Doesn't C# Extension Methods allow passing parameters by reference?

问题

Is it really impossible to create an extension method in C# where the instance is passed as a reference?

Here’s a sample VB.NET console app:

Imports System.Runtime.CompilerServices

Module Module1
  Sub Main()
    Dim workDays As Weekdays

    workDays.Add(Weekdays.Monday)
    workDays.Add(Weekdays.Tuesday)

    Console.WriteLine("Tuesday is a workday: {0}", _ 
      CBool(workDays And Weekdays.Tuesday))
    Console.ReadKey()
  End Sub
End Module

<Flags()> _
Public Enum Weekdays
  Monday = 1
  Tuesday = 2
  Wednesday = 4
  Thursday = 8
  Friday = 16
  Saturday = 32
  Sunday = 64
End Enum

Module Ext
  <Extension()> _
  Public Sub Add(ByRef Value As Weekdays, ByVal Arg1 As Weekdays) 
    Value = Value + Arg1
  End Sub
End Module

Note the Value parameter is passed ByRef.

And (almost) the same in C#:

using System;

namespace CS.Temp
{
  class Program
  {
    public static void Main()
    {
      Weekdays workDays = 0;

      workDays.Add(Weekdays.Monday); // This won't work
      workDays.Add(Weekdays.Tuesday);

      // You have to use this syntax instead...
      // workDays = workDays | Weekdays.Monday;
      // workDays = workDays | Weekdays.Tuesday;

      Console.WriteLine("Tuesday is a workday: {0}", _ 
        System.Convert.ToBoolean(workDays & Weekdays.Tuesday));
      Console.ReadKey();
    }
  }

  [Flags()]
  public enum Weekdays : int
  {
    Monday = 1,
    Tuesday = 2,
    Wednesday = 4,
    Thursday = 8,
    Friday = 16,
    Saturday = 32,
    Sunday = 64
  }

  public static class Ext
  {
    // Value cannot be passed by reference? 
    public static void Add(this Weekdays Value, Weekdays Arg1) 
    {
      Value = Value | Arg1;
    }
  }
}

The Add extension method doesn’t work in C# because I can’t use the ref keyword. Is there any workaround for this?

回答1:

No. In C#, you cannot specify any modifiers (like 'out' or ref) other than this for the first parameter of an extension method - you can for the others. Not familiar with the VB Syntax but it seems to be using a declarative approach to mark an extension method.

When you call it, you do not specify the first this parameter. Hence marking the parameter as out or ref doesnt make sense as You can't specify the modifier when you call it like you'd do for normal methods

void MyInstanceMethod(ref SomeClass c, int data) { ... } // definition

obj.MyInstanceMethod(ref someClassObj, 10);              // call

void MyExtensionMethod(this SomeClass c, int data) {.... } // defn

c.MyExtensionMethod(10);                                 // call

I think the trouble you're having here is related to value types being immutable. If Weekdays was a reference type, it would work out alright. For immutable types (structs), the defacto way is to return a new instance with the required value. E.g. See the Add method on the struct DateTime, it returns a new DateTime instance whose value = receiver DateTime instance's value + param value.

public DateTime Add( TimeSpan value )

回答2:

Yikes - you're making a mutable immutable struct. It breaks what people expect to see in C#, but if you must, then you can always directly call the method:

Ext.Add(ref value, arg1);

Any extension method is directly callable.

Also, a clarification:

SomeReferenceType value = ...;
SomeReferenceType copy = value;
value.ExtensionMethodByRef(...);
// this failing is semantically ridiculous for reference types, which
// is why it makes no sense to pass a `this` parameter by ref.
object.ReferenceEquals(value, copy);

回答3:

Strange that VB.NET allows this and C# doesn't...

However, although it might make sense from a technical point of view (since an extension method is just a static method), I think it doesn't feel right, because extension methods are used as if they were instance methods, and instance methods can't modify the this reference.

来源：https://stackoverflow.com/questions/1259103/doesnt-c-sharp-extension-methods-allow-passing-parameters-by-reference

标签

vb.net

extension-methods

byref