IList<mutable_struct> vs mutable_struct[]

Question

Ok, let's have some code:

//I complie nicely
ValueType[] good = new ValueType[1];
good[0].Name = "Robin";

//I don't compile: Cannot modify expression because its not a variable
IList<ValueType> bad = new ValueType[1];
bad[0].Name = "Jerome";

struct ValueType
{
    public string Name;
}

What precisely is going on behind the scenes that is causing the compiler to baulk?

//Adding to watch the following
good.GetType().Name //Value = "ValueType[]" It's a ValueType array.
bad.GetType().Name  //Value = "ValueType[]" Also a ValueType array.

The compiler is stopping me from modifying a member of the copy of the object I want to change. But why is a copy being made from this array?

A little more research throws up:

var guess = (ValueType[]) bad;
guess[0].Name="Delilah";

Now, what do you think bad[0].Name is? That's right, it's "Delilah".

Answer 1

Why is a value type being copy-returned from the IList<ValueType> but not from the array

Because an array is a built-in construct known to the compiler. Its operator [] has a built-in semantic, which provides the compiler with a modifiable reference inside the array itself.

When the compiler deals with an interface, on the other hand, all it knows it that it gets back a copy of a value type, which you are trying to modify. In other words, the compiler views IList's operator [] and array's operator [] differently.

Note: it goes without saying that this exercise is of purely academic value, because mutable structs are evil.