Most efficient way to parse a flagged enum to a list

If you genuinely just want the strings, can't get much simpler than:

string[] flags = role.ToString().Split(',');

This is simpler than using LINQ and is still just a single line of code. Or if you want a list instead of an array as in the sample in the question you can convert the array into a list:

List<string> flags = new List<string>(role.ToString().Split(','));

In my case I needed a generic solution and came up with this:

value.ToString().Split(',').Select(flag => (T)Enum.Parse(typeof(T), flag)).ToList();

Enum.Parse will handle the concatenated values outputted by ToString just fine. Proof using the Immediate window:

? System.Enum.Parse(typeof(System.AttributeTargets), "Class, Enum")
Class | Enum

(the second line is the output, which is different in the debugger/immediate window from the generic Enum.ToString() output).

Try this:

public void SetRoles(Enums.Roles role)
{
  List<string> result = new List<string>();
  foreach(Roles r in Enum.GetValues(typeof(Roles)))
  {
    if ((role & r) != 0) result.Add(r.ToString());
  }
}

List<string> GetRoleNames(Roles roles) =>
    Enum.GetValues(typeof(Roles))
        .Cast<Roles>()
        .Where(role => roles.HasFlag(role))
        .Select(role => role.ToString())
        .ToList();

void TestRoleSelection()
{
    var selectedRoles = (Roles)6;
    var roleNames = GetRoleNames(selectedRoles);
    Console.WriteLine(string.Join(",", roleNames));
    // Output: Admin,User
}

[Flags]
enum Roles
{
    SuperAdmin = 1,
    Admin = 2,
    User = 4,
    Anonymous = 8
}