Can't change struct's members value inside generic collections

Question

Imagine this struct :

        struct Person
        {
             public string FirstName { get; set; }
             public string LastName { get; set; }
        }

And following code :

        var list = new List<Person>();
        list.Add(new Person { FirstName = "F1", LastName = "L1" });
        list.Add(new Person { FirstName = "F2", LastName = "L2" });
        list.Add(new Person { FirstName = "F3", LastName = "L3" });

        // Can't modify the expression because it's not a variable
        list[1].FirstName = "F22";

When I want to change Property's value it gives me the following error:

Can't modify the expression because it's not a variable

While, when I tried to change it inside an array such as Person[] it worked without any error.Is there any problem with my code when using with generic collections?

Answer 1

When you return the struct via the List[] indexer, it returns a copy of the entry. So if you assigned the FirstName there, it would just be thrown away. Hence the compiler error.

Either rewrite your Person to be a reference type class, or do a full reassignment:

Person person = list[1];
person.FirstName = "F22";
list[1] = person;

Generally speaking, mutable structs bring about issues such as these that can cause headaches down the road. Unless you have a really good reason to be using them, you should strongly consider changing your Person type.

Why are mutable structs “evil”?

Answer 2

Obviously a part of the question is still unanswered. What is difference between List<Person> and Person[]. In term of getting element by index the List calls indexer (method) which returns copy of value-type instance, in opposite array by index returns not a copy but managed pointer to element at the index (used special IL instruction ldelema).

Of course mutable value-types are evil as mentioned in other answers. Look at the simple example.

var en = new {Ints = new List<int>{1,2,3}.GetEnumerator()};
while(en.Ints.MoveNext())
{
    Console.WriteLine(x.Ints.Current);
}

Surprised?