[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
I intend to ask about generic operator+ overloading but not in typical "can I do operator+ for generic type" way.
Questions are on the bottom
I recently started to create matrix class in C# and after a while It came to me that I cannot do simple T + T !
Thus, I googled and googled and found several workarounds.
Create Expression link
Create abstract class abstract class Matrix<T>{//some code}. Create 'protected virtual method Add(T itemToAdd)' and then create
operator like this : T operator+(T item1, T item2){return item1.Add(item2);}(most posts on stack) and then inherit this method in class Matrix : Matrix<int> here
Use method Add such as : T Add(T first, T second){ dynamic output = first + second; return output;} (somewhere on stack)
First one just does not suited me so I tried second one but then I run onto serious problems like:
(A LOT of )repetative code - I created 4 classes for : int, double, long, Complex - my own type
Creating multiple extension methods and so on.
Third one is just so unsafe that I rejected it immidietlay.
After my struggling I came to realise : 'Why don't I use RTTI and reflection?' I know, it is expensive in running time but why not use static constructor to do this?
Here is my idea (pseudocode):
class Matrix<T>{
static Func<T,T,T> Add;
static Matrix
{
if(T is int)
Add = (first,second) = > ((int)first)+((int)second);
else if(T is long)
Add = (first,second) = > ((long)first)+((long)second);
// and so on for built-in types
else
{ // T is not built-in type
if(typeof(T).GetMethods().Contains("op_Addition"))
{
Add = (first,second) => typeof(T).getMethod("op_Addition").invoke(first,second);
}
}
}
I know that reflection is costly but it will do it only one time (per type)!
And before you start argue : I am going to code T is int like this :
var type = typeof(T);
if(type==typeof(int)) // code
I am aware that I cannot explicitly convert T to int but there must be some sort of 'work around'. Problem is that (for example) Int32 has not explicit 'method' for operator+ hence, reflection is not of much use.
After all that introduction I have two questions :
Is it a good approach or do you see major flaws in it?
Is it doable? I don't want to start creating code without knowing for sure that my lambda function WILL work.
EDIT 1+2
I changed my code to generic.
I figured that maybe you need an usage of my class, here you are :
Matrix<int> matrix = new Matrix(1,1); // creates int-based matrix
Matrix<MyClass> matrix2 = new Matrix(1,1); // creates some other type matrix
ANSWER
based on dasblinkenlight's answer I managed to do this :
public interface ITypeTratis<T>
{
T Add(T a, T b);
T Mul(T a, T b);
T Sub(T a, T b);
T Div(T a, T b);
bool Eq(T a, T b);
}
public class IntTypeTratis : ITypeTratis<int>
{
//code for int
}
public class DoubleTypeTratis : ITypeTratis<double>
{
//code for double
}
internal class TypeTraits<T> : ITypeTratis<T>
{
public Func<T, T, T> AddF;
public Func<T, T, T> MulF;
public Func<T, T, T> DivF;
public Func<T, T, T> SubF;
public Func<T, T, bool> EqF;
public T Add(T a, T b) => AddF(a, b);
public bool Eq(T a, T b) => EqF(a, b);
public T Mul(T a, T b) => MulF(a, b);
public T Sub(T a, T b) => SubF(a, b);
public T Div(T a, T b) => DivF(a, b);
}
public class Matrix<T>
{
private static IDictionary<Type, object> traitByType = new Dictionary<Type, object>()
{
{typeof (int), new IntTypeTratis()},
{typeof (double), new DoubleTypeTratis()}
};
static Matrix()
{
Debug.WriteLine("Robie konstruktor dla " + typeof(T));
var type = typeof(T);
if (!traitByType.ContainsKey(type))
{
MethodInfo add, sub, mul, div, eq;
if ((add = type.GetMethod("op_Addition")) == null)
throw new NotSupportedException("Addition is not implemented");
if ((sub = type.GetMethod("op_Subtraction")) == null)
throw new NotSupportedException("Substraction is not implemented");
if ((mul = type.GetMethod("op_Multiply")) == null)
throw new NotSupportedException("Multiply is not implemented");
if ((div = type.GetMethod("op_Division")) == null)
throw new NotSupportedException("Division is not implemented");
if ((eq = type.GetMethod("op_Equality")) == null)
throw new NotSupportedException("Equality is not implemented");
var obj = new TypeTraits<T>
{
AddF = (a, b) => (T)add.Invoke(null, new object[] { a, b }),
SubF = (a, b) => (T)sub.Invoke(null, new object[] { a, b }),
MulF = (a, b) => (T)mul.Invoke(null, new object[] { a, b }),
DivF = (a, b) => (T)div.Invoke(null, new object[] { a, b }),
EqF = (a, b) => (bool)eq.Invoke(null, new object[] { a, b })
};
traitByType[type] = obj;
}
}
}
And this is exactly what I was looking for.
