How to use a method with an out parameter in a lambda expression

Question

I have a method which looks the following way:

bool GetIdByName(string name, out ID id)

I would like to use it inside a lambda expression, to get several 'ids' by a number of 'names':

var ids = names.Select(name => idService.GetIdByName(name, out id));

In this case I will find all the bool values inside my 'ids' variable, which is not what I want. Is it also possible to get the out parameter 'id' of each call into it?

Why are you using an out parameter for id? Why not simply return the id itself? Are there cases when no id exists? — Yuval Itzchakov
– Yuval Itzchakov, Commented Aug 24, 2015 at 9:31
The existing method which gives back the id was not written by me and I don't want to touch it. Yes there are cases where no id exists. — Christoph Brückmann
– Christoph Brückmann, Commented Aug 24, 2015 at 9:44

Matías Fidemraizer · Accepted Answer · 2015-08-24 09:33:22Z

5

You can use a delegate with body for this:

IEnumerable<ID> ids = names.Select
(
    name =>
    {
        ID id;
        GetName(name, out id);

        return id;
    }
);

answered Aug 24, 2015 at 9:33

Matías Fidemraizer

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dcastro · Accepted Answer · 2015-08-24 09:29:20Z

2

I would factor the call to GetIdByName into a method, so that it becomes more composable.

var ids = names.Select(GetId);

private static ID GetId(string name)
{
    ID id;
    idService.GetIdByName(name, out id);
    return id;
}

answered Aug 24, 2015 at 9:29

dcastro

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Comments

Dmitrii Bychenko · Accepted Answer · 2015-08-24 09:33:01Z

2

Are looking for something like that?

var ids = names
  .Select(name => {
    ID id = null; 

    if (!idService.GetIdByName(name, out id))
      id = null; // name doesn't corresponds to any ID

    return id;
  })
  .Where(id => id != null);

In case ID is a struct (and so not nullable):

  var ids = names
    .Select(name => {
      ID id = null; 
      Boolean found = idService.GetIdByName(name, out id);

      return new {
        Found = found,
        ID = id
      };
    })
    .Where(data => data.Found)
    .Select(data => data.id);

answered Aug 24, 2015 at 9:33

Dmitrii Bychenko

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