Is there a way to use operators as functions without declaring them manually?
Func<bool, bool> not = x => !x;
Similar to (+)/(-) in Haskell. Would be handy in various LINQ scenarios involving conditional query construction. Maybe some C# (4.0+) trick I don't know about.
Edit: here is a sample to clarify what I mean:
int MyWhere(Func<bool, bool, bool> cond) {}
a usual call would be:
MyWhere((x,y) => x && y)
a cool call would be (assuming Haskell style):
MyWhere((&&))
No.
You can find the definitions (F12 in VS) for the operators you can overload, and there exists a
// struct Double
public static bool operator ==(double left, double right);
But there is no way to call Double.operator==(a, b).
The Framework does offer a a.Equals(b) for this particualr case, but there is no a.Add(b).
For documentary purposes: user-defined operator are functions, however the C# language explicitly prohibits explicit calls (don't know the rationale). E.g.:
public class X
{
public static X operator+(X a, X b)
{
return new X();
}
}
public class Program
{
public static void Main(string[] args)
{
var q = new X() + new X();
X.op_Addition(q,q); // error CS0571:
// `Program.X.operator +(Program.X, Program.X)': cannot explicitly
// call operator or accessor
}
}
If you were writing IL, you'd write this:
newobj instance void class X::'.ctor'()
newobj instance void class X::'.ctor'()
call class X class X::op_Addition(class X, class X)
stloc.0
If i understand what you are want to do right, you can use System.Reflection and Delegate.CreateDelegate to create a delegate to the operator method.
Of course i never testet it, but operator methods are methods so it should work.
This could also be automated ( To save time compared to the straight forward method ).
See:
http://msdn.microsoft.com/de-de/library/53cz7sc6.aspx
Related
In c++ 20 we can use concepts which works like the where keyword in c# but I'm wondering how can i check T has some operator overloaded on it.
c++ Example:-
template <class T>
concept SupportPlusEqual = requires (T t) { t += t };
template <class T>
requires SupportPlusEqual <T>
void Add (T& lhs, T& rhs)
{
lhs += rhs;
}
In this code T must have += operator defined.
Is there any way to do the same in c#?
I think the closest equivalent is if you use .NET 7, interfaces can now specify operators with the new static abstract and virtual members feature. For example:
public interface IHasAdd<T> where T : IHasAdd<T>
{
static abstract T operator +(T a, T b);
}
To use it, make a type like this:
public class Foo : IHasAdd<Foo>
{
public Foo(int someValue)
{
SomeValue = someValue;
}
public int SomeValue { get; init; }
public static Foo operator +(Foo a, Foo b)
{
return new Foo(a.SomeValue + b.SomeValue);
}
}
An example method that can use the interface:
public T AddTogether<T>(T first, T second)
where T : IHasAdd<T>
{
return first + second;
}
Which means you can now do this:
var result = AddTogether(new Foo(1), new Foo(2));
//result.SomeValue == 3
You can only restrict generic types to interfaces, there is no way to require a specific operator to be implemented.
From .Net 7 there is the generic math feature that accomplishes something similar. This includes a bunch of interfaces, like IAdditionOperators<TSelf,TOther,TResult>, that can be used as a generic type restriction, and should also have associated operators defined to perform the corresponding operator.
Note that c++ templates and c# generic while superficially similar are implemented quite differently. While c++ resolves everything in compile time, c# includes the generic types in the CIL code, and is only converted to machine code when jitted.
You can't use covariance for value types:
Func<string> refTypeFunc = () => "foo";
Func<int> valueTypeFunc = () => 123;
Func<object> a = refTypeFunc; // works
Func<object> b = valueTypeFunc; // doesn't work
This answer by Jon Skeet explains why:
Basically, variance applies when the CLR can ensure that it doesn't need to make any representational change to the values. References all look the same - so you can use an IEnumerable<string> as an IEnumerable<object> without any change in representation; the native code itself doesn't need to know what you're doing with the values at all, so long as the infrastructure has guaranteed that it will definitely be valid.
