Create variable of class without the parameters of it's constructor - c#

I have a BaseClass with a constructor and some parameters and then I want to do a strategy where depending of an enum, it creates one derived class or another (of that BaseClass), but with the same parameters. Is there any way to refactor this ? Thanks !
public enum GameMode
{
ModeA,
ModeB
}
public abstract class BaseClass
{
public BaseClass(int a, string b, char c)
{
}
}
public class FirstGameMode : BaseClass
{
public FirstGameMode(int a, string b, char c) : base(a, b, c)
{
}
}
public class SecondGameMode: BaseClass
{
public SecondGameMode(int a, string b, char c) : base(a, b, c)
{
}
}
public class TestingPurpose
{
private GameMode _gameMode;
private BaseClass _baseClass;
public void Init()
{
if (_gameMode == GameMode.ModeA)
{
// They use the same variables !
_baseClass = new FirstGameMode(5, "Hello", 'c');
}
else
{
// They use the same variables !
_baseClass = new SecondGameMode(5, "Hello", 'c');
}
}
}
I tried with some reflection but still I couldn't do it.
I would like to have something like
public void Init()
{
BaseMatchMode type;
if (_gameMode == GameMode.ModeA)
{
type = typeof(FirstGameMode);
}
else
{
type = typeof(SecondGameMode);
}
_baseClass = new type(5, "Hello", 'c');
}

You could use a factory delegate method;
Func<int a, string b, char c, BaseClass> factory;
if (_gameMode == GameMode.ModeA)
{
factory= (a, b, c) => new FirstGameMode(a, b, c);
}
else
{
factory= (a, b, c) => new SecondGameMode(a, b, c);
}
_baseClass = factory(5, "Hello", 'c');
For such an simple example it would probably be easier to just skip the factory method and create your objects directly. But this technique is sometimes useful if you want to add some abstraction between components.
You could also create a factory class instead of just using a delegate. There are also dependency injection (DI) / Inversion of Control (IoC) frameworks that are intended to solve the problem of specifying what implementation of interfaces/base classes other components should use.

Related

C# type safe union [duplicate]

[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 call methods of data member through a generic method

Considering the classes below, I would like to be able to write:
B b = new B();
b.f(1); // calls b._a.f(int)
b.f("howdy!"); // calls b._a.f(string)
Class A cannot be modified.
public class A
{
public void f(int i) { }
public void f(string s) { }
}
public class B
{
public void f<T>(T v)
{
_a.f(v); // fails
}
A _a = new A();
}
If you really want to simplify your f pass-through down to a single method, you'll have to use run-time type checking. I don't see any other way around it. Generics aren't going to work here.
public void f(object v)
{
if (v is int i)
_a.f(i);
else if (v is string s)
_a.f(s);
else
throw new InvalidOperationException();
}
I gather that you are hesitant to just expose _a because there are some methods on it that you would rather not be accessible to the caller? Could you introduce an interface, or would that be considered a modification to class A?
public interface IFoo
{
void f(int i);
void f(string s);
}
public class A : IFoo
{
...
}
public class B
{
public IFoo A => _a;
}

Is there a way to restrict functions in a C# class to only use a specific signature?

I am writing a class that should ideally have multiple methods of the same signature. Is there a way to force the class to check its methods if all of them follow the same signature?
It would be ideal if the check could be done at compile-time/during build
If you assume the signature to be int <methodName>(string, int, char)
public class Conditions {
// no error
int MethodA(string a, int b, char c)
{
return 0;
}
// no error
int MethodB(string a, int b, char c)
{
return 1;
}
// should throw error because return type does not match signature
string MethodC(string a, int b, char c)
{
return "Should throw an error for this function";
}
}
}
It's sort of cheating, but if you require the developer to register their methods, you can force a compile time error by requiring the method to match a delegate.
This is essentially how event handlers and callbacks work.
namespace Framework
{
public delegate int MyApiSignature(int a, string b, char c);
public class Core
{
static public void RegisterMethod(MyApiSignature method)
{
//Doesn't even have to actually do anything
}
}
}
namespace Custom
{
using Framework;
class Foo
{
public Foo()
{
Core.RegisterMethod(MethodA); //Works
Core.RegisterMethod(MethodB); //Compile-time error
}
public int MethodA(int a, string b, char c)
{
return 0;
}
public int MethodB(int a, string b, byte c)
{
return 0;
}
}
}
You could make a unit test:
[TestMethod]
public void Conditions_MethodsHaveCorrectSignature()
{
var whitelist = new List<string> { "Finalize", "MemberwiseClone" };
var t = typeof(Conditions);
var m = t.GetMethods(BindingFlags.NonPublic | BindingFlags.Instance);
foreach (var item in m.Where(x => !whitelist.Contains(x.Name)))
{
Assert.AreEqual(typeof(int), item.ReturnType);
CollectionAssert.AreEquivalent(new List<Type> { typeof(string), typeof(int), typeof(char) },
item.GetParameters().Select(x => x.ParameterType).ToList());
}
}
Not directly. You could write an analyzer for it using Roslyn, or you could write a unit test that checks the signatures via reflection.

