Develop Reference

Implicit conversion with a generic type failing for an interface type

private struct Maybe<T>
{
private readonly T value;
private readonly bool hasValue;
private Maybe(T value)
{
this.value = value;
hasValue = true;
}
public static implicit operator Maybe<T>(T value) =>
value == null ? new Maybe<T>() : new Maybe<T>(value);
}
private static Maybe<byte> OK()
{
return 5;
}
private static Maybe<IEnumerable<byte>> NotOK()
{
var e = new[] { 1, 2, 3 }.Select(x => (byte)x);
Console.WriteLine(e.GetType().Name);
return e;
}
Fiddle (don't use): https://dotnetfiddle.net/NxAw9l
Updated fiddle: https://dotnetfiddle.net/NrARTl
Some generic type is failing for implicit conversion at above code. See the Ok() and NotOk() function calls and return types. A complex generic type is failing and I dont' understand why. I have simplified this from a function of a return type of IEnumerable<IEnumerable<T>>. This IEnumerable<T> still fails. I think If I can understand why this fails, I'd solve the real one too I suppose. Thanks for your help and time.
Here is the error message if you'd like:
Error CS0029 Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<byte>' to 'Maybe<System.Collections.Generic.IEnumerable<byte>>'
Update: Returning Byte[] from the NotOK() can't work because in my real source code I have a LINQ query I have to depend on its lazy-deferred execution (i.e It has to be strictly returning IEnumerable) (see alike answer => https://stackoverflow.com/a/63880804/5917087).

The C# standard currently does not allow implicit conversions from or to interfaces.
This is a well-known problem when implementing a Maybe<T> (or Optional<T>, as it is often called) type in C#. There is an ongoing discussion about this on the C# language github forum:
https://github.com/dotnet/roslyn/issues/14186
As a workaround, you could make the Maybe<T> constructor internal and add a static non-generic helper class:
private static class Maybe
{
public static Maybe<T> From<T>(T value) =>
value == null ? new Maybe<T>() : new Maybe<T>(value);
}
which allows you to use type inference and write Maybe.From(a), which is a bit shorter than new Maybe<IEnumerable<byte>>(a).

I'm going to extend #Heinzi's answer:
You could also use extension methods:
static class MaybeExtensions
{
public static Maybe<T> AsMaybe<T>(this T value)
{
return new Maybe<T>(value);
}
public static Maybe<TResult> AsMaybe<T, TResult>(this T value)
where T : unmanaged
where TResult : unmanaged
{
return new Maybe<TResult>(Unsafe.As<T, TResult>(ref value));
}
}
And in your caller methods, you could use them like:
private static Maybe<IEnumerable<byte>> NotOK()
{
var e = new[] { 1, 2, 3 }.Select(x => (byte)x);
return e.AsMaybe();
}
private static Maybe<byte> OK()
{
return 5.AsMaybe<int, byte>();
}
// Alternatively
private static Maybe<byte> OK()
{
return ((byte)5).AsMaybe();
}
You need the AsMaybe<T, TResult> overload for value types that can convert to each other. For example when you do 5.AsMaybe() it returns Maybe<int>, if your method's return type is Maybe<byte> you will need to convert Maybe<int> to Maybe<byte>, and the overload does that for you.
Now, the type conversion operator in Maybe<T> becomes redundant. And you can use var instead of full type name:
Maybe<int> obj1 = 5; // use operator
var obj2 = 5.AsMaybe(); // use extension method

You can't define a conversion to/from an interface type, If you change your example to use List<T> rather than IEnumerable<T> it will compile - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/conversions#user-defined-conversions

Change this :
private static Maybe<IEnumerable<byte>> NotOK()
{
IEnumerable<byte> a = new byte[] { 1, 2 };
return a;
}
into this :
private static Maybe<IEnumerable<byte>> NotOK()
{
var a = new byte[] { 1, 2 };
return a;
}
The struct :
private struct Maybe<T>
{
private readonly T value;
private readonly bool hasValue;
private Maybe(T value)
{
this.value = value;
hasValue = true;
}
public static implicit operator Maybe<T>(T value)
{
return value == null ? new Maybe<T>() : new Maybe<T>(value);
}
public bool HasValue(){
return this.hasValue;
}
public T GetValue(){
return this.value;
}
}
private static Maybe<byte> OK()
{
return 5;
}
private static Maybe<IEnumerable<byte>> NotOK()
{
Byte[] a = new byte[] { 1, 2 };
Console.WriteLine(a.GetType().Name);
return a;
}
Usage :
public static void Main(string[] args){
var t1 = OK();
var t2 = NotOK();
Console.WriteLine("t1 type is " + t1.GetType().Name);
Console.WriteLine("t2 type is " + t2.GetType().Name);
if(t2.HasValue())
{
List<byte> search = t2.GetValue().Where(b => b > 0).ToList();
foreach(byte num in search){
Console.WriteLine(num);
}
}
}
The reference IEnumerable<byte> a doesn't change the type, you can continue to var or byte[] and the query with LINQ, after, see in the full example
See the full example :
https://dotnetfiddle.net/V8RHQe

IEnumerable is an interface. Compiler does not know which type to work with. Put ToList() to end of your select like follows :
private static Maybe<IEnumerable<byte>> NotOK()
{
var e = new[] { 1, 2, 3 }.Select(x => (byte)x).ToList();
Console.WriteLine(e.GetType().Name);
return e;
}
To understand what is happening try to create a method like follow in your class and watch compiler to cry :)
public static implicit operator Maybe<IEnumerable<T>>(IEnumerable<T> value)
{
return value == null ? new Maybe<IEnumerable<T>>() : new Maybe<IEnumerable<T>>(value);
}

Generic class for enumerations - casting issue

I want to write a generic class that accepts enumerations. Since this class is intended to implement some interfaces, the main aim is to be able to treat enumerations as other objects implementing those interfaces(e.g. for list extensions, etc). Hence, for a sample enum
public enum QEnum : int
{
xlNoValue = 0,
xlSomeValue = 1
}
public static class QEnumExtensions
{
public static string toString(this QEnum xThis)
{
...
}
public static QEnum toEnum(this string xThis)
{
...
}
}
I would like to declare a generic class such as
public class QEnumHolder<T> where T : struct, IConvertible
{
private T mxVal = default(T);
public QEnumHolder()
{
if (!typeof(T).IsEnum) throw new NotSupportedException();
}
public QEnumHolder(T xVal)
{
if (!typeof(T).IsEnum) throw new NotSupportedException();
mxVal = xVal;
}
static public implicit operator QEnumHolder<T>(T xVal)
{
return new QEnumHolder<T>(xVal);
}
static public implicit operator T(QEnumHolder<T> xVal)
{
return (T)xVal.mxVal;
}
public string toString()
{
if (mxVal is QEnum) return ((QEnum)Convert.ToInt32(mxVal)).toString();
...
}
public void fromString(string xString)
{
if (mxVal is QEnum)
mxVal = (???)xString.toEnum(); // problem
}
}
All of the enumerations that we use implement
toString() function which returns a "nice" string that can go into comboBoxes, etc
conversion of string to enumeration, as above
hence the structure of toString/toEnum is pretty much given. The problem is with the last code line marked "problem". I have no idea how to tell the compiler that in this branch, the return type of toEnum() and T will be the same.
I tried to circumvent the problem by declaring mxVal as int and using Convert.ToInt32 everywhere. However, then I run into problem in the operator T where the compiler has objections against converting int to a T (the compiler can't know that T will be enum, hence I can't use none of the "int to enum conversion" discussions here on SO).