For value types, that doesn't work - to treat an IEnumerable<int> as an IEnumerable<object>, the code using the sequence would have to know whether to perform a boxing conversion or not.
Well, poop. At least for Func you can do this:
Func<object> c = () => valueTypeFunc();
However, such an easy way out can't be used for most cases. Say I have an interface defined as:
interface ICovariant<out T>
{
Func<T> InnerFunc { get; }
}
Now if I have an ICovariant<T>, I can't cast it to ICovariant<object>, and I see no easy way out of it. I know T can be an object - everything can. What can I do in this case? If there's no easy workaround, what else would be the best approach?
You'll have to make a special implementation of your covariant interface to do that conversion for you. Something like this:
public class Boxer<T, U> : ICovariant<T> where U : struct, T
{
public Boxer( ICovariant<U> foo )
{
mFoo = foo;
}
public Func<T> CallMe => () => mFoo.CallMe();
private readonly ICovariant<U> mFoo;
}
This now allows you to wrap value-type implementations of the ICovariant<T> interface. If you find all the generic parameters obnoxious to type, you can create a static method to do the deduction for you:
static void BoxIt<T, U>( IFoo<U> fooU, out IFoo<T> fooT ) where U : struct, T
{
fooT = new Boxer<T, U>( fooU );
}
I would like to determine if a parameter has this modifier using Reflection.I have looked at properties of ParameterInfo class, but couldn't find anything useful.I know the extension methods are just syntactic sugars but I believe there should be a way to determine whether a method is an extension method.
The only thing that distinguishes extension methods from other static methods (that are defined in a static, public class) is this modifier.
For example this is not an extension method:
public static int Square(int x) { return x * x; }
But this is:
public static int Square(this int x) { return x * x; }
So how can I distinguish between two methods using Reflection or something else if possible?
It's not exactly the same, but you can check whether the method has the ExtensionAttribute applied to it.
var method = type.GetMethod("Square");
if (method.IsDefined(typeof(ExtensionAttribute), false))
{
// Yup, it's an extension method
}
Now I say it's not exactly the same, because you could have written:
[Extension]
public static int Square(int x) { return x * x; }
... and the compiler would still pick it up as an extension method. So this does detect whether it's an extension method (assuming it's in a static top-level non-generic type) but it doesn't detect for certain whether the source code had the this modifier on the first parameter.
I'm writing, in C#, an interpreter for a dynamic language, and implementing primitive functions as an abstract class Primitive with a virtual Apply method, where each actual primitive function will be a subclass that overrides Apply.
(An alternative would be to only have the class Primitive and store a function pointer for Apply. However, making it a virtual method seems likely to be slightly faster, and this code will be run very frequently, so a small speedup is worth having.)
Obviously I could go ahead and create a full-blown class file for each primitive function, but I can't help feeling there ought to be a slightly more concise way of doing things than creating dozens of tiny class files.
In Java I'd use the anonymous subclass syntax to create and instantiate a subclass all in one expression, but I don't think C# has an exact counterpart.
What is the best way of doing this in C#?
Firstly, I wouldn't assume that a virtual method call will be faster than a delegate. Maybe it will, maybe it won't - but if performance is really that important to you, you should measure that. It would be really simple to code this using lambda expressions, particularly if all you're trying to represent is a function:
public static readonly Func<int, int> Addition = (x, y) => x + y;
public static readonly Func<int, int> Subtraction = (x, y) => x - y;
// etc
(I'm just guessing at the sorts of operation here, as we don't know the details.)