Yes, your approach will work fine.
Your static constructor will run for each type parameter T, ensuring that Add is set correctly.
You may want to separate out the addition logic into a separate class outside your matrix, and use that class to run operations based on type for your matrix. For example, if you also need multiplication, you could build a ITypeTraits<T> interface that has Add and Multiply:
public interface ITypeTraits<T> {
T Add(T a, T b);
T Mul(T a, T b);
}
Now you can build implementations of ITypeTraits<T> for individual types, e.g.
public class IntTypeTraits : ITypeTraits<int> {
public int Add(int a, int b) { return a+b; }
public int Mul(int a, int b) { return a*b; }
}
public class LongTypeTraits : ITypeTraits<long> {
public long Add(long a, long b) { return a+b; }
public long Mul(long a, long b) { return a*b; }
}
... // and so on
make a dictionary out of them
static readonly IDictionary<Type,object> traitByType = new Dictionary<Type,object> {
{typeof(int), new IntTypeTraits() }
, {typeof(long), new LongTypeTraits() }
... // and so on
};
and get the one you need to perform operations:
ITypeTraits<T> traits = (ITypeTraits<T>)traitByType(typeof(T));
T first = ...
T second = ...
T sum = traits.Add(first, second);
T prod = traits.Mul(first, second);
We can do this natively in C# 11 / .NET 7 (or above):
class Matrix<T> where T : INumber<T> // or just IAdditionOperators<T,T,T>
{
T x, y, z; // just to show we can do things
public T Sum() => x + y + z;
}
What is wrong with #3? You can just check for type, like so:
public abstract class Matrix<T>
{
public static HashSet<Type> AllowAdd = new HashSet<Type>
{
typeof(int),
typeof(long),
typeof(string),
typeof(double),
};
public T Add<T>(T first, T second)
{
if(!AllowAdd.Contains(typeof(T)))
{
throw new Exception(string.Format("Cannot preform addition for type: {0}", typeof(T).Name));
}
dynamic result = (dynamic)first + (dynamic)second;
return (T)result;
}
}
Bulding on dasblinkenlight's answer, here's my version of it. The benefit is that it doesn't need a dictionary lookup, instead making the type system do it. Should be faster, I think, but I haven't measured it. Also a bit less typing.
public abstract class MatrixBase
{
protected static class OperationDict<T>
{
private static Func<T,T,T> _notSupported = (a, b) => { throw new NotSupportedException(string.Format("Type {0} not supported for Matrix operations!", typeof(T))); };
public static Func<T, T, T> Add = _notSupported;
public static Func<T, T, T> Multiply = _notSupported;
}
static MatrixBase()
{
OperationDict<int>.Add = (a, b) => a + b;
OperationDict<int>.Multiply = (a, b) => a * b;
OperationDict<decimal>.Add = (a, b) => a + b;
OperationDict<decimal>.Multiply = (a, b) => a * b;
// Etc. for all supported types
}
}
public class Matrix<T> : MatrixBase
{
public T DoAdd(T a, T b)
{
return OperationDict<T>.Add(a, b);
}
}
I think you are on the right path, in order to avoid using reflection, you are required to somehow inform the compiler that you know "T" has the "+" operator, however, this feature does not yet exist in C#, so this is impossible to implement without runtime type checking or imposing other constraints.
