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.

Answer 1

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;
}