There's no particularly tiny syntax for subclasses in C#, but for semi-singletons like this
I find nested classes work well... similar to Java enums:
public abstract class Primitive
{
public static readonly Primitive Addition = new AdditionPrimitive();
public static readonly Primitive Subtraction = new SubtractionPrimitive();
// Prevent outside instantiation
private Primitive()
{
}
public abstract int Apply(int x, int y);
// Anything else you want
private class AdditionPrimitive : Primitive
{
public override int Apply(int x, int y)
{
return x + y;
}
}
private class SubtractionPrimitive : Primitive
{
public override int Apply(int x, int y)
{
return x - y;
}
}
}
This question already has answers here:
Equivalent implicit operators: why are they legal?
(2 answers)
Closed 8 years ago.
In C#, why are there no compilation errors when the "same" user-defined conversion exists twice? (once in the source class and once in the target class?)
For example, if I try to compile the following code I get no compilation errors:
namespace TestConversionOverloading
{
public class A
{
public int value = 1;
public static explicit operator B(A a)
{
B b = new B();
b.value = a.value + 6;
return b;
}
}
public class B
{
public int value = 2;
public static explicit operator B(A a)
{
B newB = new B();
newB.value = a.value;
return newB;
}
}
public class program
{
public static void Main() {}
}
}
However, if I try to explicitly convert A to B, I do get a compilation error. Say I add the following to Main() and try to compile:
A a = new A();
B b = ((B)a);
I'll get the following:
Ambiguous user defined conversions
'TestConversionOverloading.A.explicit operator TestConversionOverloading.B(TestConversionOverloading.A)'
and
'TestConversionOverloading.B.explicit
operator
TestConversionOverloading.B(TestConversionOverloading.A)'
when converting from 'TestConversionOverloading.A' to 'TestConversionOverloading.B'
So why not give an error straight from definition? Could there be a way to use either conversion?
I wouldn't speculate on why it makes sense for the language to allow this, but if you are in control of both classes, the obvious solution is to get rid of one of the operators.
If you can't, here's a way to disambiguate using reflection.
First, create a delegate that binds to the intended operator:
// Picks the conversion operator declared in class A.
var method = typeof(A).GetMethod("op_Explicit", new[] { typeof(A) });
var converter = (Func<A, B>)Delegate.CreateDelegate(typeof(Func<A, B>), method);
And then use the delegate as:
A a = ...
B b = converter(a);
According to the spec, this is the expected behavior.
Heavily compressing the original text, here's what happens in this case: the compiler will find all operators that could convert A to B in both class definitions. This would enlist A operator B(A a) and B operator B(A a). Then,
If no such operator exists, or if more than one such operator exists, then the conversion is ambiguous and a compile-time error occurs.
So why not give an error straight from definition? Because both definitions are OK, but it's their use that makes the problem arise.
Could there be a way to use either conversion? I don't see an easy way to do this. I'm thinking of bypassing the compiler, emitting IL by hand. That way I think you can instruct the program to use one operator or the other. Not sure if this is entirely feasible, though. A tool like Reflector could help.
While there's some beauty using operator-based conversions, either one of the classes will lose one operator, or you could change to constructor-based conversions or a more straightforward syntax of ToA(A a) and FromA(A a). Or maybe Eric Lippert could enlighten us with some language cleverness!
Look at the IL code generated per "public static implicit operator B(A a)" code line:
.method public hidebysig specialname static
class TestConversionOverloading.B op_Explicit(class TestConversionOverloading.A a) cil managed
So here is the answer on the first question: Implicit/explicit convertion operators are syntactic sugar. In MSIL they are look like usual methods (and they are). There is nothing criminal when two different classes have methods with identical signature as it does not violate anything. Though conversion operator call cannot be compiled in this case.
And as it was mentioned you can use reflection to get MethodInfo of either method.
Remember that one of the conflicting conversions could be generic, and could be useful for other combinations of generic parameters.
You can even have conflicting conversions defined in the SAME class:
class C<T>
{
implicit operator int() { return 0; }
implicit operator T() { return default(T); }
}
C<int> c;
int i = c;
If the compiler complained about this, you'd lose the ability for C<string> to convert to both string and int.