If you look at the C# source code for List<T>, I think List<T>.AddRange() has optimizations that a simple loop doesn’t address. So, an extension method should simply check to see if the IList<T> is a List<T>, and if so use its native AddRange(). Poking around the source code, you see the .NET folks do similar … Read more
Please refer to the updated and optimized C# 7 version. I didn’t want to remove the VB.NET version so I just posted it in a separate answer. Go to updated version Seems it’s not supported, I implemented by myself, FYI, hope it to be helpful: I updated the VB version and from now on it … Read more
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.