Add a column to an IEnumerable in c#

Question

I don't think I can actually add a field (column) to an existing IEnumerable. But what I want is a new IEnumerable that is derived from an existing IEnumerable with a calculated field. The pseudocode in WebMatrix using Web Pages looks like:

var db = Database.Open("LOS");
var ie = db.Query(sqlAssignments);

// make a new ie2 that has an extra field
// ??? ie2 <=== ie with new field c = ie.a + ie.b

var grid = new WebGrid( ie2, extra parameters );

I know how to do this looping through all the rows in ie. But I'm hoping there's something more elegant.

Momus

IEnumerable simply signifies an enumerable type, i.e. a consumer can enumerate through it's constituent items. It has no concept of 'fields' or 'columns'. – Aplanospore

@Adam - yet I think more people are likely to understand that title than, for example: "Use a projection to create a new sequence based on an old sequence, but with additional members"... – Epidaurus

Answer 1

How about:

var ie2 = ie.Select(x => new { x.Foo, x.Bar, Sum = x.Abc + x.Def });
var grid = new WebGrid(ie2);

Epidaurus

this works great, but bizarrely has led to a new problem where the WebGrid no longer automatically sorts :( . I decided to make it a new question. – Momus

@Momus - just use ie2.ToList() – Epidaurus

I tried ToList, but it still didn't sort. Rather than morph this question into the new question, i asked a new question. #5339835 – Momus

that's kind of a pain: what if you have like 20 columns or more? – Iphigenia

Answer 2

You can do this using Select. Try this:

ie2 = ie.Select( e => new { IE = e, NewParam =  e.X+e.Y });

Obelia

The one comment I have about this is that most data-binding only looks one level down; you usually need to flatten the properties of e, otherwise you'll just have a column named e with e.ToString() as the values. – Epidaurus

If that is the case, you will have to flatten as your answer depicts. If it only looks down one level, let's hope the number of properties on e is manageable :) – Wenn

I didn't get this to immediately work, and decided to flatten it. Fortunately, there are only about 10 properties on e. – Momus

Answer 3

ie2 = ie.Select(v => new {v.a, v.b, c = v.a + v.b});

Glower

Answer 4

Using Linq!

var newIe = from item in ie
            select new {item.NewField, item.OldFiedl1 etc... }

Also, probably best (if you intend to use outside this method) to make that anonymous type named.

Geithner

Answer 5

First of all, the IEnumerable is probably a list of something - an object. That is the object you can extend.

You can probably do something like this:

var ie = db.Query( ... );
var ie2 = ie.Select(i => new MyIe2Object {
   Prop1 = i.Prop1,
   NewProp = i.Prop1 + i.Prop2
});

Latona

When LINQ gets involved, I don't think it is true to guess a "list" - but sure, extending the something would work, too – Epidaurus

The point is that it's not really about extending the IEnumerable, but the something. – Carrasco