Direct Calls in Visual Basic .NET

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The IL instruction set contains three instructions intended for the direct method calls (well, direct in the sense that all these instructions directly specify the method being called; some purists would not consider a virtual call direct, because under the hood it is done via the v-table): jmp <token> (0x27). Abandon the current method and jump to the target method, specified by <token>, transferring the current arguments in their present state (which may be different from the original state, because the calling method could change the argument values before the jump). Everything else of the current method, including local variables and locally allocated memory, is abandoned. At the moment jmp is invoked, the evaluation stack must be empty, and the arguments are transferred automatically. Because of this, the signature of the target method must exactly match the signature of the method invoking jmp. This instruction should not be used within EH blocks .try, catch, filter, fault, or finally blocks, discussed in 14 or within a synchronized region (the code segment protected by a thread lock, such as a mutex). The jmp instruction is unverifiable. call <token> (0x28). Call an instance or static method nonvirtually. You can also call a virtual method, but in this case it is called not through the type s v-table. (If this sounds somehow vague to you, you might want to return to 10 and, more precisely, to the sample file Virt_not.il.) The real difference between virtual and nonvirtual instance methods becomes obvious when you create an instance of a class, cast it to the parent type of the class, and then call instance methods on this child-posing-as-parent instance. The nonvirtual methods are called directly, bypassing the child type s v-table,
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CHAPTER 13 IL INSTRUCTIONS
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so the parent s methods will be called in this case. Virtual methods are called through the v-table, and hence the overriding child s methods will be called. To confirm this, carry out a simple experiment: open the sample file Virt_not.il in a text editor, and change callvirt instance void A::Bar( ) to call instance void A::Bar( ). Then recompile the sample, and run it. callvirt <token> (0x6F). Call the virtual method specified by <token>. This type of method call is conducted through the instance s v-table. It is possible to call a nonvirtual instance method using callvirt. In this case, the method is called directly simply because the method cannot be found in the v-table. But unlike call, the callvirt instruction first checks the validity of the object reference (this pointer) before doing anything else, which is a very useful feature. The C# compiler exploits it shamelessly, emitting callvirt to call both virtual and nonvirtual instance methods of classes. I say of classes because callvirt requires an object reference as the this pointer and will not accept a managed pointer to a value type instance. To use callvirt with an instance of a value type, you need to box the instance first, thus converting it to a class instance carrying the v-table. CLR 2.0 considers nonvirtual calls of virtual methods unverifiable except in the following cases: If the called method is final (cannot be overridden) If the instance reference is that of a boxed value type If the parent class of the called method is sealed If the calling method and the called method are instance methods of the same class If the instance pointer is a managed pointer to a value type These exceptions cover almost all cases when the called virtual method is guaranteed not to be overridden. I say almost because there is at least one such case not covered when the type of the object reference is reliably traceable and the called method belongs to this type: .class public A { ... .method public virtual void f() { ... } } .class public B extends A { ... .method public virtual void f() { ... } } ... newobj instance void B::.ctor() dup // The objects on the stack are known to be B (not B's descendants cast to B) call instance void B::f() // should be verifiable, the type matches the object call instance void A::f() // unverifiable the type doesn't match the object ...
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