Can I write this lambda with IEnumerable rather than IEnumerable?

Question

Is it possible to write this lambda in a more generic way? It works fine, but seems too specifically typed; the content of the IEnumerables are not touched, but should be preserved in the return type.

static readonly Func<
    (IEnumerable<string>, IEnumerable<string>),
    (IEnumerable<string>, IEnumerable<string>),
    (IEnumerable<string>, IEnumerable<string>)
> JoinListTuples = (a, b) =>
    (a.Item1.Concat(b.Item1),
        b.Item2.Concat(b.Item2));

I was thinking of something like this, but it doesn't compile:

static readonly Func<
    (IEnumerable<T>, IEnumerable<U>),
    (IEnumerable<T>, IEnumerable<U>),
    (IEnumerable<T>, IEnumerable<U>)
> JoinListTuples = (a, b) =>
    (a.Item1.Concat(b.Item1),
        b.Item2.Concat(b.Item2));

The lambda is fine. The declaration of the field you initialize with the lambda is a problem. — Ben Voigt, May 31 '18 at 23:42
Possible duplicate of [Is it possible to define a generic lambda in C#?](https://stackoverflow.com/questions/2404993/is-it-possible-to-define-a-generic-lambda-in-c) — Colin Young, May 31 '18 at 23:44
Possible duplicate of [How create a generic Func delegate](https://stackoverflow.com/questions/26758915/how-create-a-generic-func-delegate) — Evan Trimboli, May 31 '18 at 23:44
Please **do not** edit your questions to show/contain answers, — Camilo Terevinto, Jun 01 '18 at 00:29

score 4 · Accepted Answer · edited Jun 01 '18 at 05:08

Types can be generic, methods can be generic, but fields cannot be generic.

A workaround when you really need to have this generically as a delegate type is to make it a field of a generic class:

public static class HelperClass<T, U> {
    public static readonly Func<
        (IEnumerable<T>, IEnumerable<U>),
        (IEnumerable<T>, IEnumerable<U>),
        (IEnumerable<T>, IEnumerable<U>)
    > JoinListTuples = (a, b) =>
        (a.Item1.Concat(b.Item1),
            b.Item2.Concat(b.Item2));
}

But for most purposes, it would suffice to just turn it into a method instead. Perhaps something like this:

static (IEnumerable<T>, IEnumerable<U>) JoinListTuples<T, U>(
    (IEnumerable<T>, IEnumerable<U>) a,
    (IEnumerable<T>, IEnumerable<U>) b)
{
    return (a.Item1.Concat(b.Item1),
        b.Item2.Concat(b.Item2));
}

;) ill upvote because you did a better job of explaining – TheGeneral May 31 '18 at 23:46 — TheGeneral, May 31 '18 at 23:46

score 2 · Answer 2 · answered May 31 '18 at 23:43

2

I'm not sure if this will help you

However, something like this maybe

public class SomeHelper<T,U>
{

    public static readonly Func<
        (IEnumerable<T>, IEnumerable<U>),
        (IEnumerable<T>, IEnumerable<U>),
        (IEnumerable<T>, IEnumerable<U>)> 
            JoinListTuples = (a, b) => 
               (a.Item1.Concat(b.Item1),b.Item2.Concat(b.Item2));

}

Note, untested and not in front of VS

answered May 31 '18 at 23:43

TheGeneral

79,002
9
103
141

1

Heh, you beat me to it. Yes, that compiles exactly the way you have it now. – May 31 '18 at 23:44

Can I write this lambda with IEnumerable rather than IEnumerable?

2 Answers2