Question

I have a simple class which has boolean field:

public struct Foo { bool isAvailable; }

Now I have a List of foos:

List < Foo >  list = new List< Foo >();

Later on I enumerate each foo in the list and try to update its isAvailable field:

foreach(Foo foo in list) {
    foo.isAvailable = true; 
}

But the above code never updates the list. What am I doing wrong here and what’s its remedy.

Answer 1

It’s because Foo is a mutable struct.

When you fetch the value from the list, it’s making a copy – because that’s how value types behave. You’re changing the copy, leaving the original value unchanged.

Suggestions:

• You probably should be using a class
• Don’t create mutable structs. They behave in ways which can be hard to predict, or at least not the way you might expect when you’re not explicitly thinking about it.

While you could change your code to iterate over the list in a different way and replace the value each time, it’s generally a bad idea to do so. Just use a class… or project your list to a new list with the appropriate values.

---

Original answer, when Foo was a class

It should work fine. For example, here’s a short but complete program which does work:

using System.Collections.Generic;

public class Foo
{
    public bool IsAvailable { get; set; }
    public string Name { get; set; }

    public override string ToString()
    {
        return Name + ": " + IsAvailable;
    }
}

class Test
{
    static void Main()
    {
        List<Foo> list = new List<Foo>()
        {
            new Foo { Name = "First", IsAvailable = true },
            new Foo { Name = "Second", IsAvailable = false },
            new Foo { Name = "Third", IsAvailable = false },
        };

        Console.WriteLine("Before:");
        list.ForEach(Console.WriteLine);
        Console.WriteLine();

        foreach (Foo foo in list)
        {
            foo.IsAvailable = true;
        }

        Console.WriteLine("After:");
        list.ForEach(Console.WriteLine);
    }
}

Try to adapt your current code to a similar short but complete program which doesn’t work, post that, and we can work out what’s going on.