I was full of doubt about local functions at first. They break some dogmas for me. For example, return from the method isn't the last statement anymore.

Then I started to use them. Now I can say that this idea makes code more readable and keep classes more coherent.

Before introducing local functions in C# 7, extracting part of the code required a private method. Of course, one can say that it's still hidden from the outside, but it always felt awkward.

Take this code:

class PersonCreator {
  public Person CreatePerson(string login, string email)
  {
    if (!string.IsNullOrWhiteSpace(login) && EmailIsValid(email))
        return new Person(login, email);

    return null;
  }

  private bool EmailIsValid(string email) =>
    Regex.IsMatch(email,
        @"^[^@\s]+@[^@\s]+\.[^@\s]+$");
}

public record Person(string Login, string Email);

This approach isn't flawed. But like EmailIsValid method shouldn't be part of the PersonCreator class, even a private one.

How can we refactor this to a local function?

Our method could look like this:

Person CreatePerson(string login, string email)
{
  if (!string.IsNullOrWhiteSpace(login) && EmailIsValid())
    return new Person(login, email);

  return null;

  bool EmailIsValid() =>
    Regex.IsMatch(email,
        @"^[^@\s]+@[^@\s]+\.[^@\s]+$");
}

We can skip a parameter because of closure.

In the end, it will get compiled to the private method anyway. But I found this code cleaner and easier to maintain.

What's your approach to local functions?