Does it make sense to pass a "reference type" to a method as a parameter with 'ref' key?

Question

Possible Duplicate:
C#: What is the use of “ref” for Reference-type variables?

Hi,

Or it is just nonsense as it is already a reference type but not a value type?

Thanks!

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

5

It lets you change the reference variable itself, in addition to the object it's pointing to.

It makes sense if you think you might make the variable point to a different object (or to null) inside your method.

Otherwise, no.

edited Jun 20 '20 at 09:12

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answered May 11 '11 at 07:40

user541686

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So, if I DO NOT pass as with 'ref' keyword and if I change its reference to null, then I return ffrom the method the object won't be null; correct? – pencilCake May 11 '11 at 08:02
@pencilCake: The reference won't be null, no. (An *object* is never "null".) – user541686 May 11 '11 at 08:09
So only in the scope of the method it will be behaved as it is null but once the method is left, it will still be as it was -not null-? – pencilCake May 11 '11 at 08:38

vgru · Answer 2 · 2011-05-11T07:45:59.730

If makes a difference, because it allows the method to change the instance your variable is pointing to.

In other words, you use it when you want to make your variable point to a different instance of your reference type.

private static void WithoutRef(string s)
{
    s = "abc";
}

private static void WithRef(ref string s)
{
    s = "abc";
}

private static void Main()
{
    string s = "123";

    WithoutRef(s);
    Console.WriteLine(s); // s remains "123"

    WithRef(ref s);
    Console.WriteLine(s); // s is now "abc"
}

score 3 · Answer 3 · answered May 11 '11 at 07:40

When passing a reference type as ref, you are passing the reference as a reference, and this might make sense. It means that the method can replace the reference, if it wishes to:

public void CallRef()
{
    string value = "Hello, world";
    DoSomethingWithRef(ref value);
    // Value is now "changed".
}

public void DoSomethingWithRef(ref string value) 
{
    value = "changed";
}

score 0 · Answer 4 · edited May 23 '17 at 11:45

0

ref in C# allows you to modify the actual variable.

Check out this question - What is the use of "ref" for reference-type variables in C#? - including this example

Foo foo = new Foo("1");

void Bar(ref Foo y)
{
    y = new Foo("2");
}

Bar(ref foo);
// foo.Name == "2"

edited May 23 '17 at 11:45

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answered May 11 '11 at 07:38

Stuart

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ref in C# is the same as ByRef in VB, no difference. – user541686 May 11 '11 at 07:39
I've edited the text - decided it was confusing anyways - and included the example from the duplicate question. – Stuart May 11 '11 at 07:41
@Stuart: Took off my downvote. :) – user541686 May 11 '11 at 07:42
It's cool - good to learn - think I'd misunderstood something in VB - I'm a C++ coder at heart :) – Stuart May 11 '11 at 07:45
Not sure what the original answer said, as there's no edit history. But Mehrdad is indeed correct. `ByRef` in VB.NET is equivalent to C#'s `ref`. `ByVal` in VB.NET is equivalent to omitting a qualifier in C#, as passing by value is the default in both languages. – Cody Gray - on strike May 11 '11 at 07:53
+1 back to you for pointing out the dupe. – slugster May 11 '11 at 07:56

score 0 · Answer 5 · edited May 11 '11 at 08:01

0

It does if you want the incoming variable that is being passed in to have its pointer changed.

edited May 11 '11 at 08:01

user541686

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answered May 11 '11 at 07:39

Adam Dymitruk

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@phoog: To regard references as pointers is a fairly good analogy. What do you think the relevant difference is here in this case? – Cody Gray - on strike May 11 '11 at 07:52
@Cody Gray: I'm paraphrasing Eric Lippert. For one thing, pointers identify a memory address, so you need to pin the pointer's target so the GC won't move it. Also, from the 2nd link below, a pointer variable is for "[manipulating] a variable itself as data, rather than manipulating the value of that variable as data." http://blogs.msdn.com/b/ericlippert/archive/2009/02/17/references-are-not-addresses.aspx; http://blogs.msdn.com/b/ericlippert/archive/2011/03/07/references-and-pointers-part-one.aspx; http://blogs.msdn.com/b/ericlippert/archive/2011/03/10/references-and-pointers-part-two.aspx – phoog May 11 '11 at 18:40

phoog · Answer 6 · 2011-05-11T07:46:19.240

0

It's not nonsense. When you do that, you're passing the reference by reference.

Example:

class X
{
     string y;
     void AssignString(ref string s)
    {
        s = "something";
    }
    void Z()
    {
        AssignString(ref this.y};
    }
}

edited May 11 '11 at 07:46

answered May 11 '11 at 07:39

phoog

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score -1 · Answer 7 · answered May 11 '11 at 07:43

Consider the following code. What do you expect shall be the output from this program?

string s = "hello world";
Console.WriteLine(s);

foo(s);
Console.WriteLine(s);

bar(ref s);
Console.WriteLine(s);

void foo(string x)
{
    x = "foo";
}

void bar(ref string x)
{
    x = "bar";
}

The output is:

hello world
hello world
bar

When calling the method bar, you're passing the reference to string s by reference (instead of by value), which means that s will then change at the call site.

Does it make sense to pass a "reference type" to a method as a parameter with 'ref' key?

7 Answers7

It lets you change the reference variable itself, in addition to the object it's pointing to.

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