Can't change value in foreach from IEnumerable<Model>

Question

How to change value of an object in foreach from IEnumerable<Model>.

Code:

public IEnumerable<Model> ListDaftarPjsp()
{
    IEnumerable<Model> list = from x in db.PJSPEvaluation
                              select new Model
                              {
                                  foo = x.Foo,
                                  bar = x.Bar               
                              };
    foreach (Model item in list) {
        item.condition = "example";
    }
    return list;
}

public class Model{
    public string foo{ get; set; }
    public string bar { get; set; }
    public string condition{ get; set; }
}

I already create Model. Then I am looping result using foreach, then set it. But the Return for conditionstill not changing? how to set condition inside foreach then return it for result

Answer 1

IEnumerable<T> is a query, not a collection. While there is some sort of collection at the other end, the query itself is not the collection. The nature of the collection you are targeting will determine whether or not you can modify the contents.

The general rule of thumb is that you can't expect an IEnumerable<T> to return the same list of objects twice, or even expect that you will be able to enumerate across it more than once - it is perfectly valid (if unusual) for an IEnumerable<T> to enumerate once only and refuse to enumerate a second or third time.

In this case what you have is actually a database query of type IQueryable<Model> that is cast to IEnumerable<Model>. It's still an IQueryable<Model> which means that each time you enumerate across it you will get (probably) the same list of data but in completely new objects. Changing one of the objects won't change all of the objects for the same source record, nor change the contents of the underlying record itself.

If you are trying to modify the returned objects without changing the underlying records (seems to be the case) then you need to materialize the query into a collection in memory. There are a few ways to do this depending on what you're expecting to do with the returned data.

The simplest is to convert the query to an array using the .ToArray() extension method:

public Model[] ListDaftarPjsp()
{
    var query = from x in db.PJSPEvaluation
                select new Model
                {
                    foo = x.Foo,
                    bar = x.Bar               
                };

    var list = query.ToArray();

    foreach (Model item in list) 
    {
        item.condition = "example";
    }

    return list;
}

Now the records are in an array in memory and enumeration of that array can be done multiple times returning the same exact objects instead of fetching new copies of the data from the database every time.