Why can I use initializer syntax with readonly properties

Question

I have a Manager class with two properties as below:

public class Manager()
{
  private string _name;
  private List<int> _reportingEmployeesIds;
  public string Name { get { return _name; }}
  public List<int> ReportingEmployeesIds { get {return _reportingEmployeesIds; } }  

I am trying to create an instance of the Manager class as follows

Manager m = new Manager 
{  
   Name = "Dave", // error, expected
   ReportingEmployeesIds = {2345, 432, 521} // no compile error - why?
};

The set property is missing from both the properties but the compiler allows setting ReportingEmployeesIds whereas does not allow setting the Name property (Error: Property or indexer Manager.Name can not be assigned to, it is readonly).

Why is this the case? Why doesn't the compiler complain about the ReportingEmployeesIds being readonly.

Answer 1

The ReportingEmployeesIds = {2345, 432, 521} doesn't set the property. It is shorthand for calling Add(...) with each of the items. You can always Add, even for a readonly list property.

For it to be a set it would need to be:

ReportingEmployeesIds = new List<int> {2345, 432, 521}

Instead, the line:

Manager m = new Manager {Name = "Dave", ReportingEmployeesIds = {2345, 432, 521} }

is essentially:

var m = new Manager();
m.Name = "Dave";
var tmp = m.ReportingEmployeesIds;
tmp.Add(2345);
tmp.Add(432);
tmp.Add(521);