Indexers in List vs Array

Question

How are the Indexers are defined in List and Arrays.

List lists=new List(); where MyStruct is a Structure.

Answer 1

The final point:

lists[0]._x=5

is actually just a restatement of your earlier point:

arr[0] gives a reference to the first Structure item.But lists[0] gives me a copy of it.

If you edited a copy of it, the change would be lost into the ether, i.e.

var throwawayCopy = lists[0];
throwawayCopy._x = 5;
// (and never refer to throwawayCopy again, ever)

Since that is almost certainly not what you intended, the compiler doesn't let you. However, mutable structs are evil. A better option here would be don't use mutable structs. They bite.

---

Taking this down a level, to a simple but concrete example:

using System;
struct Evil
{
    public int Yeuch;
}
public class Program
{
    public static void Main()
    {
        var pain = new Evil[] { new Evil { Yeuch = 2 } };
        pain[0].Yeuch = 27;
        Console.WriteLine(pain[0].Yeuch);
    }
}

This compiles (looking at the last 2 lines here) as:

L_0026: ldloc.0 <== pain
L_0027: ldc.i4.0 <== 0
L_0028: ldelema Evil <== get the MANAGED POINTER to the 0th element
                           (not the 0th element as a value)
L_002d: ldc.i4.s 0x1b <== 27
L_002f: stfld int32 Evil::Yeuch <== store field

L_0034: ldloc.0 <== pain
L_0035: ldc.i4.0 <== 0
L_0036: ldelema Evil <== get the MANAGED POINTER to the 0th element
                           (not the 0th element as a value)
L_003b: ldfld int32 Evil::Yeuch <== read field
L_0040: call void [mscorlib]System.Console::WriteLine(int32) <== writeline
L_0045: ret 

It never actual talks to the struct as a value - no copies, etc