将其投射到DateTime?，这将导致DefaultIfEmpty创建一个默认集合，如果该集合为空，则该集合将包含空值.

Cast it to DateTime? that will cause DefaultIfEmpty creates a default collection that contains null value if the collection is empty.

CreatedDate = ctaMatch
    .Select(d => (DateTime?)d.CreatedDate)
    .DefaultIfEmpty()
    .FirstOrDefault()

PS:可以省略DefaultIfEmpty，因为它后面是FirstOrDefault.

PS: The DefaultIfEmpty can be omitted, because it's followed by FirstOrDefault.