Using unary expressions with property functions

[Andrej Mitrovic]

So I was a bit surprised today to find out that this sample C# code works:

class LibraryItem
{
    private int _numCopies;

    // Property
    public int NumCopies
    {
        get { return _numCopies; }
        set { _numCopies = value; }
    }
    
    public void BorrowItem(string name)
    {
        NumCopies--;
    }  
}

Apparently you can use the -- and ++ unary ops on property functions, which might be convenient, I guess. But this won't work in D:

class LibraryItem
{
    private int _numCopies;

    @property
    int NumCopies()
    {
        return _numCopies;
    }
    
    @property
    void NumCopies(int value)
    {
        _numCopies = value;
    }
    
    public void BorrowItem(string name)
    {
        NumCopies--;
    }
}

void main()
{
}

I get this back: Error: this.NumCopies() is not an lvalue

So D tries to call the getter method "int NumCopies()" and apply -- on it. One way around this is to make the getter method return by reference:

import std.stdio;

class LibraryItem
{
    private int _numCopies;

    @property
    ref int NumCopies()
    {
        return _numCopies;
    }
    
    @property
    void NumCopies(int value)
    {
        _numCopies = value;
    }
    
    public void BorrowItem(string name)
    {
        NumCopies--;
        writeln(NumCopies);     // writes -1 after the call
    }
}

void main()
{
    auto lib = new LibraryItem();
    lib.BorrowItem("TDPL");
}

So this works. But now the getter method acts both as a getter and a setter, which contradicts the idea of having separate get/set methods, don't you agree?

I'm not sure how C# works. Does it use the getter method and return a reference for the -- op like in the last D example, or does it rewrite the whole operation as "NumCopies(-1)"? Maybe we can have a functionality like that in D without having to change the getter method to return a reference.. What do you think?