Can we define implicit conversions of enums in c#?
There is a solution. Consider the following:
public sealed class AccountStatus
{
public static readonly AccountStatus Open = new AccountStatus(1);
public static readonly AccountStatus Closed = new AccountStatus(2);
public static readonly SortedList<byte, AccountStatus> Values = new SortedList<byte, AccountStatus>();
private readonly byte Value;
private AccountStatus(byte value)
{
this.Value = value;
Values.Add(value, this);
}
public static implicit operator AccountStatus(byte value)
{
return Values[value];
}
public static implicit operator byte(AccountStatus value)
{
return value.Value;
}
}
The above offers implicit conversion:
AccountStatus openedAccount = 1; // Works
byte openedValue = AccountStatus.Open; // Works
This is a fair bit more work than declaring a normal enum (though you can refactor some of the above into a common generic base class). You can go even further by having the base class implement IComparable & IEquatable, as well as adding methods to return the value of DescriptionAttributes, declared names, etc, etc.
I wrote a base class (RichEnum<>) to handle most fo the grunt work, which eases the above declaration of enums down to:
public sealed class AccountStatus : RichEnum<byte, AccountStatus>
{
public static readonly AccountStatus Open = new AccountStatus(1);
public static readonly AccountStatus Closed = new AccountStatus(2);
private AccountStatus(byte value) : base (value)
{
}
public static implicit operator AccountStatus(byte value)
{
return Convert(value);
}
}
The base class (RichEnum) is listed below.
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Resources;
namespace Ethica
{
using Reflection;
using Text;
[DebuggerDisplay("{Value} ({Name})")]
public abstract class RichEnum<TValue, TDerived>
: IEquatable<TDerived>,
IComparable<TDerived>,
IComparable, IComparer<TDerived>
where TValue : struct , IComparable<TValue>, IEquatable<TValue>
where TDerived : RichEnum<TValue, TDerived>
{
#region Backing Fields
/// <summary>
/// The value of the enum item
/// </summary>
public readonly TValue Value;
/// <summary>
/// The public field name, determined from reflection
/// </summary>
private string _name;
/// <summary>
/// The DescriptionAttribute, if any, linked to the declaring field
/// </summary>
private DescriptionAttribute _descriptionAttribute;
/// <summary>
/// Reverse lookup to convert values back to local instances
/// </summary>
private static SortedList<TValue, TDerived> _values;
private static bool _isInitialized;
#endregion
#region Constructors
protected RichEnum(TValue value)
{
if (_values == null)
_values = new SortedList<TValue, TDerived>();
this.Value = value;
_values.Add(value, (TDerived)this);
}
#endregion
#region Properties
public string Name
{
get
{
CheckInitialized();
return _name;
}
}
public string Description
{
get
{
CheckInitialized();
if (_descriptionAttribute != null)
return _descriptionAttribute.Description;
return _name;
}
}
#endregion
#region Initialization
private static void CheckInitialized()
{
if (!_isInitialized)
{
ResourceManager _resources = new ResourceManager(typeof(TDerived).Name, typeof(TDerived).Assembly);
var fields = typeof(TDerived)
.GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
.Where(t => t.FieldType == typeof(TDerived));
foreach (var field in fields)
{
TDerived instance = (TDerived)field.GetValue(null);
instance._name = field.Name;
instance._descriptionAttribute = field.GetAttribute<DescriptionAttribute>();
var displayName = field.Name.ToPhrase();
}
_isInitialized = true;
}
}
#endregion
#region Conversion and Equality
public static TDerived Convert(TValue value)
{
return _values[value];
}
public static bool TryConvert(TValue value, out TDerived result)
{
return _values.TryGetValue(value, out result);
}
public static implicit operator TValue(RichEnum<TValue, TDerived> value)
{
return value.Value;
}
public static implicit operator RichEnum<TValue, TDerived>(TValue value)
{
return _values[value];
}
public static implicit operator TDerived(RichEnum<TValue, TDerived> value)
{
return value;
}
public override string ToString()
{
return _name;
}
#endregion
#region IEquatable<TDerived> Members
public override bool Equals(object obj)
{
if (obj != null)
{
if (obj is TValue)
return Value.Equals((TValue)obj);
if (obj is TDerived)
return Value.Equals(((TDerived)obj).Value);
}
return false;
}
bool IEquatable<TDerived>.Equals(TDerived other)
{
return Value.Equals(other.Value);
}
public override int GetHashCode()
{
return Value.GetHashCode();
}
#endregion
#region IComparable Members
int IComparable<TDerived>.CompareTo(TDerived other)
{
return Value.CompareTo(other.Value);
}
int IComparable.CompareTo(object obj)
{
if (obj != null)
{
if (obj is TValue)
return Value.CompareTo((TValue)obj);
if (obj is TDerived)
return Value.CompareTo(((TDerived)obj).Value);
}
return -1;
}
int IComparer<TDerived>.Compare(TDerived x, TDerived y)
{
return (x == null) ? -1 :
(y == null) ? 1 :
x.Value.CompareTo(y.Value);
}
#endregion
public static IEnumerable<TDerived> Values
{
get
{
return _values.Values;
}
}
public static TDerived Parse(string name)
{
foreach (TDerived value in _values.Values)
if (0 == string.Compare(value.Name, name, true) || 0 == string.Compare(value.DisplayName, name, true))
return value;
return null;
}
}
}
You can't do implict conversions (except for zero), and you can't write your own instance methods - however, you can probably write your own extension methods:
public enum MyEnum { A, B, C }
public static class MyEnumExt
{
public static int Value(this MyEnum foo) { return (int)foo; }
static void Main()
{
MyEnum val = MyEnum.A;
int i = val.Value();
}
}
This doesn't give you a lot, though (compared to just doing an explicit cast).
One of the main times I've seen people want this is for doing [Flags] manipulation via generics - i.e. a bool IsFlagSet<T>(T value, T flag); method. Unfortunately, C# 3.0 doesn't support operators on generics, but you can get around this using things like this, which make operators fully available with generics.
struct PseudoEnum
{
public const int
INPT = 0,
CTXT = 1,
OUTP = 2;
};
// ...
var arr = new String[3];
arr[PseudoEnum.CTXT] = "can";
arr[PseudoEnum.INPT] = "use";
arr[PseudoEnum.CTXT] = "as";
arr[PseudoEnum.CTXT] = "array";
arr[PseudoEnum.OUTP] = "index";