A better design would be to use some naming convention, put all your enum extension methods in one and the same static class, and bind these functions inside your holder class static constructor. Something like this:
public static partial class MyEnumExtensions
{
public static MyEnumHolder<T> ToHolder<T>(this T source)
where T : struct, IConvertible
{
return new MyEnumHolder<T>(source);
}
}
public class MyEnumHolder<T> where T : struct, IConvertible
{
static readonly Func<T, string> toStringFunc;
static readonly Func<string, T> toEnumFunc;
static MyEnumHolder()
{
if (!typeof(T).IsEnum) throw new NotSupportedException();
// Use your naming conventions
var name = typeof(T).Name;
toStringFunc = (Func<T, string>)Delegate.CreateDelegate(typeof(Func<T, string>),
typeof(MyEnumExtensions).GetMethod("toString", new[] { typeof(T) }));
toEnumFunc = (Func<string, T>)Delegate.CreateDelegate(typeof(Func<string, T>),
typeof(MyEnumExtensions).GetMethod("to" + name, new[] { typeof(string) }));
}
private T value;
public MyEnumHolder() { value = default(T); }
public MyEnumHolder(T value) { this.value = value; }
static public implicit operator MyEnumHolder<T>(T x) { return new MyEnumHolder<T>(x); }
static public implicit operator T(MyEnumHolder<T> x) { return x.value; }
public string toString()
{
return toStringFunc(value);
}
public void fromString(string xString)
{
value = toEnumFunc(xString);
}
}
Sample enum definitions (could be in separate files, but must be inside the same project):
public enum MyEnumA { A1, A2, A3 }
partial class MyEnumExtensions
{
public static string toString(this MyEnumA x)
{
//...
return x.ToString();
}
public static MyEnumA toMyEnumA(this string x)
{
//...
return (MyEnumA)Enum.Parse(typeof(MyEnumA), x);
}
}
and
public enum MyEnumB { B1, B2, B3 }
partial class MyEnumExtensions
{
public static string toString(this MyEnumB x)
{
//...
return x.ToString();
}
public static MyEnumB toMyEnumB(this string x)
{
//...
return (MyEnumB)Enum.Parse(typeof(MyEnumB), x);
}
}
test:
var a = MyEnumA.A1.ToHolder();
var sA = a.toString();
a.fromString("A2");
var b = MyEnumB.B2.ToHolder();
var sB = b.toString();
b.fromString("B1");

mxVal = (T)(object)xString.toEnum();

Discriminated union in C#

[Note: This question had the original title "C (ish) style union in C#"
but as Jeff's comment informed me, apparently this structure is called a 'discriminated union']
Excuse the verbosity of this question.
There are a couple of similar sounding questions to mine already in SO but they seem to concentrate on the memory saving benefits of the union or using it for interop.
Here is an example of such a question.
My desire to have a union type thing is somewhat different.
I am writing some code at the moment which generates objects that look a bit like this
public class ValueWrapper
{
public DateTime ValueCreationDate;
// ... other meta data about the value
public object ValueA;
public object ValueB;
}
Pretty complicated stuff I think you will agree. The thing is that ValueA can only be of a few certain types (let's say string, int and Foo (which is a class) and ValueB can be another small set of types. I don't like treating these values as objects (I want the warm snugly feeling of coding with a bit of type safety).
So I thought about writing a trivial little wrapper class to express the fact that ValueA logically is a reference to a particular type. I called the class Union because what I am trying to achieve reminded me of the union concept in C.
public class Union<A, B, C>
{
private readonly Type type;
public readonly A a;
public readonly B b;
public readonly C c;
public A A{get {return a;}}
public B B{get {return b;}}
public C C{get {return c;}}
public Union(A a)
{
type = typeof(A);
this.a = a;
}
public Union(B b)
{
type = typeof(B);
this.b = b;
}
public Union(C c)
{
type = typeof(C);
this.c = c;
}
/// <summary>
/// Returns true if the union contains a value of type T
/// </summary>
/// <remarks>The type of T must exactly match the type</remarks>
public bool Is<T>()
{
return typeof(T) == type;
}
/// <summary>
/// Returns the union value cast to the given type.
/// </summary>
/// <remarks>If the type of T does not exactly match either X or Y, then the value <c>default(T)</c> is returned.</remarks>
public T As<T>()
{
if(Is<A>())
{
return (T)(object)a; // Is this boxing and unboxing unavoidable if I want the union to hold value types and reference types?
//return (T)x; // This will not compile: Error = "Cannot cast expression of type 'X' to 'T'."
}
if(Is<B>())
{
return (T)(object)b;
}
if(Is<C>())
{
return (T)(object)c;
}
return default(T);
}
}
Using this class ValueWrapper now looks like this
public class ValueWrapper2
{
public DateTime ValueCreationDate;
public Union<int, string, Foo> ValueA;
public Union<double, Bar, Foo> ValueB;
}
which is something like what I wanted to achieve but I am missing one fairly crucial element - that is compiler enforced type checking when calling the Is and As functions as the following code demonstrates
public void DoSomething()
{
if(ValueA.Is<string>())
{
var s = ValueA.As<string>();
// .... do somethng
}
if(ValueA.Is<char>()) // I would really like this to be a compile error
{
char c = ValueA.As<char>();
}
}
IMO It is not valid to ask ValueA if it is a char since its definition clearly says it is not - this is a programming error and I would like the compiler to pick up on this. [Also if I could get this correct then (hopefully) I would get intellisense too - which would be a boon.]
In order to achieve this I would want to tell the compiler that the type T can be one of A, B or C
public bool Is<T>() where T : A
or T : B // Yes I know this is not legal!
or T : C
{
return typeof(T) == type;
}
Does anyone have any idea if what I want to achieve is possible? Or am I just plain stupid for writing this class in the first place?

