Null-conditional with-expressions

[mrwensveen]

I would like to be able to use the with expression on nullable record types, as if calling an instance method on that type. For example, suppose I have a class Book and a record Person:

class Book
{
  string Name { get; set; }
  Person? Author { get; set; }
}

record Person(string FirstName, string LastName);

I might want to set the Author's FirstName, but only when it's not null:

book.Author = book.Author is null ? null : book.Author with { FirstName = "John" };

Or I might want to copy (clone) the author from another Book instance:

void CopyAuthor(Book other)
{
  Author = other.Author is null ? null : other.Author with { };
}

Under the hood, both use a generated <Clone>$() method, so technically null-conditional calls could be done as (for the second example):

Author = other.Author?.'<Clone>$'();

This is specifically prohibited because of the generated method's name.

My proposal is to allow null-conditional with-expressions:

Author = other.Author? with { FirstName = "John" };

setting Author to null when other.Author is null, or applying the with expression (clone-and-set) when it isn't null.