If you don't care about the performance, you could use dynamic:
(dynamic)first + (dynamic)second
but that will take several reflection performance hits in every operation
Or you could use some other more complex approach that caches the specific methods in a dictionary, but you won't escape calling at least .GetType() in your add's implementation
In the following code. The function getInstance dynamically creates instance of of Aaa, Bbb and Ccc. However, how to dynamically generate the Union3<...>.Case2(Bbb)?
Union3<Aaa, Bbb, Ccc> getInstance(int i)
{
Type t = getType(i); // one of Aaa, Bbb, Ccc
var instance = Activator.CreateInstance(t);
// .... set property values
return ???? // Union3<Aaa, Bbb, Ccc>.Case?(instance) // 1 to 3 to replace the "?"
}
public abstract class Union3<A, B, C>
{
public abstract T Match<T>(Func<A, T> f, Func<B, T> g, Func<C, T> h);
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);
}
}
}
You have to use Type.MakeGenericType when dynamically creating generic types:
Union3<Aaa, Bbb, Ccc> getInstance<Aaa, Bbb, Ccc>(int i)
{
Type t = typeof(Aaa); // implementation of getType(i) left out
var instance = Activator.CreateInstance(t);
Type unionType = typeof(Union3<,,>).MakeGenericType(typeof(Aaa), typeof(Bbb), typeof(Ccc));
var nestedTypes = unionType.GetNestedTypes();
object unionInstance = null;
Type toCreate = null;
if (t == typeof(Aaa))
toCreate= nestedTypes.Single(x => x.Name == "Case1").MakeGenericType(typeof(Aaa), typeof(Bbb), typeof(Ccc));
else if (t == typeof(Bbb))
toCreate= nestedTypes.Single(x => x.Name == "Case2").MakeGenericType(typeof(Aaa), typeof(Bbb), typeof(Ccc));
else if (t == typeof(Ccc))
toCreate= nestedTypes.Single(x => x.Name == "Case3").MakeGenericType(typeof(Aaa), typeof(Bbb), typeof(Ccc));
else
throw new NotImplementedException();
unionInstance = Activator.CreateInstance(toCreate, instance);
return (Union3<Aaa, Bbb, Ccc>)unionInstance;
}
The end result of this will be an instance of one of the 3 inner classes with the correct Item Type.
Is there something similar to this C++ template?
template <int A>
class B
{
int f()
{
return A;
}
}
I want to make every instance of B<1>, B<2>, etc (eg tuple) a different type.
The short answer is no.
It doesn't fit the way C# generics, as apposed to C++ templates, work.
The .net generics are not a language feature, they are a runtime feature. The runtime knows how to instantiate generics from special generic bytecode which is rather restricted compared to what C++ templates can describe.
Compare this with C++ templates, which basically instantiate the whole AST of the class using substituted types. It'd be possible to add AST based instantiation to the runtime, but it'd certainly be much more complex than the current generics.
Without features like value-type arrays (which only exist in unsafe code), recursive template instantiation or template specialization using such parameters wouldn't be very useful either.
A workaround to this limitation is to define a class which itself exposes the literal value you are interested in. For example:
public interface ILiteralInteger
{
int Literal { get; }
}
public class LiteralInt10 : ILiteralInteger
{
public int Literal { get { return 10; } }
}
public class MyTemplateClass< L > where L: ILiteralInteger, new( )
{
private static ILiteralInteger MaxRows = new L( );
public void SomeFunc( )
{
// use the literal value as required
if( MaxRows.Literal ) ...
}
}
Usage:
var myObject = new MyTemplateClass< LiteralInt10 >( );
C# does not support non-type generic parameters like C++ does.
C# generics are far simpler and less capable than C++ templates. MSDN has a succinct list of Differences Between C++ Templates and C# Generics.
C# generics are specialized at run-time, whereas C++ templates are processed at compile-time to make an entirely new type. Given this, the runtime simply doesn't have the features to process non-type parameters (it's not just a C# issue).
