我注意到传递给全局函数的参数总是使用“call”，但重载的成员函数调用总是使用“callvirt”。
为什么在将该类传递给全局函数（Print（））时，它总是调用与基类有关的重载函数而不是派生类？
似乎它选择在编译时运行Print的哪个重载函数，是否有特别的东西让我决定在运行时不应该解决这个问题？
这里有一些代码可以证明我的意思：
Module Module1
Class BaseClass
Friend Overridable Sub Print()
Console.WriteLine("BaseClass.Print")
End Sub
End Class
Class DerivedClass
Inherits BaseClass
Friend Overrides Sub Print()
Console.WriteLine("DerivedClass.Print")
End Sub
End Class
Sub Print(iObject As Object)
Console.WriteLine("Object")
End Sub
Sub Print(iClass1 As BaseClass)
Console.WriteLine("BaseClass")
End Sub
Sub Print(iClass2 As DerivedClass)
Console.WriteLine("DerivedClass")
End Sub
Sub Main()
Dim tBaseClass As New BaseClass
Dim tDerivedClass As New DerivedClass
Dim tBaseClassRef As BaseClass
Dim tObjPtr As Object
Console.WriteLine()
Console.WriteLine("Test 1")
Console.WriteLine()
'in IL it always uses callvirt for an overloaded member function
'from MSDN: The callvirt instruction calls a late-bound method on an object.
' That is, the method is chosen based on the runtime type of obj rather than the compile-time class visible in the method pointer.
'prints "BaseClass.print"
'callvirt instance void Overloading.Module1/BaseClass::Print()
tBaseClass.Print()
'in IL it uses "call", even though Print() is overloaded? Why is this?
'We slip by here because this type is not late bound
'prints "BaseClass"
'call void Overloading.Module1::Print(class Overloading.Module1/BaseClass)
Print(tBaseClass)
Console.WriteLine()
Console.WriteLine("Test 2")
Console.WriteLine()
'prints "DerivedClass.print"
'callvirt instance void Overloading.Module1/BaseClass::Print()
tDerivedClass.Print()
'call works out okay here too because we're still not late bound
'prints "DerivedClass"
'call void Overloading.Module1::Print(class Overloading.Module1/BaseClass)
Print(tDerivedClass)
Console.WriteLine()
Console.WriteLine("Test 3")
Console.WriteLine()
tBaseClassRef = tBaseClass
tBaseClassRef.Print()
'prints "BaseClass.print"
'callvirt instance void Overloading.Module1/DerivedClass::Print()
'call took our word for it that tBaseClassRef is BaseClass typed
'which is correct
Print(tBaseClassRef)
'prints "BaseClass"
'call void Overloading.Module1::Print(class Overloading.Module1/DerivedClass)
Console.WriteLine()
Console.WriteLine("Test 4")
Console.WriteLine()
tBaseClassRef = tDerivedClass
tBaseClassRef.Print()
'prints "DerivedClass.print"
'IL_0098: callvirt instance void Overloading.Module1/BaseClass::Print()
'Callvirt correctly handles our tBaseClass having a derived class's type
Print(tBaseClassRef)
'prints "BaseClass" <!>
'IL_009f: call void Overloading.Module1::Print(class Overloading.Module1/BaseClass)
'"Call" is ill-equipped to handle our Derived class
Console.WriteLine()
Console.WriteLine("Test 5")
Console.WriteLine()
tObjPtr = tDerivedClass
tObjPtr.Print()
'(I don't expect this to work, but the error message surprised me)
' -- unhandled exception -- "Public member 'Print' on type 'DerivedClass' <??> not found.
'IL instructions: http://en.wikipedia.org/wiki/List_of_CIL_instructions
'a. where is it getting DerivedClass from for the exception?
'b. Why did LateCall know what type it was, but was unable to find the method?
' [ILDASM]
' IL_00be: ldloc.3 //Load local [3] object tObjPtr)
' IL_00bf: ldnull // push NULL to stack (no string)
' IL_00c0: ldstr "Print" // push string object for literal string -- is this what we're using as our object?
' IL_00c5: ldc.i4.0 //Push False
' IL_00c6: newarr [mscorlib]System.Object
' IL_00cb: ldnull // no string array
' IL_00cc: ldnull // no class array
' IL_00cd: ldnull // no bool array
' IL_00ce: ldc.i4.1 //Push True
' IL_00cf: call object [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.NewLateBinding::LateCall(object,
' class [mscorlib]System.Type,
' string,
' object[],
' string[],
' class [mscorlib]System.Type[],
' bool[],
' bool)
Console.WriteLine("Marker") 'divider so I can figure out what it is in idasm
Print(tObjPtr)
' prints "object"
'again, why use "call" instead of "callvirt"? There is a more specific definition of Print that can handle this
'[ILDASM]
' IL_00e0: ldloc.3
' IL_00e1: call object [mscorlib]System.Runtime.CompilerServices.RuntimeHelpers::GetObjectValue(object)
' IL_00e6: call void Overloading.Module1::Print(object)
End Sub
End Module
编辑：另外，请分享您的来源，我想了解更多关于IL的工作原理。如果重要的话，这将使用.NET 4.0。
答案 0 :(得分：1)
callvirt指令调用对象的后期绑定方法。
这意味着：在实例objet上，该方法“属于”;在Module方法的情况下（如果是类的共享方法），则不涉及实例 - ＆gt;不需要callvirt。
这里有2个概念，重载和覆盖（这是一种特定的重载形式）;只有后者才需要callvirt。
至于如何选择打印过载，您可以看到related documentation