Collection Collection type When you define only Get, also through special writing assignment

Type definition:

Attribute definitions:

use:

Question: Clearly define the Get, but it can be assigned by a special way? ? ?

It may relate to the underlying principles

Code:

Quote:
using System.Net.Mail;

Create an object:
new MailMessage { To = { string }, Subject = string, Body = string, IsBodyHtml = bool };

PS: If the great God to know, hoping to explain it in the comments or to related links, thank
PPS: Posted on 1908211822