I've distilled this question down to the simplest code sample I can think of. Why is the explicit operator not invoked from the generic method?
class Program
{
static void Main(string[] args)
{
A a = new A();
B b = (B)a; // works as expected
b = Cast<B>(a);
}
static TResult Cast<TResult>(object o)
{
return (TResult)o; // throws Invalid Cast Exception
}
}
class A
{
}
class B
{
public static explicit operator B(A a)
{
return new B();
}
}
Because a generic method has one set of IL, based around TResult. It doesn't do different IL per-caller. And the generic TResult does not have any operators.
Also: the operator here would need to be between object and TResult, and you can't define operators involving object.
For example: Cast<int> and Cast<string> and Cast<Guid> all have the exact same IL.
You can cheat with dynamic:
return (TResult)(dynamic)o;
What it comes down to is that the implicit and explicit operators aren't true conversion operators; they're entirely compile time syntactic sugar. Once the code is compiled nothing about the conversion operators remains.
When the compiler sees:
B b = (B) new A();
It says, "is there any native conversion (implicit or explicit) from an A to a B?" (This would be the case if A extended B, for example), or for one of the few special cased language conversions such as double to int (implicit) or int to double (explicit).)
If not, it then looks for user defined conversion operators (it only looks in the definition of A and B, it doesn't look in the definition of C for an implicit conversion from A and to B, for example). If it finds one, then it injects that operator as a static method call, so the code ends up looking like:
B b = B.SomeAutogneratedName(new A());
That way by the time you get to runtime it's just executing another method, something the runtime knows how to do. The only actual runtime conversion operators that are allowed are the handful baked into the language (i.e. from any base type to a parent type, and between certain primitive types).
Related
I have an own class and I need to use a bunch of other classes that I cannot modify. I have written implicit conversions from my own type to the other types:
public class MyClass
{
public static implicit operator YourClass(MyClass m) => new YourClass();
public static implicit operator HisClass(MyClass m) => new HisClass();
}
(In reality all classes have members, but I omit them here in favour of brevity.)
Now I can do this:
var mine = new MyClass();
YourClass yours = mine; // implicit conversion takes place
So far, so good.
Now I want to write a conversion method from some generic class (that I cannot modify) instance of MyType to any of the abovementioned other classes, so that I can do this:
var listOfMine = new List<MyClass>();
List<YourClass> listOfYours = Convert<YourClass>(listOfMine);
(In reality the generic class in question is not List.)
I'd love to write it like this:
public static List<T> Convert<T>(List<MyClass> listOfMine)
where T : MyClass_has_implicit_conversion_to
{
List<T> listOfT = new List<T>();
foreach (var mine in listOfMine)
listOfT.Add(mine); // implicit conversion takes place
return listOfT;
}
, i.e. tell the compiler: T can be any class for which there is an implicit conversion from MyClass to T.
But since such a type constraint does not exist, I cannot tell the compiler that a conversion from MyClass to T is legal. So it complains:
compiler error CS1503: Argument 'mine' cannot convert from MyClass to T.
So what do I do?
I know I can pacify the compiler by doing
listOfT.Add((T)(object)mine);
, but then I get a runtime error - my implicit conversion is never called in that case.
If I need to do the conversion in a different way (i.e. explicit conversion, method like ToYourClass, ..), I can live with that.
But I need the conversion outside of the Convert<T> method, so that I do not need to repeat myself if I create several such Convert<T> methods (e.g. one for List<T>, one for T[], one for Tuple<int,T>, ...).
Charles Mager's comment
Your convert method can't be done as the compiler doesn't know of any relationship between the two types. So you need to specify a function to convert between them.
gave me the following idea:
public static List<T> Convert<T>(List<MyClass> listOfMine, Func<MyClass, T> Convert)
{
List<T> listOfT = new List<T>();
foreach (var mine in listOfMine)
listOfT.Add(Convert(mine)); // manual conversion done here
return listOfT;
}
That's quite alright, I can live with that.
