C# 中的 HashSet 是否有等效的 AddRange

Question

With a list you can do:

list.AddRange(otherCollection);

There is no add range method in a HashSet . What is the best way to add another ICollection to a HashSet ?

Answer 1

For HashSet<T> , the name is UnionWith .

This is to indicate the distinct way the HashSet works. You cannot safely Add a set of random elements to it like in Collections , some elements may naturally evaporate.

I think that UnionWith takes its name after "merging with another HashSet ", however, there's an overload for IEnumerable<T> too.

Answer 2

This is one way:

public static class Extensions
{
    public static bool AddRange<T>(this HashSet<T> source, IEnumerable<T> items)
    {
        bool allAdded = true;
        foreach (T item in items)
        {
            allAdded &= source.Add(item);
        }
        return allAdded;
    }
}

Answer 3

You can also use CONCAT with LINQ. This will append a collection or specifically a HashSet<T> onto another.

    var A = new HashSet<int>() { 1, 2, 3 };  // contents of HashSet 'A'
    var B = new HashSet<int>() { 4, 5 };     // contents of HashSet 'B'

    // Concat 'B' to 'A'
    A = A.Concat(B).ToHashSet();    // Or one could use: ToList(), ToArray(), ...

    // 'A' now also includes contents of 'B'
    Console.WriteLine(A);
    >>>> {1, 2, 3, 4, 5}

NOTE: Concat() creates an entirely new collection. Also, UnionWith() is faster than Concat().

" ... this ( Concat() ) also assumes you actually have access to the variable referencing the hash set and are allowed to modify it, which is not always the case. " – @PeterDuniho