I don't really like the type-checking and type-casting solutions provided above, so here's 100% type-safe union which will throw compilation errors if you attempt to use the wrong datatype:
using System;
namespace Juliet
{
class Program
{
static void Main(string[] args)
{
Union3<int, char, string>[] unions = new Union3<int,char,string>[]
{
new Union3<int, char, string>.Case1(5),
new Union3<int, char, string>.Case2('x'),
new Union3<int, char, string>.Case3("Juliet")
};
foreach (Union3<int, char, string> union in unions)
{
string value = union.Match(
num => num.ToString(),
character => new string(new char[] { character }),
word => word);
Console.WriteLine("Matched union with value '{0}'", value);
}
Console.ReadLine();
}
}
public abstract class Union3<A, B, C>
{
public abstract T Match<T>(Func<A, T> f, Func<B, T> g, Func<C, T> h);
// private ctor ensures no external classes can inherit
private Union3() { }
public sealed class Case1 : Union3<A, B, C>
{
public readonly A Item;
public Case1(A item) : base() { this.Item = item; }
public override T Match<T>(Func<A, T> f, Func<B, T> g, Func<C, T> h)
{
return f(Item);
}
}
public sealed class Case2 : Union3<A, B, C>
{
public readonly B Item;
public Case2(B item) { this.Item = item; }
public override T Match<T>(Func<A, T> f, Func<B, T> g, Func<C, T> h)
{
return g(Item);
}
}
public sealed class Case3 : Union3<A, B, C>
{
public readonly C Item;
public Case3(C item) { this.Item = item; }
public override T Match<T>(Func<A, T> f, Func<B, T> g, Func<C, T> h)
{
return h(Item);
}
}
}
}

I like the direction of the accepted solution but it doesn't scale well for unions of more than three items (e.g. a union of 9 items would require 9 class definitions).
Here is another approach that is also 100% type-safe at compile-time, but that is easy to grow to large unions.
public class UnionBase<A>
{
dynamic value;
public UnionBase(A a) { value = a; }
protected UnionBase(object x) { value = x; }
protected T InternalMatch<T>(params Delegate[] ds)
{
var vt = value.GetType();
foreach (var d in ds)
{
var mi = d.Method;
// These are always true if InternalMatch is used correctly.
Debug.Assert(mi.GetParameters().Length == 1);
Debug.Assert(typeof(T).IsAssignableFrom(mi.ReturnType));
var pt = mi.GetParameters()[0].ParameterType;
if (pt.IsAssignableFrom(vt))
return (T)mi.Invoke(null, new object[] { value });
}
throw new Exception("No appropriate matching function was provided");
}
public T Match<T>(Func<A, T> fa) { return InternalMatch<T>(fa); }
}
public class Union<A, B> : UnionBase<A>
{
public Union(A a) : base(a) { }
public Union(B b) : base(b) { }
protected Union(object x) : base(x) { }
public T Match<T>(Func<A, T> fa, Func<B, T> fb) { return InternalMatch<T>(fa, fb); }
}
public class Union<A, B, C> : Union<A, B>
{
public Union(A a) : base(a) { }
public Union(B b) : base(b) { }
public Union(C c) : base(c) { }
protected Union(object x) : base(x) { }
public T Match<T>(Func<A, T> fa, Func<B, T> fb, Func<C, T> fc) { return InternalMatch<T>(fa, fb, fc); }
}
public class Union<A, B, C, D> : Union<A, B, C>
{
public Union(A a) : base(a) { }
public Union(B b) : base(b) { }
public Union(C c) : base(c) { }
public Union(D d) : base(d) { }
protected Union(object x) : base(x) { }
public T Match<T>(Func<A, T> fa, Func<B, T> fb, Func<C, T> fc, Func<D, T> fd) { return InternalMatch<T>(fa, fb, fc, fd); }
}
public class Union<A, B, C, D, E> : Union<A, B, C, D>
{
public Union(A a) : base(a) { }
public Union(B b) : base(b) { }
public Union(C c) : base(c) { }
public Union(D d) : base(d) { }
public Union(E e) : base(e) { }
protected Union(object x) : base(x) { }
public T Match<T>(Func<A, T> fa, Func<B, T> fb, Func<C, T> fc, Func<D, T> fd, Func<E, T> fe) { return InternalMatch<T>(fa, fb, fc, fd, fe); }
}
public class DiscriminatedUnionTest : IExample
{
public Union<int, bool, string, int[]> MakeUnion(int n)
{
return new Union<int, bool, string, int[]>(n);
}
public Union<int, bool, string, int[]> MakeUnion(bool b)
{
return new Union<int, bool, string, int[]>(b);
}
public Union<int, bool, string, int[]> MakeUnion(string s)
{
return new Union<int, bool, string, int[]>(s);
}
public Union<int, bool, string, int[]> MakeUnion(params int[] xs)
{
return new Union<int, bool, string, int[]>(xs);
}
public void Print(Union<int, bool, string, int[]> union)
{
var text = union.Match(
n => "This is an int " + n.ToString(),
b => "This is a boolean " + b.ToString(),
s => "This is a string" + s,
xs => "This is an array of ints " + String.Join(", ", xs));
Console.WriteLine(text);
}
public void Run()
{
Print(MakeUnion(1));
Print(MakeUnion(true));
Print(MakeUnion("forty-two"));
Print(MakeUnion(0, 1, 1, 2, 3, 5, 8));
}
}

I wrote some blog posts on this subject that might be useful:
Union Types in C#
Implementing Tic-Tac-Toe Using State Classes
Let's say you have a shopping cart scenario with three states: "Empty", "Active" and "Paid", each with different behavior.
You create have a ICartState interface that all states have in common (and it could just be an empty marker interface)
You create three classes that implement that interface. (The classes do not have to be in an inheritance relationship)
The interface contains a "fold" method, whereby you pass a lambda in for each state or case that you need to handle.
You could use the F# runtime from C# but as a lighter weight alternative, I have written a little T4 template for generating code like this.
Here's the interface:
partial interface ICartState
{
ICartState Transition(
Func<CartStateEmpty, ICartState> cartStateEmpty,
Func<CartStateActive, ICartState> cartStateActive,
Func<CartStatePaid, ICartState> cartStatePaid
);
}
And here's the implementation:
class CartStateEmpty : ICartState
{
ICartState ICartState.Transition(
Func<CartStateEmpty, ICartState> cartStateEmpty,
Func<CartStateActive, ICartState> cartStateActive,
Func<CartStatePaid, ICartState> cartStatePaid
)
{
// I'm the empty state, so invoke cartStateEmpty
return cartStateEmpty(this);
}
}
class CartStateActive : ICartState
{
ICartState ICartState.Transition(
Func<CartStateEmpty, ICartState> cartStateEmpty,
Func<CartStateActive, ICartState> cartStateActive,
Func<CartStatePaid, ICartState> cartStatePaid
)
{
// I'm the active state, so invoke cartStateActive
return cartStateActive(this);
}
}
class CartStatePaid : ICartState
{
ICartState ICartState.Transition(
Func<CartStateEmpty, ICartState> cartStateEmpty,
Func<CartStateActive, ICartState> cartStateActive,
Func<CartStatePaid, ICartState> cartStatePaid
)
{
// I'm the paid state, so invoke cartStatePaid
return cartStatePaid(this);
}
}
Now let's say you extend the CartStateEmpty and CartStateActive with an AddItem method which is not implemented by CartStatePaid.
And also let's say that CartStateActive has a Pay method that the other states dont have.
Then here's some code that shows it in use -- adding two items and then paying for the cart:
public ICartState AddProduct(ICartState currentState, Product product)
{
return currentState.Transition(
cartStateEmpty => cartStateEmpty.AddItem(product),
cartStateActive => cartStateActive.AddItem(product),
cartStatePaid => cartStatePaid // not allowed in this case
);
}
public void Example()
{
var currentState = new CartStateEmpty() as ICartState;
//add some products
currentState = AddProduct(currentState, Product.ProductX);
currentState = AddProduct(currentState, Product.ProductY);
//pay
const decimal paidAmount = 12.34m;
currentState = currentState.Transition(
cartStateEmpty => cartStateEmpty, // not allowed in this case
cartStateActive => cartStateActive.Pay(paidAmount),
cartStatePaid => cartStatePaid // not allowed in this case
);
}
Note that this code is completely typesafe -- no casting or conditionals anywhere, and compiler errors if you try to pay for an empty cart, say.

