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Calling it is simple:. At runtime, Visual Basic sees that the object implements IDynamicMetaObjectProvider and thus passes control to the DLR, which then communicates with Python and executes the method passing along the array that was defined in Visual Basic as an argument to the method. The key is to implement the IDynamicMetaObjectProvider interface, in which case the Visual Basic and C compilers will recognize that the object has special dynamic semantics.
DynamicObject class which already implements this interface and just override a couple of methods. If you have an object of type IEnumerable Of Apple and want to assign it to an IEnumerable Of Fruit , that should be legal, because every Apple is a Fruit enforced by an inheritance relationship. Unfortunately, before Visual Basic , generic variance was not supported in the compiler, even though it actually was supported in the Common Language Runtime CLR. In Visual Basic , the code in Figure 4 would generate a compile error or if Option Strict is off, a runtime exception on the Dim enabledOnly line.
The workaround was to call the. Cast extension method, as shown here:. This is no longer necessary, because in Visual Basic , the IEnumerable interface has been marked as covariant by using the Out modifier:. A type can actually have both. For example, the Func delegate discussed earlier has both contravariant parameters things that get passed in and a covariant parameter for the return type :.
You can use the In and Out modifiers on your custom interfaces and delegates. Many commonly used interfaces and delegates in the. Situations that used to cause compile errors or require a call to.
Cast Of T should work just fine in Visual Basic Optional parameters provide a handy productivity feature that lets developers make more flexible methods and avoid polluting a class with numerous overloads of a method. One limitation in the past was that optional parameters could not be nullable or indeed any non-intrinsic structure type.
Visual Basic now lets you define optional parameters of any value type:. In this case, units is of type Nullable Of Integer and backgroundColor is a non-intrinsic structure type, but they can still be used as optional parameters.
Visual Basic also provides better support for optional parameters that are generic. A PIA is a. The coolest thing about all the features in Visual Basic is that you can even use them in projects that target the. NET Framework 2. NET Framework 3. This means that implicit line continuation, array literals, collection initializers, statement lambdas, auto-implemented properties and so on will all work in existing projects without having to retarget to the.
NET Framework 4. The one exception is Embed Interop Types, which has a dependency on types that are only in the. NET Framework versions 2. Also, the types that are marked as variant are only marked that way in the.
Cast Of T when targeting versions 2. To change the current target framework for an application, double-click My Project, click the Compile tab, click Advanced Compile Options and then select from the combo box at the bottom. Instead, the compiler looks at which assembly provided the definition of System. Object typically mscorlib and which framework that assembly is targeting, then stamps that value in your output assembly.
This is the same mechanism the compiler uses when building Silverlight assemblies. As you can see, Visual Basic has many powerful features that let you be more productive while writing fewer lines of code, offloading more work to the compiler.
The Visual Basic team would love to hear your feedback on what we can do to make Visual Basic even better, so send us your comments and questions on Microsoft Connect. To learn more about the language and IDE features, check out the content on msdn. Want more Visual Basic? You got it. Jonathan Aneja is a program manager on the Entity Framework team at Microsoft. Previously he was the program manager for the Visual Basic compiler during the releases of Visual Basic and Visual Basic He has been at Microsoft for four years.
Thanks to the following technical experts for reviewing this article: Dustin Campbell, Jason Malinowski and Lucian Wischik.
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