Question

When I build the following, I get the error Cannot modify members of 'pd' because it is a 'foreach iteration variable'.

foreach (var w in tt.Weeks)
  foreach (var d in w.Days)
    foreach (var pd in d.Periods)
      if (pd.Type == TimetablePeriod.Types.Registration) pd.ClassId = clsId;

I understand why this error occurs, but I’m trying to think of a neat way to rewrite the code. I could use nested for loops, but is there a nice LINQ solution?

(In case it’s not clear, I’m trying to set the ClassId of all periods in all days of all weeks to clsId, wherever the PeriodType is TimetablePeriod.Types.Registration.)

Answer 1

I assume you have a struct like this:

public struct Period
{
    public SomeType ClassId{get;set;}
}

This means that any iteration over a collection using either LINQ or foreach will work on copies of the values. So if you modify them you only modify a temporary copy which will then be discarded. The original struct will remain untouched.

For this and many other reasons mutable structs should be avoided unless you have a really good reason to use them. (Search for “Mutable structs evil” and you’ll find a lot)

In your case you can simply replace struct with class. You then get reference semantics.

Or you can use a for loop, which gives you direct access. But it will still not compile with the naive implementation:

for(int i=i;i<list.Count;i++)
  list[i].ClassId=something;

Because the indexer on lists is a property, which copies its results. With arrays this would work since the indexer of an array returns a managed reference(or something similar). For lists you need to do:

for(int i=i;i<list.Count;i++)
{
  var copy=list[i];
  copy.ClassId=something;
  list[i]=copy;
}