I have written a library for doing this at https://github.com/mcintyre321/OneOf
Install-Package OneOf
It has the generic types in it for doing DUs e.g. OneOf<T0, T1> all the way to
OneOf<T0, ..., T9>. Each of those has a .Match, and a .Switch statement which you can use for compiler safe typed behaviour, e.g.:
```
OneOf<string, ColorName, Color> backgroundColor = getBackground();
Color c = backgroundColor.Match(
str => CssHelper.GetColorFromString(str),
name => new Color(name),
col => col
);
```

I am not sure I fully understand your goal. In C, a union is a structure that uses the same memory locations for more than one field. For example:
typedef union
{
float real;
int scalar;
} floatOrScalar;
The floatOrScalar union could be used as a float, or an int, but they both consume the same memory space. Changing one changes the other. You can achieve the same thing with a struct in C#:
[StructLayout(LayoutKind.Explicit)]
struct FloatOrScalar
{
[FieldOffset(0)]
public float Real;
[FieldOffset(0)]
public int Scalar;
}
The above structure uses 32bits total, rather than 64bits. This is only possible with a struct. Your example above is a class, and given the nature of the CLR, makes no guarantee about memory efficiency. If you change a Union<A, B, C> from one type to another, you are not necessarily reusing memory...most likely, you are allocating a new type on the heap and dropping a different pointer in the backing object field. Contrary to a real union, your approach may actually cause more heap thrashing than you would otherwise get if you did not use your Union type.

char foo = 'B';
bool bar = foo is int;
This results in a warning, not an error. If you're looking for your Is and As functions to be analogs for the C# operators, then you shouldn't be restricting them in that way anyhow.

If you allow multiple types, you cannot achieve type safety (unless the types are related).
You can't and won't achieve any kind of type safety, you could only achieve byte-value-safety using FieldOffset.
It would make much more sense to have a generic ValueWrapper<T1, T2> with T1 ValueA and T2 ValueB, ...
P.S.: when talking about type-safety I mean compile-time type-safety.
If you need a code wrapper (performing bussiness logic on modifications you can use something along the lines of:
public class Wrapper
{
public ValueHolder<int> v1 = 5;
public ValueHolder<byte> v2 = 8;
}
public struct ValueHolder<T>
where T : struct
{
private T value;
public ValueHolder(T value) { this.value = value; }
public static implicit operator T(ValueHolder<T> valueHolder) { return valueHolder.value; }
public static implicit operator ValueHolder<T>(T value) { return new ValueHolder<T>(value); }
}
For an easy way out you could use (it has performance issues, but it is very simple):
public class Wrapper
{
private object v1;
private object v2;
public T GetValue1<T>() { if (v1.GetType() != typeof(T)) throw new InvalidCastException(); return (T)v1; }
public void SetValue1<T>(T value) { v1 = value; }
public T GetValue2<T>() { if (v2.GetType() != typeof(T)) throw new InvalidCastException(); return (T)v2; }
public void SetValue2<T>(T value) { v2 = value; }
}
//usage:
Wrapper wrapper = new Wrapper();
wrapper.SetValue1("aaaa");
wrapper.SetValue2(456);
string s = wrapper.GetValue1<string>();
DateTime dt = wrapper.GetValue1<DateTime>();//InvalidCastException

Here is my attempt. It does compile time checking of types, using generic type constraints.
class Union {
public interface AllowedType<T> { };
internal object val;
internal System.Type type;
}
static class UnionEx {
public static T As<U,T>(this U x) where U : Union, Union.AllowedType<T> {
return x.type == typeof(T) ?(T)x.val : default(T);
}
public static void Set<U,T>(this U x, T newval) where U : Union, Union.AllowedType<T> {
x.val = newval;
x.type = typeof(T);
}
public static bool Is<U,T>(this U x) where U : Union, Union.AllowedType<T> {
return x.type == typeof(T);
}
}
class MyType : Union, Union.AllowedType<int>, Union.AllowedType<string> {}
class TestIt
{
static void Main()
{
MyType bla = new MyType();
bla.Set(234);
System.Console.WriteLine(bla.As<MyType,int>());
System.Console.WriteLine(bla.Is<MyType,string>());
System.Console.WriteLine(bla.Is<MyType,int>());
bla.Set("test");
System.Console.WriteLine(bla.As<MyType,string>());
System.Console.WriteLine(bla.Is<MyType,string>());
System.Console.WriteLine(bla.Is<MyType,int>());
// compile time errors!
// bla.Set('a');
// bla.Is<MyType,char>()
}
}
It could use some prettying-up. Especially, I couldn't figure out how to get rid of the type parameters to As/Is/Set (isn't there a way to specify one type parameter and let C# figure the other one?)

