METHODS in Visual Basic .NET

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CHAPTER 10 METHODS
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.entrypoint ... newobj instance void C::.ctor() // Create instance of derived class castclass class A // Cast it to "grandparent" dup dup // We need 3 instance pointers // On stack for 3 calls
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call instance void A::Foo() callvirt instance void A::Bar() callvirt instance void A::Baz() ret } The output is the same, which proves that class C has inherited the overridden methods from class B: A::Foo B::BarBaz B::BarBaz ILAsm supports an extended form of the explicit override directive, placed within the class scope: .override <class_ref>::<method_name> with <method_ref> For example, the overriding effect would be the same in the preceding code if we defined class B like so: .class public B extends A { .method public specialname void .ctor() { ldarg.0 call instance void A::.ctor() ret } .method public void Foo() { ldstr "B::Foo" call void [mscorlib]System.Console::WriteLine(string) ret } .method public virtual void BarBaz() { ldstr "B::BarBaz" call void [mscorlib]System.Console::WriteLine(string) ret
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CHAPTER 10 METHODS
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} .override A::Bar with instance void B::BarBaz() .override A::Baz with instance void B::BarBaz() } In the extended form of the .override directive, the overriding method must be fully specified because the extended form is used within the overriding class scope, not within the overriding method scope. To tell the truth, the extended form of the .override directive is not very useful in the existing versions of the common language runtime because the overriding methods are restricted to those of the overriding class. Under these circumstances, the short form of the directive is sufficient, and I doubt that anyone would want to use the more cumbersome extended form. But I ve noticed that in this industry the circumstances tend to change. One more note: you probably have noticed that the sample Override.il looks tedious and repetitive: similar constructors of the classes and multiple calls to [mscorlib]System. Console::WriteLine(string). As was discussed in 3, version 2.0 of the ILAsm allows you to streamline the programming by means of defines, typedefs, and the special keywords .this, .base, and .nester. Have a look at the sample Override_v2.il on the Apress Web site: #define DEFLT_CTOR ".method public specialname void .ctor() {ldarg.0; call instance void .base::.ctor(); ret}" .typedef method void [mscorlib]System.Console::WriteLine(string) as PrintString .class public A { DEFLT_CTOR .method public void Foo() { ldstr "A::Foo" call PrintString ret } .method public virtual void Bar() { ldstr "A::Bar" call PrintString ret } .method public virtual void Baz() { ldstr "A::Baz" call PrintString ret } }
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CHAPTER 10 METHODS
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.class public B extends A { DEFLT_CTOR .method public void Foo() { ldstr "B::Foo" call PrintString ret } .method public virtual void BarBaz() { .override .base::Bar .override .base::Baz ldstr "B::BarBaz" call PrintString ret } } ... .class public C extends B { DEFLT_CTOR // No overrides; let's inherit everything from B } .method public static void Exec() { .entrypoint ... newobj instance void C::.ctor() // Create instance of derived class castclass class A // Cast it to "grandparent" dup dup // We need 3 instance pointers // On stack for 3 calls
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call instance void A::Foo() callvirt instance void A::Bar() callvirt instance void A::Baz() ret }
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CHAPTER 10 METHODS
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Not only is sample Override_v2.il easier to read and to type, it is compiled faster (only marginally; you will not notice any effect compiling such small sample). I will leave it to you to modify the Virt_not.il sample in the same way. Just don t forget that these syntax enhancements are specific to version 2.0 and are not supported in versions 1.0 and 1.1.
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Method Overriding and Accessibility
Can I override an inaccessible virtual method For example, if class A has private virtual method Foo, can I derive class B from A and override Foo I know I cannot call A::Foo, but I don t want to call it; I want to override it and call my own B::Foo. Can I Yes you can, says C++, exactly because you are not calling the private method Foo of A. No you cannot, says C#, because you have no access whatsoever to the private method Foo of A. Eh says Visual Basic . No, no, I m just kidding, of course. Actually, VB sides with C#. So what should the common language runtime say in this regard As usual, it finds some common ground that is acceptable to all languages. There is the special flag strict (0x0200), which controls the overridability of a virtual method. If the method is declared strict virtual, then it can be overridden only by classes that have access to it. A private strict virtual method, for example, cannot be overridden in principle, so it just as well might have been marked final. If the flag strict is not specified, then the method can be overridden without any regard to its accessibility. So C# and VB declare their methods strict virtual, C++ declares its methods virtual, and everyone is happy. An interesting thing about this situation is that the explicit overrides are always bound to the accessibility, as if all virtual methods were strict virtual. This creates a regrettable asymmetry between implicit and explicit overriding. One more note about overriding and accessibility: you cannot override a virtual method with a method that has more restricted accessibility. For example, you cannot override a public method with a family method, but you can override a family method with a public method. This rule works for both implicit and explicit overrides. I leave it to you to figure out the reasoning behind this rule.
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