With C# 10, there's something that comes sort of close:
interface IConstant
{
abstract static int Constant { get; }
}
class B<T> where T : IConstant
{
int GetConstant() => T.GetConstant();
}
class Constant1 : IConstant { static int Constant => 1; }
class Constant2 : IConstant { static int Constant => 2; }
This does not guarantee that Constant is actually constant sadly, and you need to define types for all numbers you want to use. However it does allow you to discriminate B<Constant1> and B<Constant2>
Not nearly an equivalent but something at least
Note that you need to add <EnablePreviewFeatures>true</EnablePreviewFeatures> to your .csproj for this to work, as abstract static Interface methods are still in preview
The following approach may be useful depending on what you're trying to accomplish. The size of the underlying collection is determined by an abstract property.
public abstract class TupleBase
{
protected abstract int NumElems { get; }
protected object[] values;
protected TupleBase(params object[] init)
{
if (init.Length != NumElems)
{
throw new ArgumentException($"Collection must contains {NumElems} elements", nameof(init));
}
values = new object[NumElems];
for (int i = 0; i < init.Length; i++)
{
values[i] = init[i];
}
}
protected object Get(int idx) => values[idx];
protected void Set(int idx, object value) => values[idx] = value;
}
public class Tuple1<T> : TupleBase {
protected override int NumElems => 1;
public Tuple1(T val0) : base(val0) { }
public T Get0() => (T)Get(0);
public void Set0(T value) => Set(0, value);
}
public class Tuple2<T, U> : TupleBase {
protected override int NumElems => 2;
public Tuple2(T val0, U val1) : base(val0, val1) { }
public T Get0() => (T)Get(0);
public U Get1() => (U)Get(1);
public void Set0(T value) => Set(0, value);
public void Set1(U value) => Set(1, value);
}
public class Tuple3<T, U, V> : TupleBase
{
protected override int NumElems => 3;
public Tuple3(T val0, U val1, V val2) : base(val0, val1, val2) { }
public T Get0() => (T)Get(0);
public U Get1() => (U)Get(1);
public V Get2() => (V)Get(2);
public void Set0(T value) => Set(0, value);
public void Set1(U value) => Set(1, value);
public void Set2(V value) => Set(2, value);
}
public class Tuple4<T, U, V, W> : TupleBase
{
protected override int NumElems => 4;
public Tuple4(T val0, U val1, V val2, W val3) : base(val0, val1, val2, val3) { }
public T Get0() => (T)Get(0);
public U Get1() => (U)Get(1);
public V Get2() => (V)Get(2);
public W Get3() => (W)Get(3);
public void Set0(T value) => Set(0, value);
public void Set1(U value) => Set(1, value);
public void Set2(V value) => Set(2, value);
public void Set3(W value) => Set(3, value);
}
public class Tuple5<T, U, V, W, X> : TupleBase
{
protected override int NumElems => 5;
public Tuple5(T val0, U val1, V val2, W val3, X val4) : base(val0, val1, val2, val3, val4) { }
public T Get0() => (T)Get(0);
public U Get1() => (U)Get(1);
public V Get2() => (V)Get(2);
public W Get3() => (W)Get(3);
public X Get4() => (X)Get(4);
public void Set0(T value) => Set(0, value);
public void Set1(U value) => Set(1, value);
public void Set2(V value) => Set(2, value);
public void Set3(W value) => Set(3, value);
public void Set4(X value) => Set(4, value);
}
I'm looking for a creative pattern to enumerate two IEnumerable<>'s synchronized.
If I was making something up and adding to the C# syntax I might write:
foreach(var firstItem, var secondItem in this.ListOne, this.ListTwo)
{
if (firstItem.Prop == secondItem.Prop)
WorkSomeMagic(secondItem);
DoSomethingElse(firstItem);
}
Now, obviously that doesn't exist. What patterns have people used to accomplish something similar when dealing with enumerations that aren't accessible by index? Keep in mind, what is inside my pseudo-foreach would be more complex; I simplified for the example.
You're looking for Zip, which is new in .NET 4 or you can use the implementation here:
Is there a zip-like method in .Net?
I usually do the following:
using (IEnumerator<int> e1 = this.ListOne.GetEnumerator(),
e2 = this.ListTwo.GetEnumerator()) {
while (e1.MoveNext() && e2.MoveNext()) {
...