So I've hit this same problem many times, and I just came up with a solution that gets the syntax I want (at the expense of some ugliness in the implementation of the Union type.)
To recap: we want this sort of usage at the call site.
Union<int, string> u;
u = 1492;
int yearColumbusDiscoveredAmerica = u;
u = "hello world";
string traditionalGreeting = u;
var answers = new SortedList<string, Union<int, string, DateTime>>();
answers["life, the universe, and everything"] = 42;
answers["D-Day"] = new DateTime(1944, 6, 6);
answers["C#"] = "is awesome";
We want the following examples to fail to compile, however, so that we get a modicum of type safety.
DateTime dateTimeColumbusDiscoveredAmerica = u;
Foo fooInstance = u;
For extra credit, let's also not take up more space than absolutely needed.
With all that said, here's my implementation for two generic type parameters. The implementation for three, four, and so on type parameters is straight-forward.
public abstract class Union<T1, T2>
{
public abstract int TypeSlot
{
get;
}
public virtual T1 AsT1()
{
throw new TypeAccessException(string.Format(
"Cannot treat this instance as a {0} instance.", typeof(T1).Name));
}
public virtual T2 AsT2()
{
throw new TypeAccessException(string.Format(
"Cannot treat this instance as a {0} instance.", typeof(T2).Name));
}
public static implicit operator Union<T1, T2>(T1 data)
{
return new FromT1(data);
}
public static implicit operator Union<T1, T2>(T2 data)
{
return new FromT2(data);
}
public static implicit operator Union<T1, T2>(Tuple<T1, T2> data)
{
return new FromTuple(data);
}
public static implicit operator T1(Union<T1, T2> source)
{
return source.AsT1();
}
public static implicit operator T2(Union<T1, T2> source)
{
return source.AsT2();
}
private class FromT1 : Union<T1, T2>
{
private readonly T1 data;
public FromT1(T1 data)
{
this.data = data;
}
public override int TypeSlot
{
get { return 1; }
}
public override T1 AsT1()
{
return this.data;
}
public override string ToString()
{
return this.data.ToString();
}
public override int GetHashCode()
{
return this.data.GetHashCode();
}
}
private class FromT2 : Union<T1, T2>
{
private readonly T2 data;
public FromT2(T2 data)
{
this.data = data;
}
public override int TypeSlot
{
get { return 2; }
}
public override T2 AsT2()
{
return this.data;
}
public override string ToString()
{
return this.data.ToString();
}
public override int GetHashCode()
{
return this.data.GetHashCode();
}
}
private class FromTuple : Union<T1, T2>
{
private readonly Tuple<T1, T2> data;
public FromTuple(Tuple<T1, T2> data)
{
this.data = data;
}
public override int TypeSlot
{
get { return 0; }
}
public override T1 AsT1()
{
return this.data.Item1;
}
public override T2 AsT2()
{
return this.data.Item2;
}
public override string ToString()
{
return this.data.ToString();
}
public override int GetHashCode()
{
return this.data.GetHashCode();
}
}
}

And my attempt on minimal yet extensible solution using nesting of Union/Either type.
Also usage of default parameters in Match method naturally enables "Either X Or Default" scenario.
using System;
using System.Reflection;
using NUnit.Framework;
namespace Playground
{
[TestFixture]
public class EitherTests
{
[Test]
public void Test_Either_of_Property_or_FieldInfo()
{
var some = new Some(false);
var field = some.GetType().GetField("X");
var property = some.GetType().GetProperty("Y");
Assert.NotNull(field);
Assert.NotNull(property);
var info = Either<PropertyInfo, FieldInfo>.Of(field);
var infoType = info.Match(p => p.PropertyType, f => f.FieldType);
Assert.That(infoType, Is.EqualTo(typeof(bool)));
}
[Test]
public void Either_of_three_cases_using_nesting()
{
var some = new Some(false);
var field = some.GetType().GetField("X");
var parameter = some.GetType().GetConstructors()[0].GetParameters()[0];
Assert.NotNull(field);
Assert.NotNull(parameter);
var info = Either<ParameterInfo, Either<PropertyInfo, FieldInfo>>.Of(parameter);
var name = info.Match(_ => _.Name, _ => _.Name, _ => _.Name);
Assert.That(name, Is.EqualTo("a"));
}
public class Some
{
public bool X;
public string Y { get; set; }
public Some(bool a)
{
X = a;
}
}
}
public static class Either
{
public static T Match<A, B, C, T>(
this Either<A, Either<B, C>> source,
Func<A, T> a = null, Func<B, T> b = null, Func<C, T> c = null)
{
return source.Match(a, bc => bc.Match(b, c));
}
}
public abstract class Either<A, B>
{
public static Either<A, B> Of(A a)
{
return new CaseA(a);
}
public static Either<A, B> Of(B b)
{
return new CaseB(b);
}
public abstract T Match<T>(Func<A, T> a = null, Func<B, T> b = null);
private sealed class CaseA : Either<A, B>
{
private readonly A _item;
public CaseA(A item) { _item = item; }
public override T Match<T>(Func<A, T> a = null, Func<B, T> b = null)
{
return a == null ? default(T) : a(_item);
}
}
private sealed class CaseB : Either<A, B>
{
private readonly B _item;
public CaseB(B item) { _item = item; }
public override T Match<T>(Func<A, T> a = null, Func<B, T> b = null)
{
return b == null ? default(T) : b(_item);
}
}
}
}

You could throw exceptions once there's an attempt to access variables that haven't been initialized, ie if it's created with an A parameter and later on there's an attempt to access B or C, it could throw, say, UnsupportedOperationException. You'd need a getter to make it work though.

The C# Language Design Team discussed discriminated unions in January 2017 https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-01-10.md#discriminated-unions-via-closed-types
You can vote for the feature request at https://github.com/dotnet/csharplang/issues/113

You can export a pseudo-pattern matching function, like I use for the Either type in my Sasa library. There's currently runtime overhead, but I eventually plan to add a CIL analysis to inline all the delegates into a true case statement.

It's not possible to do with exactly the syntax you've used but with a bit more verbosity and copy/paste it's easy to make overload resolution do the job for you:
// this code is ok
var u = new Union("");
if (u.Value(Is.OfType()))
{
u.Value(Get.ForType());
}
// and this one will not compile
if (u.Value(Is.OfType()))
{
u.Value(Get.ForType());
}
By now it should be pretty obvious how to implement it:
public class Union
{
private readonly Type type;
public readonly A a;
public readonly B b;
public readonly C c;
public Union(A a)
{
type = typeof(A);
this.a = a;
}
public Union(B b)
{
type = typeof(B);
this.b = b;
}
public Union(C c)
{
type = typeof(C);
this.c = c;
}
public bool Value(TypeTestSelector _)
{
return typeof(A) == type;
}
public bool Value(TypeTestSelector _)
{
return typeof(B) == type;
}
public bool Value(TypeTestSelector _)
{
return typeof(C) == type;
}
public A Value(GetValueTypeSelector _)
{
return a;
}
public B Value(GetValueTypeSelector _)
{
return b;
}
public C Value(GetValueTypeSelector _)
{
return c;
}
}
public static class Is
{
public static TypeTestSelector OfType()
{
return null;
}
}
public class TypeTestSelector
{
}
public static class Get
{
public static GetValueTypeSelector ForType()
{
return null;
}
}
public class GetValueTypeSelector
{
}
There are no checks for extracting the value of the wrong type, e.g.:
var u = Union(10);
string s = u.Value(Get.ForType());
So you might consider adding necessary checks and throw exceptions in such cases.

