Should this immutable struct be a mutable class?
c#, class, immutability, mutable, struct
Solution
A program that manipulates a phone number is a model of a process.
Therefore, make things which are immutable in the process immutable in the code. Make things which are mutable in the process mutable in the code.
For example, a process probably includes a person. A person has a name. A person can change their name while retaining their identity. Therefore, the name of a person object should be mutable.
A person has a phone number. A person can change their phone number while retaining their identity. Therefore, the phone number of a person should be mutable.
A phone number has an area code. A phone number CANNOT change its area code and retain its identity; you change the area code, you now have a different phone number. Therefore the area code of a phone number should be immutable.
Problem
I showed this struct to a fellow programmer and they felt that it should be a mutable class. They felt it is inconvenient not to have null references and the ability to alter the object as required. I would really like to know if there are any other reasons to make this a mutable class. ``` [Serializable] public struct PhoneNumber : IEquatable<PhoneNumber> { private const int AreaCodeShift = 54; private const int CentralOfficeCodeShift = 44; private const int SubscriberNumberShift = 30; private const int CentralOfficeCodeMask = 0x000003FF; private const int SubscriberNumberMask = 0x00003FFF; private const int ExtensionMask = 0x3FFFFFFF; private readonly ulong value; public int AreaCode { get { return UnmaskAreaCode(value); } } public int CentralOfficeCode { get { return UnmaskCentralOfficeCode(value); } } public int SubscriberNumber { get { return UnmaskSubscriberNumber(value); } } public int Extension { get { return UnmaskExtension(value); } } public PhoneNumber(ulong value) : this(UnmaskAreaCode(value), UnmaskCentralOfficeCode(value), UnmaskSubscriberNumber(value), UnmaskExtension(value), true) { } public PhoneNumber(int areaCode, int centralOfficeCode, int subscriberNumber) : this(areaCode, centralOfficeCode, subscriberNumber, 0, true) { } public PhoneNumber(int areaCode, int centralOfficeCode, int subscriberNumber, int extension) : this(areaCode, centralOfficeCode, subscriberNumber, extension, true) { } private PhoneNumber(int areaCode, int centralOfficeCode, int subscriberNumber, int extension, bool throwException) { value = 0; if (areaCode < 200 || areaCode > 989) { if (!throwException) return; throw new ArgumentOutOfRangeException("areaCode", areaCode, @"The area code portion must fall between 200 and 989."); } else if (centralOfficeCode < 200 || centralOfficeCode > 999) { if (!throwException) return; throw new ArgumentOutOfRangeException("centralOfficeCode", centralOfficeCode, @"The central office code portion must fall between 200 and 999."); } else if (subscriberNumber < 0 || subscriberNumber > 9999) { if (!throwException) return; throw new ArgumentOutOfRangeException("subscriberNumber", subscriberNumber, @"The subscriber number portion must fall between 0 and 9999."); } else if (extension < 0 || extension > 1073741824) { if (!throwException) return; throw new ArgumentOutOfRangeException("extension", extension, @"The extension portion must fall between 0 and 1073741824."); } else if (areaCode.ToString()[1] == '9') { if (!throwException) return; throw new ArgumentOutOfRangeException("areaCode", areaCode, @"The second digit of the area code cannot be greater than 8."); } else { value |= ((ulong)(uint)areaCode << AreaCodeShift); value |= ((ulong)(uint)centralOfficeCode << CentralOfficeCodeShift); value |= ((ulong)(uint)subscriberNumber << SubscriberNumberShift); value |= ((ulong)(uint)extension); } } public override bool Equals(object obj) { return obj != null && obj.GetType() == typeof(PhoneNumber) && Equals((PhoneNumber)obj); } public bool Equals(PhoneNumber other) { return this.value == other.value; } public override int GetHashCode() { return value.GetHashCode(); } public override string ToString() { return ToString(PhoneNumberFormat.Separated); } public string ToString(PhoneNumberFormat format) { switch (format) { case PhoneNumberFormat.Plain: return string.Format(@"{0:D3}{1:D3}{2:D4}{3:#}", AreaCode, CentralOfficeCode, SubscriberNumber, Extension).Trim(); case PhoneNumberFormat.Separated: return string.Format(@"{0:D3}-{1:D3}-{2:D4} {3:#}", AreaCode, CentralOfficeCode, SubscriberNumber, Extension).Trim(); default: throw new ArgumentOutOfRangeException("format"); } } public ulong ToUInt64() { return value; } public static PhoneNumber Parse(string value) { var result = default(PhoneNumber); if (!TryParse(value, out result)) { throw new FormatException(string.Format(@"The string ""{0}"" could not be parsed as a phone number.", value)); } return result; } public static bool TryParse(string value, out PhoneNumber result) { result = default(PhoneNumber); if (string.IsNullOrEmpty(value)) { return false; } var index = 0; var numericPieces = new char[value.Length]; foreach (var c in value) { if (char.IsNumber(c)) { numericPieces[index++] = c; } } if (index < 9) { return false; } var numericString = new string(numericPieces); var areaCode = int.Parse(numericString.Substring(0, 3)); var centralOfficeCode = int.Parse(numericString.Substring(3, 3)); var subscriberNumber = int.Parse(numericString.Substring(6, 4)); var extension = 0; if (numericString.Length > 10) { extension = int.Parse(numericString.Substring(10)); } result = new PhoneNumber( areaCode, centralOfficeCode, subscriberNumber, extension, false ); return result.value != 0; } public static bool operator ==(PhoneNumber left, PhoneNumber right) { return left.Equals(right); } public static bool operator !=(PhoneNumber left, PhoneNumber right) { return !left.Equals(right); } private static int UnmaskAreaCode(ulong value) { return (int)(value >> AreaCodeShift); } private static int UnmaskCentralOfficeCode(ulong value) { return (int)((value >> CentralOfficeCodeShift) & CentralOfficeCodeMask); } private static int UnmaskSubscriberNumber(ulong value) { return (int)((value >> SubscriberNumberShift) & SubscriberNumberMask); } private static int UnmaskExtension(ulong value) { return (int)(value & ExtensionMask); } } public enum PhoneNumberFormat { Plain, Separated } ```