Alternative to using ref in foreach?

Question

I have a modifying method with a signature like

private bool Modify(ref MyClass obj);

that will make modifications to obj and indicate succes with it's return value. Modify is not reassigning the reference (I know that this wouldn't work), just modifying instance fields, so I want to use it to do something like the following:

foreach(MyClass obj in myList)
{
    bool success = Modify(obj);
    // do things depending on success
}

I am running into a problem compiling as obj is "not being passed with the ref keyword". However, if I put the ref keyword in like so:

bool success = Modify(ref obj);

I get "cannot use obj as a ref/out because it is a 'foreach iteration variable". I understand that foreach uses an immutable iterator and that's why this doesn't work.

My question is what is the easiest alternative to make something like this work?

I have tried using

foreach(int i = 0; i < myList.Count; i++)
{
    bool success = Modify(ref myList[i]);
    // do things depending on success
}

but they I get "a property or indexer may not be passed as an out of ref parameter".

Thanks your help.

Subarid

maybe use simplefor loop instead of foreach – Bullnecked

That's not possible with foreach I guess. Because myList can be a generator, read-only list, etc. – Galsworthy

Why do you need to pass object by ref to this Modify function? – Galah

@James so that I can modify it. Right? – Subarid

You can modify the properties of obj without using ref. Using ref allows you to modify the reference itself. This is almost certainly not what you are intending. – Espouse

Define how you modify it. If you are updating properties or fields, you do not need a ref parameter. If you are completely reassigning the reference, that's a separate issue. – Sclaff

Oh wow. Thanks for clarifying. That makes a lot of sense. – Subarid

Please update your question to include how you use obj inside Modify – Aslam

First - the outter class shouldn't modify the inner class, it should be done over calling methods of inner class or by using command pattern. – Encampment

@RufusL, passing by reference has a very specific meaning and it's a mistake to suggest classes are passed by reference by default. A class' reference is passed by value. – Sclaff

@AnthonyPegram, true, I've removed that comment. thank you! – Idolah

Same, i hate this about c#. You can see the same question asked over and over with the only answer that "you shouldnt do that" – Bedelia

Answer 1

You state

Modify is not reassigning the reference

Therefore, there is no reason the Modify(ref MyClass) function needs to pass argument by ref.

You should be able to do the same "modifications", whatever that is (please clarify that) by passing the object reference by value, i.e. removing the ref keyword.

So, the fix here should be changing your Modify function signature from Modify(ref MyClass) to Modify(MyClass)

Galah

The modifications are complicated and I would argue not relevant to the example. – Subarid

Answer 2

Any type within C# is passed actually by value. When you pass an instance of a class to a method what is actually passed is not the instance itself but a reference to it which itself is passed by value. So effectivly you're passing instances of a class as reference - which is why you call them reference-types.

In your case you just modify an existing instance referenced by that reference-value in your method, no need to use the ref-keyword.

foreach(var m in myList)
{
    MyMethod(m);
}

MyMethod(MyClass instance)
{
    instance.MyProperty = ...
}

If you'd really pass the reference by reference you'd re-assign the obj on every iteration within your loop which isn't allowed within a foreach-block. This would be similar to the following:

foreach(var m in myList)
{
    m = new MyClass();
}

On the other side you could also use a classic for-loop. However you'd need a temporary variable to store the outcome of your method:

for(int i = 0; i < myList.Length; i++)
{
    var tmp = myList[i];
    MyMethod(ref tmp);
    myList[i] = tmp;
}

Lorusso

This truly answers the conceptual question being asked, whether or not it is the right solution for the OP – Galah

Answer 3

use a temp variable to bypass the message

foreach(MyClass obj in myList)
{
    MyClass objTemp = obj;
    bool success = Modify(ref objTemp);
    // do things depending on success
}

private MyMethod(ref MyClass instance)
{
    instance.MyProperty = ...
}

Zendejas

Answer 4

You need to cast your array to a Span:

int[] nums = new[] { 1, 2, 3, 4, 5 };

int i = 0;
foreach (ref var item in nums.AsSpan())
{
    item += 10;
    Console.WriteLine(nums[i++]);
}

output:
11
12
13
14
15

You can modify the iteration variable because Span Enumerator's Current property is of ref T type.

Photooffset

Answer 5

It is solved by using LINQ.

My Code:

    private static List<string> _test = new List<string>();

    public List<string> Test { get => _test; set => _test = value; }


    static void Main(string[] args)
    {
        string numString = "abcd";

        _test.Add("ABcd");
        _test.Add("bsgd");

        string result = _test.Where(a => a.ToUpper().Equals(numString.ToUpper()) == true).FirstOrDefault();

        Console.WriteLine(result + " linq");
    }

Leadsman

How does this answer the question on modifying a list within a loop? – Lorusso

I suggested a way to solve it without having to use for or foreach statement. – Leadsman

The questioner seemed to be looking for an easier way than for ref in for or foreach. – Leadsman