I am currently trying to create a Julia Runtime in .NET. Julia has types like Union{Int, String}... Etc. I am currently trying to simulate this .NET (without doing weird IL that would not be able to be called from c#).
Here is a compile time implementation of a union of structures. I will be creating more unions for object unions, and cross object and struct unions (this will be the most complex case).
public struct Union<T1,T2> where T1 : struct where T2 : struct{
private byte type;
[FieldOffset(1)] private T1 a1;
[FieldOffset(1)] private T2 a2;
public T1 A1 {
get => a1;
set {
a1 = value;
type = 1;
}
}
public T2 A2 {
get => a2;
set {
a2 = value;
type = 2;
}
}
public Union(int _ = 0) {
type = 0;
a1 = default;
a2 = default;
}
public Union(T1 a) : this() => A1 = a;
public Union(T2 a) : this() => A2 = a;
public bool HasValue => type < 1 || type > 2;
public bool IsNull => !HasValue;
public bool IsT1 => type == 1;
public bool IsT2 => type == 2;
public Type GetType() {
switch (type) {
case 1: return typeof(T1);
case 2: return typeof(T2);
default: return null;
}
}
}
You can use the above like the following:
Union<int, long> myUnion(5); \\Set int inside
myUnion.a2 = 5;
Type theTypeInside = myUnion.GetType(); //long
myUnion.a1 = 5;
theTypeInside = myUnion.GetType(); //int
I will also be creating dynamic union generators or aligned unions for the cross object and struct union.
Take a look at:Generated Struct Union Output to see the current compile time unions I am using.
If you want to create a union of any size take a look at Generator for Struct Unions
If anyone has any improvements for the above let me know! Implementing julia into .NET is an extraordinarily hard task!

I use own of Union Type.
Consider an example to make it clearer.
Imagine we have Contact class:
public class Contact
{
public string Name { get; set; }
public string EmailAddress { get; set; }
public string PostalAdrress { get; set; }
}
These are all defined as simple strings, but really are they just strings?
Of course not. The Name can consist of First Name and Last Name. Or is an Email just a set of symbols? I know that at least it should contain # and it is necessarily.
Let's improve us domain model
public class PersonalName
{
public PersonalName(string firstName, string lastName) { ... }
public string Name() { return _fistName + " " _lastName; }
}
public class EmailAddress
{
public EmailAddress(string email) { ... }
}
public class PostalAdrress
{
public PostalAdrress(string address, string city, int zip) { ... }
}
In this classes will be validations during creating and we will eventually have valid models. Consturctor in PersonaName class require FirstName and LastName at the same time. This means that after the creation, it can not have invalid state.
And contact class respectively
public class Contact
{
public PersonalName Name { get; set; }
public EmailAdress EmailAddress { get; set; }
public PostalAddress PostalAddress { get; set; }
}
In this case we have same problem, object of Contact class may be in invalid state. I mean it may have EmailAddress but haven't Name
var contact = new Contact { EmailAddress = new EmailAddress("foo#bar.com") };
Let's fix it and create Contact class with constructor which requires PersonalName, EmailAddress and PostalAddress:
public class Contact
{
public Contact(
PersonalName personalName,
EmailAddress emailAddress,
PostalAddress postalAddress
)
{
...
}
}
But here we have another problem. What if Person have only EmailAdress and haven't PostalAddress?
If we think about it there we realize that there are three possibilities of valid state of Contact class object:
A contact only has an email address
A contact only has a postal address
A contact has both an email address and a postal address
Let's write out domain models. For the beginning we will create Contact Info class which state will be corresponding with above cases.
public class ContactInfo
{
public ContactInfo(EmailAddress emailAddress) { ... }
public ContactInfo(PostalAddress postalAddress) { ... }
public ContactInfo(Tuple<EmailAddress,PostalAddress> emailAndPostalAddress) { ... }
}
And Contact class:
public class Contact
{
public Contact(
PersonalName personalName,
ContactInfo contactInfo
)
{
...
}
}
Let's try use it:
var contact = new Contact(
new PersonalName("James", "Bond"),
new ContactInfo(
new EmailAddress("agent#007.com")
)
);
Console.WriteLine(contact.PersonalName()); // James Bond
Console.WriteLine(contact.ContactInfo().???) // here we have problem, because ContactInfo have three possible state and if we want print it we would write `if` cases
Let's add Match method in ContactInfo class
public class ContactInfo
{
// constructor
public TResult Match<TResult>(
Func<EmailAddress,TResult> f1,
Func<PostalAddress,TResult> f2,
Func<Tuple<EmailAddress,PostalAddress>> f3
)
{
if (_emailAddress != null)
{
return f1(_emailAddress);
}
else if(_postalAddress != null)
{
...
}
...
}
}
In the match method, we can write this code, because the state of the contact class is controlled with constructors and it may have only one of the possible states.
Let's create an auxiliary class, so that each time do not write as many code.
public abstract class Union<T1,T2,T3>
where T1 : class
where T2 : class
where T3 : class
{
private readonly T1 _t1;
private readonly T2 _t2;
private readonly T3 _t3;
public Union(T1 t1) { _t1 = t1; }
public Union(T2 t2) { _t2 = t2; }
public Union(T3 t3) { _t3 = t3; }
public TResult Match<TResult>(
Func<T1, TResult> f1,
Func<T2, TResult> f2,
Func<T3, TResult> f3
)
{
if (_t1 != null)
{
return f1(_t1);
}
else if (_t2 != null)
{
return f2(_t2);
}
else if (_t3 != null)
{
return f3(_t3);
}
throw new Exception("can't match");
}
}
We can have such a class in advance for several types, as is done with delegates Func, Action. 4-6 generic type parameters will be in full for Union class.
Let's rewrite ContactInfo class:
public sealed class ContactInfo : Union<
EmailAddress,
PostalAddress,
Tuple<EmaiAddress,PostalAddress>
>
{
public Contact(EmailAddress emailAddress) : base(emailAddress) { }
public Contact(PostalAddress postalAddress) : base(postalAddress) { }
public Contact(Tuple<EmaiAddress, PostalAddress> emailAndPostalAddress) : base(emailAndPostalAddress) { }
}
Here the compiler will ask override for at least one constructor. If we forget to override the rest of the constructors we can't create object of ContactInfo class with another state. This will protect us from runtime exceptions during Matching.
var contact = new Contact(
new PersonalName("James", "Bond"),
new ContactInfo(
new EmailAddress("agent#007.com")
)
);
Console.WriteLine(contact.PersonalName()); // James Bond
Console
.WriteLine(
contact
.ContactInfo()
.Match(
(emailAddress) => emailAddress.Address,
(postalAddress) => postalAddress.City + " " postalAddress.Zip.ToString(),
(emailAndPostalAddress) => emailAndPostalAddress.Item1.Name + emailAndPostalAddress.Item2.City + " " emailAndPostalAddress.Item2.Zip.ToString()
)
);
That's all.
I hope you enjoyed.
Example taken from the site F# for fun and profit

How to override a calculated value?

