How to combine more than two generic lists in C# Zip?

Question

I have three (it's possible to have more than 3-4 generic list, but in this example let 3) generic lists.

List<string> list1

List<string> list2

List<string> list3

all lists have same number of elements (same counts).

I used that for combining two lists with ZIP :

var result = list1.Zip(list2, (a, b) => new {
  test1 = f,
  test2 = b
}

I used that for foreach statement, to avoid foreach each List, like

foreach(var item in result){
Console.WriteLine(item.test1 + " " + item.test2);
}

How to use simmilary with Zip for three lists ?

Thanks

EDIT:

I want like:

List<string> list1 = new List<string>{"test", "otherTest"};

List<string> list2 = new List<string>{"item", "otherItem"};

List<string> list3 = new List<string>{"value", "otherValue"};

after ZIP (I don't know method), I want to result (in VS2010 debug mode)

[0] { a = {"test"},
      b = {"item"},
      c = {"value"}
    }   

[1] { a = {"otherTest"},
      b = {"otherItem"},
      c = {"otherValue"}
    }  

How to do that ?

Answer 1

The most obvious way for me would be to use Zip twice.

For example,

var results = l1.Zip(l2, (x, y) => x + y).Zip(l3, (x, y) => x + y);

would combine (add) the elements of three List<int> objects.

Update:

You could define a new extension method that acts like a Zip with three IEnumerables, like so:

public static class MyFunkyExtensions
{
    public static IEnumerable<TResult> ZipThree<T1, T2, T3, TResult>(
        this IEnumerable<T1> source,
        IEnumerable<T2> second,
        IEnumerable<T3> third,
        Func<T1, T2, T3, TResult> func)
    {
        using (var e1 = source.GetEnumerator())
        using (var e2 = second.GetEnumerator())
        using (var e3 = third.GetEnumerator())
        {
            while (e1.MoveNext() && e2.MoveNext() && e3.MoveNext())
                yield return func(e1.Current, e2.Current, e3.Current);
        }
    }
}

The usage (in the same context as above) now becomes:

var results = l1.ZipThree(l2, l3, (x, y, z) => x + y + z);

Similarly, you three lists can now be combined with:

var results = list1.ZipThree(list2, list3, (a, b, c) => new { a, b, c });

Answer 2

There is another quite interesting solution that I'm aware of. It's interesting mostly from educational perspective but if one needs to perform zipping different counts of lists A LOT, then it also might be useful.

This method overrides .NET's LINQ SelectMany function which is taken by a convention when you use LINQ's query syntax. The standard SelectMany implementation does a Cartesian Product. The overrided one can do zipping instead. The actual implementation could be:

static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IEnumerable<TSource> source,
        Func<TSource, IEnumerable<TCollection>> selector, Func<TSource, TCollection, TResult> select)
{
    using (var e1 = source.GetEnumerator())
        using (var e2 = selector(default(TSource)).GetEnumerator())
            while (true)
                if (e1.MoveNext() && e2.MoveNext())
                    yield return select(e1.Current, e2.Current);
                else
                    yield break;
}

It looks a bit scary but it is a logic of zipping which if written once, can be used in many places and the client's code look pretty nice - you can zip any number of IEnumerable<T> using standard LINQ query syntax:

var titles = new string[] { "Analyst", "Consultant", "Supervisor"};
var names = new string[] { "Adam", "Eve", "Michelle" };
var surnames = new string[] { "First", "Second", "Third" };

var results =
    from title in titles
    from name in names
    from surname in surnames
    select $"{ title } { name } { surname }";

If you then execute:

foreach (var result in results)
    Console.WriteLine(result);

You will get:

Analyst Adam First
Consultant Eve Second
Supervisor Michelle Third

You should keep this extension private within your class because otherwise you will radically change behavior of surrounding code. Also, a new type will be useful so that it won't colide with standard LINQ behavior for IEnumerables.

For educational purposes I've created once a small c# project with this extension method + few benefits: https://github.com/lukiasz/Zippable

Also, if you find this interesting, I strongly recommend Jon Skeet's Reimplementing LINQ to Objects articles.

Have fun!

Answer 3

You can combine many lists in C# with cascade zip methods and anonymous classes and Tuple result.

List<string> list1 = new List<string> { "test", "otherTest" };
List<string> list2 = new List<string> { "item", "otherItem" };
List<string> list3 = new List<string> { "value", "otherValue" };

IEnumerable<Tuple<string, string, string>> result = list1
    .Zip(list2, (e1, e2) => new {e1, e2})
    .Zip(list3, (z1, e3) => Tuple.Create(z1.e1, z1.e2, e3));

The result is:

[0]
{(test, item, value)}
    Item1: "test"
    Item2: "item"
    Item3: "value"