In .NET is there any difference between using pointers as function parameters or using the "ref" keyword?

Question

I have written a struct and functions where I try to pass the struct by reference (i.e. struct value can be modified inside functions).

enum RoomType { Economy, Buisness, Executive, Deluxe };

struct HotelRoom
{
    public int Number;
    public bool Taken;
    public RoomType Category;

    public void Print()
    {
        String status = Taken ? "Occupied" : "available";
        Console.WriteLine("Room {0} is of {1} class and is currently {2}", 
            Number, Category, status);
    }
}

Now to pass this struct by reference I've found two ways.

//Using Pointer
private unsafe static void Reserve(HotelRoom* room)
{
    if (room->Taken)
        Console.WriteLine("Cannot reserve room {0}", room->Number);
    else
        room->Taken = true;
}      

//Using ref keyword.
private static void Reserve(ref HotelRoom room)
{
    if (room.Taken)
         Console.WriteLine("Cannot reserve room {0}", room.Number);
    else
         room.Taken = true;
}

Is there any difference? In general when should I use a pointer and when should I go for the ref keyword?

Mutable structs are evil. You should use a class, and a normal parameter. — SLaks, Apr 19 '12 at 13:44
What does the need for the `unsafe` keyword with pointers tell you about your question? — Oded, Apr 19 '12 at 13:44
It sounds like you're thinking in C++ terms. Don't. C# is not C++. — SLaks, Apr 19 '12 at 13:45
@SLaks jsut trying to tap in all optimization options of .Net, I have coded in C++ for more than 10 years so might have some effect. — Pritesh, Apr 19 '12 at 13:59
@Oded I was wondering any advantage in taking risk of unsafe — Pritesh, Apr 19 '12 at 14:00
This is not an optimization; it's actually a pessimization. Pinning is well-known to [impair GC performance](http://msdn.microsoft.com/en-us/magazine/cc163910.aspx#S2). — Raymond Chen, Apr 19 '12 at 23:44

score 3 · Answer 1 · answered Apr 19 '12 at 13:42

3

Simple rule of thumb:

Don't use pointers.

Pointers are meant for obscure interop or obtuse optimizations.
In general, you should not use unsafe code.

answered Apr 19 '12 at 13:42

SLaks

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Agreed. In .Net there is no need to use pointers. That's what `ref` and `out` are for. However, it can be necessary to use a pointer if you include unmanaged code in your project. That's why this option exists. – Nico Schertler Apr 19 '12 at 13:45
@Nico Schertler 2 places 2 be used. 1) Interop 2) optimization. – Pritesh Apr 19 '12 at 14:06
@Pritesh: However, there are few scenarios in which pointers will actually make things faster. In general, if you have to ask, you probably shouldn't. (and, as always, _measure_!) – SLaks Apr 19 '12 at 14:30
@SLaks It's like "power to use pointer comes with great responsibility" – Pritesh Apr 19 '12 at 14:42

score 1 · Accepted Answer · answered Apr 19 '12 at 13:45

1

Pointers are considered unsafe.

If you code in an unsafe context, like passing a reference by pointer to the function, someone can change your pointer to pointer somewhere else, and you get garbage data.

If you use the ref keyword, the context is "safe", and you can't change where room is pointing to, only it's data like number, taken and category.

answered Apr 19 '12 at 13:45

Nihil

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6

most important point you brought up is "someone can change your pointer to pointer somewhere else, and you get garbage data" +1 on that? – Pritesh Apr 19 '12 at 14:01
You're asking me if to +1? I'm biased.. :) – Nihil Apr 19 '12 at 14:25
I put the "?" by mistake. it is "+1", you arnswer was very good. – Pritesh Apr 19 '12 at 14:35

score 1 · Answer 3 · edited May 23 '17 at 11:55

1

Pointers are useful part of C# language. Using pointers are not difficult but should be used carefully because it might be cause of errors difficult to diagnose. Using pointers disturb garbage collector functioning especially when we use many pointers in our program. Therefore before using pointers we should consider it and maybe try to find some other ways.For details see:
http://www.c-sharpcorner.com/UploadFile/gregory_popek/WritingUnsafeCode11102005040251AM/WritingUnsafeCode.aspx
and
Working with pointers in C#

edited May 23 '17 at 11:55

Community

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answered Apr 19 '12 at 13:53

4b0

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seems it is like Hammer have to use it sometime but carefully, should not end up hitting in our own feet. – Pritesh Apr 19 '12 at 14:03

score 0 · Answer 4 · answered Apr 19 '12 at 21:19

In .net, when something is passed by ref, the called function receives a pointer which it is forbidden from copying except in certain very limited circumstances; unless the pointer is given to unsafe code, any copies of the pointer that have been made during the execution of the called function are guaranteed to be destroyed before the called function returns. If the pointer in question points to part of a heap object, the metadata associated with the call site will let the Garbage Collector know where to find a reference to that object, so that it will know not to disturb it.

Pointers created by, or passed to, unsafe functions or unmanaged code are different. If an unsafe function or unmanaged code has a pointer to something, it may store it in a fashion which outlives the function call, and which the Garbage Collector doesn't know about. There exist means by which the Garbage Collector can be explicitly told not to disturb a heap object, but the use of such techniques is fraught with peril.

Use ref for passing parameters by reference unless there's an overwhelmingly strong reason to use pointers. It's apt to be just as fast as passing pointers in cases where both techniques will consistently work correctly.

In .NET is there any difference between using pointers as function parameters or using the "ref" keyword?

4 Answers4

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