Chapter 3: Visual Basic 9.0 Language Features

The release of Microsoft Visual Basic 9.0 introduces enhancements to the language to better support Language Integrated Query (LINQ). As with C# 3.0, the changes to Visual Basic 9.0 do not require modification of the common language runtime (CLR).

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For more information about C# 3.0, see Chapter 2, “C# Language Features.”

In this chapter, we will examine the new syntax available in Visual Basic 9.0, comparing it to the C# equivalent whenever possible. As you will see, there are some differences between these languages, and some features are not present in both. Even if you are a C# programmer, please take a look at this chapter. You will discover that sometimes you can take advantage of Visual Basic 9.0 rather than C#, or at least you will be able to read Visual Basic code for LINQ.

If you prefer to use Visual Basic, remember that knowledge of the whole set of Visual Basic 8.0 features is necessary to effectively use the new Visual Basic 9.0 syntax. The ability to read C# code is also important for reading the rest of this book (because LINQ examples are written in C#) and for understanding differences between C# 3.0 and Visual Basic 9.0.

Visual Basic 9.0 and Nullable Types

Visual Basic 9.0 includes features that have been available in C# since version 2.0. One of these features-nullable types-is often useful with LINQ; therefore, we want to quickly describe nullable types here.

Since the introduction of generics in Microsoft .NET 2.0, the CLR offered the generic class Nullable(Of T As Struct ) to programmers who wanted to add the semantic of NULL to a value type. Declaring a variable of this type, you can assign the NULL “value” without having to define a new type for this sole purpose. In this way, handling NULLs in data becomes similar to how it is done in SQL.

C# 2.0 added direct support in the language to enable the use of this type: if you simply add the ? suffix to the name of the type, your compiled code will use the Nullable generic class instantiated for the requested type. Visual Basic 9.0 offers the same capabilities with a similar syntax, as you can see in Listing 3-1.

Listing 3-1: Nullable type declarations

  Dim a As Integer? = 18 Dim b As Integer? = 24 Dim c As Integer? = Nothing Dim d As Integer? d = a + c  ' d = 18 + Nothing = Nothing c = a + b  ' c = 18 + 24 = 42 

A regular Integer variable cannot be assigned to Nothing, because Integer is a value type. In Listing 3-1, we assigned c to Nothing, and we used it in a calculation to assign d. The result of a + c is Nothing, showing the three-valued logic that is typical of the nullable types. The NULL “value” (represented by Nothing in Visual Basic) is propagated into an expression, with some exceptions using logical operators.

A nullable value cannot be assigned to the corresponding non-nullable type. The code in Listing 3-2 shows the required conversions.

Listing 3-2: Nullable type conversions

  Dim k As Integer? = 16 Dim p As Integer = k    ' Compiler error Dim q As Integer = DirectCast( k, Integer ) ' Ok Dim r As Integer = CType( k, Integer )      ' Ok 

Please refer to the Visual Basic documentation for more information about the nullable types in Visual Basic 9.0. We expect this feature to be used extensively in code that manipulates data back and forth from a relational database.