So I have the following snippet of code:
private Nullable<decimal> _excessWages;
public decimal ExcessWages
{
get
{
return _excessWages ?? CalculateExcessWages();
}
set
{
if (value != CalculateExcessWages())
_excessWages = value;
else
_excessWages = null;
}
}
So basically the behavior I'm trying to implement is if a field is left blank or is assigned a value equal the calculated one use the calculated value, otherwise store the assigned value.
I have a lot of fields that need to support overriding like this. Is this the best way to accomplish this? If not what would you suggest?

I worked on this a bit based mostly on Vlad's suggestion. Turns out you can use a single generic class to abstract this. Here is the end result:
public class Overridable<T>
{
private Func<T> _calculate;
private readonly Func<T, T, bool> _compare;
protected T _t;
public Overridable(Func<T> calculate, Func<T, T, bool> compare)
{
_calculate = calculate;
_compare = compare;
}
public T Value
{
get { return _compare(_t, default(T)) ? _calculate() : _t; }
set { _t = _compare(value, _calculate()) ? default(T) : value; }
}
}
You need to pass in a compare delegate because the type isn't known until you set it in a subclass. So a simple == isn't going to cut it. I went the easy route and used a Func delegate but this could be replaced with a normal delegate if it had to be adapted for .NET 2.0 for some reason.
You'll notice I'm using default(T) instead of null. This works because the default value for a Nullable<T> is null (or more precisely, undefined but it works out to be the same).
This doesn't prevent you from trying to declare an Overridable<T> for a non-nullable type. What you'd wind up with won't through run time errors but it isn't as useful. Trying to set a Overridable<decimal>.Value to null will get you a compiler error. While setting it to default(decimal) will cause it to revert to calculating the value.
I went this route because the properties from this the class I'm using this in needs to populate a serializable object thats eventually transmitted as xml. The schema for the xml includes numeric fields defined as a mixture of integers, decimals and strings.
You then use the Overriddable class like so:
private Overridable<decimal?> _excessWages =
new Overridable<decimal?>(CalculateExcessWages, (x,y) => x == y);
public virtual decimal? ExcessWages
{
get
{
return _excessWages.Value;
}
set
{
_excessWages.Value = value;
}
}
The only problem I ran into with this was that CalculateExcessWages is a non-static method so it can't be used in a field initializer. Since all the properties in my class are non-static I had to initialize all the backing fields in the constructor.

You can make a class wrapper for this.
class OverridableValue<T>
{
public OverridableValue<T>(Func<T> calculator)
{
_calculator = calculator;
}
private Nullable<T> _t;
private Func<T> _calculator;
public T Get()
{
return return _t ?? _calculator();
}
public void Set(T value)
{
_t = (value != _calculator()) ? value : null;
}
}
It's not so syntactically sweet, but at least saves some keystrokes.
Now you can use it like this:
class Foo
{
OverridableValue<decimal> _excessWages =
new OverridableValue<decimal>(CalculateExcessWages);
public decimal ExcessWages
{
get { return _excessWages.Get(); }
set { _excessWages.Set(value); }
}
...
}
The advantage is that the whole logic is hidden at the class.

You could do this by defining a handy set/get helper method
private static T GetUtil<T>(ref Nullable<T> value, Func<T> calc) {
return value ?? calc();
}
private static void SetUtil<T>(ref Nullable<T> value, T newValue, Func<T> calc) {
if ( newValue != calc() ) {
value = newValue
} else {
value = null;
}
}
private Nullable<decimal> _excessWages;
public decimal ExcessWages
{
get { return GetUtil(ref _excessWages, CalculateExcessWages); }
set { SetUtil(ref _excessWages, value CalculateExcessWages); }
}

That looks reasonable to my eyes. The only change I might make is to cache CalculateExcessWages(), if it is expensive to do, and ok to cache:
private Nullable<decimal> _excessWages;
private Nullable<decimal> _excessWagesCalculated;
public virtual decimal ExcessWages
{
get
{
if (_excessWagesCalculated == null)
_excessWagesCalculated = CalculateExcessWages();
return _excessWages ?? _excessWagesCalculated;
}
set
{
if (_excessWagesCalculated == null)
_excessWagesCalculated = CalculateExcessWages();
if (value != _excessWagesCalculated)
_excessWages = value;
else
_excessWages = null;
}
}
But, this is more code than yours, and I think you are looking to simplify.

What's the equivalent of Java's enum in C#? [duplicate]

This question already has answers here:
C# vs Java Enum (for those new to C#)
(13 answers)
Closed 9 years ago.
What's the equivalent of Java's enum in C#?

Full Java enum functionality isn't available in C#. You can come reasonably close using nested types and a private constructor though. For example:
using System;
using System.Collections.Generic;
using System.Xml.Linq;
public abstract class Operator
{
public static readonly Operator Plus = new PlusOperator();
public static readonly Operator Minus =
new GenericOperator((x, y) => x - y);
public static readonly Operator Times =
new GenericOperator((x, y) => x * y);
public static readonly Operator Divide =
new GenericOperator((x, y) => x / y);
// Prevent other top-level types from instantiating
private Operator()
{
}
public abstract int Execute(int left, int right);
private class PlusOperator : Operator
{
public override int Execute(int left, int right)
{
return left + right;
}
}
private class GenericOperator : Operator
{
private readonly Func<int, int, int> op;
internal GenericOperator(Func<int, int, int> op)
{
this.op = op;
}
public override int Execute(int left, int right)
{
return op(left, right);
}
}
}
Of course you don't have to use nested types, but they give the handy "custom behaviour" part which Java enums are nice for. In other cases you can just pass arguments to a private constructor to get a well-known restricted set of values.
A few things this doesn't give you:
Ordinal support
Switch support
EnumSet
Serialization/deserialization (as a singleton)
Some of that could probably be done with enough effort, though switch wouldn't really be feasible without hackery. Now if the language did something like this, it could do interesting things to make switch work by making the hackery automatic (e.g. declaring a load of const fields automatically, and changing any switch over the enum type to a switch over integers, only allowing "known" cases .)
Oh, and partial types mean you don't have to have all of the enum values in the same file. If each value got quite involved (which is definitely possible) each could have its own file.

Enums are one of the few language features that is better implemented in java than c#.
In java, enums are full fledged named instances of a type, while c# enums are basically named constants.
That being said, for the basic case, they will look similar. However in java, you have more power, in that you can add behavior to the individual enums, as they are full fledged classes.
is there some feature in particular you are looking for?

