2005 Class Form1 Sub Main() Dim a = 3 Dim b = DateTime.Now ‘ Type: Object ‘ Type: Object Value: 3 {Integer} Value: Current Time Dim cust = GetCustomer(someID) MsgBox(cust.Name) ‘ Customers have FirstName and LastName fields. ‘ This code generates an exception at Run-Time. Dim procs = Process.GetProcesses For Each g In procs ‘ Compile Error: ‘g’ is not declared Console.WriteLine(g.Id) Next End Sub End Class Figure 2 Type Inference in Visual Basic 2008 Class Form1 Sub Main() Dim a = 3 Dim b = DateTime.Now ‘ Type: Integer ‘ Type: Date Value: 3 Value: Current Time Dim cust = GetCustomer(someID) MsgBox(cust.Name) ‘ Here we get a compiler error, catching the ‘ problem quickly Dim procs = Process.GetProcesses For Each g In procs ‘ Type: Process Console.WriteLine(g.Id) ‘ No compile errors Next End Sub End Class Figure 3 Cases with No Type Inference Class Form1 Sub Main() Dim d d=3 End Sub Dim e Dim f = 3 ‘ Assignment not on the declaration line ‘ Type: Object Value: 3 {Integer} ‘ Class-level ‘ Type: Object ‘ Type: Object Value: Nothing Value: 3 {Integer} Value: 3 {Integer} Why Is Type Inference Necessary? Type inference is necessary in Visual Basic 2008 because of new language constructs that enable the creation of a type that is not explicitly defined anywhere. In order to assign a variable to an expression with a non-explicitly defined type, it is necessary to infer the type, since it would be impossible to specify it in code. These non-explicitly defined types are called anonymous types. Function Func1() ‘ Type: Object Return 3 End Function End Class An object can be created and certain members initialized without being previously defined. This enables LINQ statements to This column is based on a prerelease version of Visual Basic 2008. Details contained herein are subject to change. october2007 31

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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