why would a ? appear in IEnumerable<int>?.ToList()? [duplicate]

Question

I was reviewing some code and I came across the following line of code:

List authorIntList = authorIds?.ToList();

In the example above, authorIds is an IEnumerable. What is the purpose of the ? in the line of code above? I'm not sure I've ever seen this pattern before. What does it do and in which version of .NET was it implemented?

Answer 1

That's called the "null conditional operator" -- i.e. ? and . together -- new to C# 6.0. What it means is that, if authorIds is not null, then it will call/return ToList() on it. Otherwise, it will return null. It's basically syntactic sugar so you don't have to write lengthier code like List AuthorIntList = authorIds != null ? authorIds.ToList() : null;.