Here's another interesting idea. I came up with the following Enumeration base class:
public abstract class Enumeration<T>
where T : Enumeration<T>
{
protected static int nextOrdinal = 0;
protected static readonly Dictionary<int, Enumeration<T>> byOrdinal = new Dictionary<int, Enumeration<T>>();
protected static readonly Dictionary<string, Enumeration<T>> byName = new Dictionary<string, Enumeration<T>>();
protected readonly string name;
protected readonly int ordinal;
protected Enumeration(string name)
: this (name, nextOrdinal)
{
}
protected Enumeration(string name, int ordinal)
{
this.name = name;
this.ordinal = ordinal;
nextOrdinal = ordinal + 1;
byOrdinal.Add(ordinal, this);
byName.Add(name, this);
}
public override string ToString()
{
return name;
}
public string Name
{
get { return name; }
}
public static explicit operator int(Enumeration<T> obj)
{
return obj.ordinal;
}
public int Ordinal
{
get { return ordinal; }
}
}
It's got a type parameter basically just so the ordinal count will work properly across different derived enumerations. Jon's Operator example above then becomes:
public class Operator : Enumeration<Operator>
{
public static readonly Operator Plus = new Operator("Plus", (x, y) => x + y);
public static readonly Operator Minus = new Operator("Minus", (x, y) => x - y);
public static readonly Operator Times = new Operator("Times", (x, y) => x * y);
public static readonly Operator Divide = new Operator("Divide", (x, y) => x / y);
private readonly Func<int, int, int> op;
// Prevent other top-level types from instantiating
private Operator(string name, Func<int, int, int> op)
:base (name)
{
this.op = op;
}
public int Execute(int left, int right)
{
return op(left, right);
}
}
This gives a few advantages.
Ordinal support
Conversion to string and int which makes switch statements feasible
GetType() will give the same result for each of the values of a derived Enumeration type.
The Static methods from System.Enum can be added to the base Enumeration class to allow the same functionality.

You could probably use the old typesafe enum pattern that we used in Java before we got real ones (assuming that the ones in C# really aren't classes as a comment claims). The pattern is described just before the middle of this page

//Review the sample enum below for a template on how to implement a JavaEnum.
//There is also an EnumSet implementation below.
public abstract class JavaEnum : IComparable {
public static IEnumerable<JavaEnum> Values {
get {
throw new NotImplementedException("Enumeration missing");
}
}
public readonly string Name;
public JavaEnum(string name) {
this.Name = name;
}
public override string ToString() {
return base.ToString() + "." + Name.ToUpper();
}
public int CompareTo(object obj) {
if(obj is JavaEnum) {
return string.Compare(this.Name, ((JavaEnum)obj).Name);
} else {
throw new ArgumentException();
}
}
//Dictionary values are of type SortedSet<T>
private static Dictionary<Type, object> enumDictionary;
public static SortedSet<T> RetrieveEnumValues<T>() where T : JavaEnum {
if(enumDictionary == null) {
enumDictionary = new Dictionary<Type, object>();
}
object enums;
if(!enumDictionary.TryGetValue(typeof(T), out enums)) {
enums = new SortedSet<T>();
FieldInfo[] myFieldInfo = typeof(T).GetFields(BindingFlags.Static | BindingFlags.DeclaredOnly | BindingFlags.Public);
foreach(FieldInfo f in myFieldInfo) {
if(f.FieldType == typeof(T)) {
((SortedSet<T>)enums).Add((T)f.GetValue(null));
}
}
enumDictionary.Add(typeof(T), enums);
}
return (SortedSet<T>)enums;
}
}
//Sample JavaEnum
public class SampleEnum : JavaEnum {
//Enum values
public static readonly SampleEnum A = new SampleEnum("A", 1);
public static readonly SampleEnum B = new SampleEnum("B", 2);
public static readonly SampleEnum C = new SampleEnum("C", 3);
//Variables or Properties common to all enums of this type
public int int1;
public static int int2 = 4;
public static readonly int int3 = 9;
//The Values property must be replaced with a call to JavaEnum.generateEnumValues<MyEnumType>() to generate an IEnumerable set.
public static new IEnumerable<SampleEnum> Values {
get {
foreach(var e in JavaEnum.RetrieveEnumValues<SampleEnum>()) {
yield return e;
}
//If this enum should compose several enums, add them here
//foreach(var e in ChildSampleEnum.Values) {
// yield return e;
//}
}
}
public SampleEnum(string name, int int1)
: base(name) {
this.int1 = int1;
}
}
public class EnumSet<T> : SortedSet<T> where T : JavaEnum {
// Creates an enum set containing all of the elements in the specified element type.
public static EnumSet<T> AllOf(IEnumerable<T> values) {
EnumSet<T> returnSet = new EnumSet<T>();
foreach(T item in values) {
returnSet.Add(item);
}
return returnSet;
}
// Creates an enum set with the same element type as the specified enum set, initially containing all the elements of this type that are not contained in the specified set.
public static EnumSet<T> ComplementOf(IEnumerable<T> values, EnumSet<T> set) {
EnumSet<T> returnSet = new EnumSet<T>();
foreach(T item in values) {
if(!set.Contains(item)) {
returnSet.Add(item);
}
}
return returnSet;
}
// Creates an enum set initially containing all of the elements in the range defined by the two specified endpoints.
public static EnumSet<T> Range(IEnumerable<T> values, T from, T to) {
EnumSet<T> returnSet = new EnumSet<T>();
if(from == to) {
returnSet.Add(from);
return returnSet;
}
bool isFrom = false;
foreach(T item in values) {
if(isFrom) {
returnSet.Add(item);
if(item == to) {
return returnSet;
}
} else if(item == from) {
isFrom = true;
returnSet.Add(item);
}
}
throw new ArgumentException();
}
// Creates an enum set initially containing the specified element(s).
public static EnumSet<T> Of(params T[] setItems) {
EnumSet<T> returnSet = new EnumSet<T>();
foreach(T item in setItems) {
returnSet.Add(item);
}
return returnSet;
}
// Creates an empty enum set with the specified element type.
public static EnumSet<T> NoneOf() {
return new EnumSet<T>();
}
// Returns a copy of the set passed in.
public static EnumSet<T> CopyOf(EnumSet<T> set) {
EnumSet<T> returnSet = new EnumSet<T>();
returnSet.Add(set);
return returnSet;
}
// Adds a set to an existing set.
public void Add(EnumSet<T> enumSet) {
foreach(T item in enumSet) {
this.Add(item);
}
}
// Removes a set from an existing set.
public void Remove(EnumSet<T> enumSet) {
foreach(T item in enumSet) {
this.Remove(item);
}
}
}

enum , or do you need something in particular that Java enums have but c# doesn't ?