LINQ identity function?

Question

Just a little niggle about LINQ syntax. I\'m flattening an IEnumerable> with SelectMany(x => x).

My problem i

Answer 1

With C# 6.0 and if you reference FSharp.Core you can do:

using static Microsoft.FSharp.Core.Operators

And then you're free to do:

SelectMany(Identity)

Answer 2

Does this work in the way you want? I realize Jon posted a version of this solution, but he has a second type parameter which is only necessary if the resulting sequence type is different from the source sequence type.

public static IEnumerable<T> Flatten<T>(this IEnumerable<T> source)
    where T : IEnumerable<T>
{
    return source.SelectMany(item => item);
}

Answer 3

You can get close to what you need. Instead of a regular static function, consider an Extension Method for your IEnumerable<T>, as if the identity function is of the collection, not the type (a collection can generate the identity function of its items):

public static Func<T, T> IdentityFunction<T>(this IEnumerable<T> enumerable)
{
     return x => x;
}

with this, you don't have to specify the type again, and write:

IEnumerable<IEnumerable<T>> deepList = ... ;
var flat = deepList.SelectMany(deepList.IdentityFunction());

This does feel a bit abusive though, and I'd probably go with x=>x. Also, you cannot use it fluently (in chaining), so it will not always be useful.

Answer 4

Unless I misunderstand the question, the following seems to work fine for me in C# 4:

public static class Defines
{
   public static T Identity<T>(T pValue)
   {
      return pValue;
   }

   ...

You can then do the following in your example:

var result =
   enumerableOfEnumerables
   .SelectMany(Defines.Identity);

As well as use Defines.Identity anywhere you would use a lambda that looks like x => x.

Answer 5

I'd go with a simple class with a single static property and add as many as required down the line

    internal class IdentityFunction<TSource>
    {
        public static Func<TSource, TSource> Instance
        {
            get { return x => x; }
        }
    }

    SelectMany(IdentityFunction<Foo>.Instance)

Answer 6

With C# 6.0 things are getting better. We can define the Identity function in the way suggested by @Sahuagin:

static class Functions
{
    public static T It<T>(T item) => item;
}

and then use it in SelectMany the using static constructor:

using Functions;

...

var result = enumerableOfEnumerables.SelectMany(It);

I think it looks very laconic in the such way. I also find Identity function useful when building dictionaries:

class P
{
    P(int id, string name) // sad, we are not getting Primary Constructors in C# 6.0
    {
        ID = id;
        Name = id;
    }

    int ID { get; }
    int Name { get; }

    static void Main(string[] args)
    {
        var items = new[] { new P(1, "Jack"), new P(2, "Jill"), new P(3, "Peter") };
        var dict = items.ToDictionary(x => x.ID, It);
    }
}