Question

I have list of structure. I want to modify a particular data from the structure.
And the structure is at the particular index location of the List.

I want to do something like this:

struct sample
{
  int a;
  string name;
}

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


for(int i=0; i < list.Count; i++)
{
  list[i].a = someotherlist[i].data;
}

Answer 1

The problem is that the list indexer creates a copy of the struct, i.e. it is really:

for(int i=0; i < list.Count; i++)
{
  var completelyIsolatedClone = list[i];
  completelyIsolatedClone.a = someotherlist[i].data;
}

The compiler is preventing you making an obvious mistake. The code you have uses the get, mutates a separate copy of the data, but never puts it back – so your change doesn’t do anything useful. Note that the Mono folks think it would be nice if it worked your way, though: http://tirania.org/blog/archive/2012/Apr-11.html

Note that an array works differently; with an array you are touching the struct in place, so the following would work fine:

for(int i=0; i < list.Count; i++)
{
  someArray[i].a = someotherlist[i].data;
}

Another approach is to copy the data out (the get accessor), mutate it, and put it back:

for(int i=0; i < list.Count; i++)
{
  var completelyIsolatedClone = list[i];
  completelyIsolatedClone.a = someotherlist[i].data;
  list[i] = completelyIsolatedClone;
}

or better, acoid mutable structs completely, perhaps with a method that applies the change to a new copy:

for(int i=0; i < list.Count; i++)
{
  list[i] = list[i].SetA(someotherlist[i].data);
}

where SetA creates a new struct, like DateTime.AddDays etc, i.e.

public SomeType SetA(int a) {
    return new SomeType(this.X, this.Y, a, this.Z);
}