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There isn't any cool LINQ sugar for creating unions. The Enumerable.Union() method is usually called like this:

var bigList = list1.Union(list2);

The alternative is to call Enumerable.Union() which can be more readable:

var bigList = Enumerable.Union(list1, list2);

However neither of these methods are very stylish (more importantly, readable) when scaling out The following is probably the best method:

var reallyBigList = list1.Union(list2).Union(list3);

Which can result in some messy method chaining. Alternatives need incidental variables:

var list1and2 = list1.Union(list2);
var reallyBigList = list1and2.Union(list3);


var list1and2 = Enumerable.Union(list1, list2);
var reallyBigList = Enumerable.Union(list1and2, list2);

Is there a clean way of setting up these more complex unions? Would an extension like Enumerable.Union(params IEnumerable<T> collections) (used like var reallyBigList = Enumerable.Union(list1, list2, list3)) be better?

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2 Answers 2

I don't consider the method chaining option messy at all. Sure, something like

 var reallyBigList = (from ... where ... select ...).Union(from ... where ... select)...

can easily get unreadable, but, on the other hand,

 var mp3s = from ... where ... select ...
 var videos = from ... where ... select ...
 var alreadyProcessed = from ... where ... select ...

 var toDo = mp3s.Union(videos).Except(alreadyProcessed)

reads quite naturally. So, when using well-named intermediate variables, the method chaining approach is extremely readable.

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The method chaining style is pretty readable, in my opinion. Certainly you could write your own extension method to take many IEnumerable<T>.

Here's an example using iteration. Recursion works too, but I didn't like it as much.

public static IEnumerable<T> MyUnion<T>(
    this IEnumerable<T> original, params IEnumerable<T>[] toUnion)
    var enumerable = original;
    foreach (var other in toUnion)
        enumerable = enumerable.Union(other);

    return enumerable;
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or you could flatten the toUnion array to factor away the loop - return original.Union(toUnion.SelectMany(x => x)); –  MattDavey Feb 14 '13 at 9:31
@MattDavey: Note that joining sets in toUnion and then doing the union has a different performance impact than computing the union in a loop. Which one will be faster depends on the context. –  MainMa Feb 14 '13 at 9:36
@MainMa agreed, I didn't make any claims about performance, I was merely pointing out there was a more concise way to write the code. –  MattDavey Feb 14 '13 at 10:59
I don't think the call to AsEnumerable() is necessary here. –  svick Feb 14 '13 at 11:05

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