Double dispatch in C# 4.0 - dynamic keyword?

I realise this has been asked before, but I didn't find a clear agreement on the best solution.
Is using dynamic (like below) the best way to do this?
I guess its best to avoid dynamic whenever possible to help catch problems at compile-time.
(classA1 and classA2 implement Interface A and likewise for B)
public static void Foo(InterfaceA a, InterfaceB b)
{
Foo((dynamic)a, (dynamic)b);
}
static void Foo(classA1 a, classB1 b) { //some code }
static void Foo(classA2 a, classB2 b) { //some code }
static void Foo(classA1 a, classB2 b) { //some code }
static void Foo(classA2 a, classB1 b) { //some code }
Or similarly...
public static void Foo(InterfaceA a, InterfaceB b)
{
((dynamic) a).Foo(b);
}
public classA1
{
void Foo(classB1 b) { //some code }
}
//repeated for other cases
Is using dynamic (like below) the best way to do this?
Well it's a way to do it - so long as the execution-time types will always end up with something that overload resolution will be happy with.
You might want to put a backstop method of
static void Foo(object x, object y)
in case none of the methods are applicable (e.g. a is a non-ClassA1/ClassA2 implementation). It's not going to help you if both values are null, mind you...
I would usually attempt to redesign so that this isn't required, but it's hard to know the best solution without more context.
Are 'classA1' and so on implementations of InterfaceA? If so then why not just declare the Foo functions as accepting InterfaceA and InterfaceB and cast them within to the concrete implementation expected by the function? E.g.,
static void Foo(InterfaceA a, InterfaceB b) {
classA1 c1 = a as classA1;
classB1 b1 = b as classB1;
// ... etc
}
Dynamic isn't intended to be used this way.
You could do something AWFUL with reflection - but I'm sure it's not better than doing dynamic:
void Main()
{
var a = "hello";//5;
var b = "hello";
var type1 = a.GetType();
var type2 = b.GetType();
var t = typeof(FooClass);
var methods = t.GetMethods();
foreach(var method in methods)
{
var parameters = method.GetParameters();
if(parameters.Length == 2)
{
if(parameters[0].ParameterType == type1
&& parameters[1].ParameterType == type2)
{
method.Invoke(this, new object[]{ a, b });
}
}
}
}
public static class FooClass
{
public static void Foo(int i, string s)
{
"Foo1".Dump();
}
public static void Foo(string s, string s2)
{
"Foo2".Dump();
}
}
C# has traditionally been a statically-typed language. The dynamic keyword adds dynamic typing to the language. The usual advice is to use "dynamic" sparingly. Here may be a case where you need it.
Generics won't cut it as this won't compile:
private void button1_Click(object sender, EventArgs e)
{
Foo(new classA1(), new classB2());
}
static void Foo<T, T1>(T a, T1 b) where T: InterfaceA
where T1: InterfaceB
{
Foo2(a, b);
}
static void Foo2(classA1 a, classB1 b) { }
static void Foo2(classA2 a, classB2 b) { }
static void Foo2(classA1 a, classB2 b) { }
static void Foo2(classA2 a, classB1 b) { }
interface InterfaceA { }
interface InterfaceB { }
class classA1 : InterfaceA { }
class classA2 : InterfaceA { }
class classB1 : InterfaceB { }
class classB2 : InterfaceB { }

Calling constructor from other constructor in same class

I have a class with 2 constructors:
public class Lens
{
public Lens(string parameter1)
{
//blabla
}
public Lens(string parameter1, string parameter2)
{
// want to call constructor with 1 param here..
}
}
I want to call the first constructor from the 2nd one. Is this possible in C#?
Append :this(required params) at the end of the constructor to do 'constructor chaining'
public Test( bool a, int b, string c )
: this( a, b )
{
this.m_C = c;
}
public Test( bool a, int b, float d )
: this( a, b )
{
this.m_D = d;
}
private Test( bool a, int b )
{
this.m_A = a;
this.m_B = b;
}
Source Courtesy of csharp411.com
Yes, you'd use the following
public class Lens
{
public Lens(string parameter1)
{
//blabla
}
public Lens(string parameter1, string parameter2) : this(parameter1)
{
}
}
The order of constructor evaluation must also be taken into consideration when chaining constructors:
To borrow from Gishu's answer, a bit (to keep code somewhat similar):
public Test(bool a, int b, string c)
: this(a, b)
{
this.C = c;
}
private Test(bool a, int b)
{
this.A = a;
this.B = b;
}
If we change the evalution performed in the private constructor, slightly, we will see why constructor ordering is important:
private Test(bool a, int b)
{
// ... remember that this is called by the public constructor
// with `this(...`
if (hasValue(this.C))
{
// ...
}
this.A = a;
this.B = b;
}
Above, I have added a bogus function call that determines whether property C has a value. At first glance, it might seem that C would have a value -- it is set in the calling constructor; however, it is important to remember that constructors are functions.
this(a, b) is called - and must "return" - before the public constructor's body is performed. Stated differently, the last constructor called is the first constructor evaluated. In this case, private is evaluated before public (just to use the visibility as the identifier).

Categories