Csharp 如何在 VB.NET 中实现包含 struct 体参数的 C# 委托

这是extremely hacky，但事实证明(在.NET 7中)使用Delegate.CreateDelegate()从其签名实际接受ref参数的静态方法创建带有in参数的类型的委托是可能的.

例如，如果我创建(用C#):

struct Dummy {}
delegate int ReceiveDummy(in Dummy dummy);

static class DummyActions
{
    //https://github.com/dotnet/csharplang/blob/main/proposals/csharp-7.2/readonly-ref.md#metadata-representation-of-in-parameters
    static int ActuallyReceiveDummy(ref Dummy dummy)
    {
        return dummy.GetHashCode();
    }
    
    public static ReceiveDummy CreateReceiveDummy()
    {
        return (ReceiveDummy)
            Delegate.CreateDelegate(typeof(ReceiveDummy), 
                                    typeof(DummyActions).GetMethod(nameof(DummyActions.ActuallyReceiveDummy), 
                                                                   BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic));
    }
}

我可以做到:

var receiveMethod = DummyActions.CreateReceiveDummy();
Console.WriteLine(receiveMethod(new Dummy()));

A它起作用了！演示#1here.

翻译成VB.NET，我得到以下结果:

Friend Class DummyActions
    ' https://github.com/dotnet/csharplang/blob/main/proposals/csharp-7.2/readonly-ref.md#metadata-representation-of-in-parameters
    Private Shared Function ActuallyReceiveDummy(ByRef dummy As Dummy) As Integer
        ' Do whatever you want here but don't modify dummy!
        Return dummy.GetHashCode() 
    End Function

    Public Shared Function CreateReceiveDummy() As ReceiveDummy
        Return CType([Delegate].CreateDelegate(GetType(ReceiveDummy), 
            GetType(DummyActions).GetMethod(NameOf(DummyActions.ActuallyReceiveDummy), 
                BindingFlags.Static Or BindingFlags.Public Or BindingFlags.NonPublic)), ReceiveDummy)
    End Function
End Class

这使得在VB.NET中实现以下功能成为可能:

Dim receiveMethod = DummyActions.CreateReceiveDummy()
Console.WriteLine(receiveMethod(New Dummy()))

样机小提琴#2 here

备注:

• 根据design documents for in in C# 7.2名

  如果将System.Runtime.CompilerServices.IsReadOnlyAttribute应用于byref参数，则意味着该参数是in参数.

  此外，如果方法是抽象的或虚拟的，则此类参数(且仅此类参数)的签名必须为modreq[System.Runtime.InteropServices.InAttribute].

  动机:这样做是为了确保在方法覆盖/实现In参数的情况下匹配.

  同样的要求也适用于委托中的Invoke个方法.

  动机:这是为了确保现有的编译器在创建或分配委托时不能简单地忽略readonly.

  原来in C#编译器不允许你手动应用IsReadOnlyAttribute，如果你try 了，你会得到一个编译器错误Do not use 'System.Runtime.CompilerServices.IsReadOnlyAttribute'. This is reserved for compiler usage.(演示#3 here).但VB.NET编译器没有这样的限制，因此，如果您想要针对判断此属性的某个可能的Delegate.CreateDelegate()future 版本"保护"您的方法，您可以手动添加它:

  ' https://github.com/dotnet/csharplang/blob/main/proposals/csharp-7.2/readonly-ref.md#metadata-representation-of-in-parameters
  Private Shared Function ActuallyReceiveDummy(<System.Runtime.CompilerServices.IsReadOnlyAttribute()> ByRef dummy As Dummy) As Integer
      ' Do whatever you want here but don't modify dummy!
      Return dummy.GetHashCode() 
  End Function
  

  VB.NET编译器似乎不知道或以任何方式说明此属性.

  演示#4 here.

• 整个 idea 都是围绕着框架防止in个参数被修改这一原则进行的，因此是超级老套的.因此，您需要手动注意不要以任何方式修改您的ByRef dummy as Dummy参数.

As an alternative，如果您愿意编写一点C#代码，那么按照this answerxSjoerd van Kreel到102中的建议，您可以用C#编写一个adapter扩展方法，该方法从通过值获取Dummy的委托创建ReceiveDummy委托:

public delegate int ReceiveDummyByValue(Dummy dummy);

public static class DummyActions
{
    public static ReceiveDummy AsReceiveByReference(this ReceiveDummyByValue func) => (in Dummy d) => func(d);
}

然后在您的主要VB.NET代码中使用它，例如，如下所示:

Private Shared Function ActuallyReceiveDummy(ByVal d As Dummy) As Integer
    ' Return whatever is required for dummy
    Return d.GetHashCode()
End Function

Public Shared Function CreateReceiveDummy() As ReceiveDummy
    Return (New ReceiveDummyByValue(AddressOf ActuallyReceiveDummy)).AsReceiveByReference()
End Function