Make DateTimeOffset a readonly struct

[benaadams]

16 byte struct; currently problematic to use with in

Also when DateTimeOffset is used as a readonly field

System\DateTimeOffset.cs(47):        public static readonly DateTimeOffset MinValue = new DateTimeOffset(DateTime.MinTicks, TimeSpan.Zero);
System\DateTimeOffset.cs(48):        public static readonly DateTimeOffset MaxValue = new DateTimeOffset(DateTime.MaxTicks, TimeSpan.Zero);
System\DateTimeOffset.cs(49):        public static readonly DateTimeOffset UnixEpoch = new DateTimeOffset(DateTime.UnixEpochTicks, TimeSpan.Zero);

/cc @jkotas @stephentoub

[benaadams]

Throws if

new TimeSpan(0, _offsetMinutes, 0).Ticks % TimeSpan.TicksPerMinute != 0

and returns

new TimeSpan(0, _offsetMinutes, 0).Ticks / TimeSpan.TicksPerMinute

So if the _offsetMinutes is valid it doesn't change it, and if invalid it throws (are other checks); so the assignment is redundant (also prevents it being readonly)

[benaadams]

Offset => new TimeSpan(0, _offsetMinutes, 0);
ClockDateTime => new DateTime((_dateTime + Offset).Ticks, DateTimeKind.Unspecified)

long utcTicks = ClockDateTime.Ticks - Offset.Ticks;
return  new DateTime(utcTicks, DateTimeKind.Unspecified);

Again if the validation passes, it will just return the original value; so the assignment is redundant (also prevents it being readonly)

[jkotas]

Thanks

[jkotas]

(We will also need matching change in reference assemblies in CoreFX.)

[benaadams]

(We will also need matching change in reference assemblies in CoreFX.)

On it