Shadows vs Overloads in VB.NET

When we have new in C#, that personally I see only as a workaround to override a property that does not have a virtual/overridable declaration, in VB.NET we have two "concepts" Shadows and Overloads.

In which case prefer one to another?

回答1:

There are three closely related concepts; overriding, shadowing and overloading.

Overriding is when you make a new implementation for a virtual method.

Shadowing is when you make a new non-virtual implementation for a method.

Overloading is when you add a method with the same name but different parameters.

All three concepts are available both in C# and VB.

回答2:

I have actually confirmed by compiling the same code with Shadows vs Overloads for a method with an identical name and signature in the base class and looking at the output from ildasm for both. The only difference is the Overloads case specifies hidebysig.

The significance of this is best explained by Jon Skeet in this answer.

But simply it means there is only a real difference if the base class has overloads of the method being redefined:

Shadows will cause all of those overloads to be uncallable through the derived class, where as
Overloads only replaces the one method.

Note that this is only a language construct and not enforced by the CLI (i.e. C# and VB.NET enforce this but other languages may not).

A simple code example:

Module Module1

Sub Main()
    Dim a1 As C1 = New C2
    Dim a2 As New C2
    a1.M1()
    a2.M1()
    a1.M2()
    a2.M2()
    a1.M3()
    a2.M3()

    a1.M1(1)
    ' Overloads on M1() allows the M1(int) to be inherited/called.
    a2.M1(1)
    a1.M2(1)
    ' Shadows on M2() does not allow M2(int) to be called.
    'a2.M2(1)
    a1.M3(1)
    ' Shadows on M3() does not allow M3(int) to be called, even though it is Overridable.
    'a2.M3(1)

    If Debugger.IsAttached Then _
        Console.ReadLine()
End Sub

End Module

Class C1
Public Sub M1()
    Console.WriteLine("C1.M1")
End Sub
Public Sub M1(ByVal i As Integer)
    Console.WriteLine("C1.M1(int)")
End Sub
Public Sub M2()
    Console.WriteLine("C1.M2")
End Sub
Public Sub M2(ByVal i As Integer)
    Console.WriteLine("C1.M2(int)")
End Sub
Public Overridable Sub M3()
    Console.WriteLine("C1.M3")
End Sub
Public Overridable Sub M3(ByVal i As Integer)
    Console.WriteLine("C1.M3(int)")
End Sub
End Class

Class C2
Inherits C1
Public Overloads Sub M1()
    Console.WriteLine("C2.M1")
End Sub
Public Shadows Sub M2()
    Console.WriteLine("C2.M2")
End Sub
Public Shadows Sub M3()
    Console.WriteLine("C2.M3")
End Sub
' At compile time the different errors below show the variation.
' (Note these errors are the same irrespective of the ordering of the C2 methods.)
' Error: 'Public Overrides Sub M1(i As Integer)' cannot override 'Public Sub M1(i As Integer)' because it is not declared 'Overridable'.
'Public Overrides Sub M1(ByVal i As Integer)
'    Console.WriteLine("C2.M1(int)")
'End Sub
' Errors: sub 'M3' cannot be declared 'Overrides' because it does not override a sub in a base class.
'         sub 'M3' must be declared 'Shadows' because another member with this name is declared 'Shadows'.
'Public Overrides Sub M3(ByVal i As Integer)
'    Console.WriteLine("C2.M3(int)")
'End Sub
End Class

The output of the above:

C1.M1
C2.M1
C1.M2
C2.M2
C1.M3
C2.M3
C1.M1(int)
C1.M1(int)
C1.M2(int)
C1.M3(int)

The output shows the Shadows calls are used when C2 is called directly and not when called indirectly through C1.

回答3:

Shadows is for cases where your base class is Function SomeMethod() As String and you want to have Function SomeMethod() As Integer. Basically, to change the return type.

Overloads is for case where your base class is Function SomeMethod() As String and you want to add a parameter such as Function SomeMethod(ByVal value As Integer) As String.