}
}
Or write an extension method:
public static void EnumerateWith<T>(this IEnumerable<T> left,
IEnumerable<T> right, Action<T,T> del) {
using (IEnumerator<T> l = left.GetEnumerator(),
r = right.GetEnumerator()) {
while (l.MoveNext() && r.MoveNext()) {
del(l.Current,r.Current);
}
}
}
ListOne.EnumerateWith(ListTwo, (left, right) => {
...
});
As with any general C# question, this will probably have 10 good answers posted before VS2008 even loads. Instead of that rat race, I'll come up with an offbeat "anti-pattern" you should never use. In fact, anyone writing mission critical code, please stop reading now.
using System;
using System.Collections.Generic;
using System.Linq;
class EnumTwoLists
{
static void Main(string[] args)
{
var left = new List<int>();
var right = new List<DateTime>();
var demo = new LinqAbuse<int, DateTime>(left, right);
demo.Populate(40, s => s * s, d => new DateTime(2009, d / 31 + 1, d % 31 + 1));
demo.Enumerate( (s, d) => Console.WriteLine(String.Format("Executing arbitrary code with {0} and {1}", s, d)) );
}
}
class LinqAbuse<T1, T2>
{
public LinqAbuse(List<T1> l, List<T2> r)
{
left = l;
right = r;
}
List<T1> left;
List<T2> right;
public void Populate(int size, Func<int, T1> leftGenerator, Func<int, T2> rightGenerator)
{
new int[size].Aggregate(0, (index, empty) => PopulateWrapper(left, right, leftGenerator, rightGenerator, index));
}
int PopulateWrapper(List<T1> left, List<T2> right, Func<int, T1> leftGenerator, Func<int, T2> rightGenerator, int index)
{
left.Add(leftGenerator(index));
right.Add(rightGenerator(index));
return ++index;
}
public void Enumerate(Action<T1, T2> loopBody)
{
left.Join(right, l => "", r => "",
(l, r) => ActionWrapper(l, r, loopBody),
new CartesianComparer<object>(right.Count))
.ToList();
}
object ActionWrapper(T1 x, T2 y, Action<T1, T2> action)
{
action(x, y);
return null;
}
}
class CartesianComparer<T> : IEqualityComparer<T>
{
public CartesianComparer(int _size)
{
size = _size;
equalsCounter = (size * (size - 1) >> 1) + size; // Combinations(size, 2) + (size - trueCounter)
}
private int size;
private int equalsCounter;
private int trueCounter = 0;
public bool Equals(T x, T y)
{
if (0 < --equalsCounter)
return false;
equalsCounter = size - ++trueCounter;
return true;
}
public int GetHashCode(T obj)
{
return 0;
}
}
Aww, isn't she cute? (alternate caption: Mommy, why is Anders crying?)
Ignoring checks for nulls and whatnot:
IEnumerable<T1> first;
IEnumerable<T2> second;
using (IEnumerator<T1> e1 = first.GetEnumerator()) {
using (IEnumerator<T2> e2 = second.GetEnumerator()) {
while (e1.MoveNext() && e2.MoveNext()) {
// do something eith e1.Current and e2.Current
}
}
}
I know this question is old but for anyone coming to this question now you can build on Jason's answer and JaredPar's answer with C#7's ValueTuple, which will give you a syntax similar to the original question. You may need to install the nuget package System.ValueTuple.
If you declare an extension method something like this:
internal static class EnumerableExtensions
{
internal static IEnumerable<(T1, T2)> EnumerateWith<T1, T2>(this IEnumerable<T1> first, IEnumerable<T2> second)
{
using (var firstEnumerator = first.GetEnumerator())
using (var secondEnumerator = second.GetEnumerator())
{
while(firstEnumerator.MoveNext() && secondEnumerator.MoveNext())
{
yield return (firstEnumerator.Current, secondEnumerator.Current);
}
}
}
}
Then you can use it like this:
List<Foo> foos = new List<Foo>()
{
new Foo(),
new Foo(),
new Foo()
};
List<Bar> bars = new List<Bar>()
{
new Bar(),
new Bar(),
new Bar()
};
foreach((Foo foo, Bar bar) in foos.EnumerateWith(bars))
{
Console.WriteLine(foo.ID);
Console.WriteLine(bar.ID);
}