Usage:
var listOfMine = new List<MyClass>();
List<YourClass> listOfYours = Convert<YourClass>(listOfMine, x => x);
x => (YourClass)x would be more explicit but not necessary since I have an implicit conversion defined. I could also define and use any Func<MyClass, YourClass> / Func<MyClass, HisClass> instead of implicit or explicit conversions.
Casting mine to dynamic is a nice idea as well (thanks Charlieface), but it's a source of errors due to missing compiler checks, so I'd like to avoid it if there are other ways.
Some code for context:
class a
{
}
class b
{
public a a{get;set;}
public static implicit operator a(b b)
{
return b.a;
}
}
a a=null;
b b=null;
a = b;
//compiler: cannot apply operator '==' to operands of type tralala...
bool c = a == b;
Is it possible to use == operator on different type instances, where one can implicitly convert to another? What did i miss?
Edit:
If types must be the same calling ==, then why
int a=1;
double b=1;
bool c=a==b;
works?
The implicit operator only works for assignment.
You want to overload the equality (==) operator, as such:
class a
{
public static bool operator ==(a x, b y)
{
return x == y.a;
}
public static bool operator !=(a x, b y)
{
return !(x == y);
}
}
class b
{
public a a{get;set;}
public static implicit operator a(b b)
{
return b.a;
}
}
This should then allow you to compare two objects of type a and b as suggested in your post.
var x = new a();
var y = new b();
bool c = (x == y); // compiles
Note:
I recommmend simply overriding the GetHashCode and Equals method, as the compiler warns, but as you seem to want to supress them, you can do that as follows.
Change your class declaration of a to:
#pragma warning disable 0660, 0661
class a
#pragma warning restore 0660, 0661
{
// ...
}
Is it possible to use == operator on
different type instances, where one
can implicitly convert to another?
Yes.
What did i miss?
Here's the relevant portion of the specification. You missed the highlighted word.
The predefined reference type equality
operators require [that] both operands
are reference-type values or the
literal null. Furthermore, a standard
implicit conversion exists from the
type of either operand to the type of
the other operand.
A user-defined conversion is by definition not a standard conversion. These are reference types. Therefore, the predefined reference type equality operator is not a candidate.
If types must be the same calling ==,
then why [double == int] works?
Your supposition that the types must be the same is incorrect. There is a standard implicit conversion from int to double and there is an equality operator that takes two doubles, so this works.
I think you also missed this bit:
It is a compile-time error to use the
predefined reference type equality
operators to compare two references
that are known to be different at
compile-time. For example, if the
compile-time types of the operands are
two class types A and B, and if
neither A nor B derives from the
other, then it would be impossible for
the two operands to reference the same
object. Thus, the operation is
considered a compile-time error.
I would imagine that you need to actually override the == operator for the types you are interested in. Whether the compile/runtime will still complain even if the types are implicity convertable is something you'll have to experiment with.
public static bool operator ==(a a, b b)
{
//Need this check or we can't do obj == null in our Equals implementation
if (((Object)a) == null)
{
return false;
}
else
{
return a.Equals(b);
}
}
Alternatively just use Equals implementations like ole6ka suggests and ensure that the implementation does the type casting you need.
http://msdn.microsoft.com/en-us/library/8edha89s.aspx
In each case, one parameter must be
the same type as the class or struct
that declares the operator (...)
Use this
bool c = a.Equals(b);
i want to understand why the C# language decided to make this test expression as an error.
interface IA { }
interface IB { }
class Foo : IA, IB { }
class Program
{
static void testFunction<T>(T obj) where T : IA, IB
{
IA obj2 = obj;
if (obj == obj2) //ERROR
{
}
}
static void Main(string[] args)
{
Foo myFoo = new Foo();
testFunction(myFoo);
Console.ReadLine();
}
}
In the testFunction, i can make an object called obj2 and set it to obj implicitly without casting it. But why cant i then check the two objects to see if they are the same, without casting it? They obviously implement the same interface, so why is it an error?
You can check to see if they're the same object by using Object.ReferenceEquals or Object.Equals.
However, since your constraints (IA and IB interfaces) don't enforce that the type is necessarily a reference type, there's no guarantee that the equality operator can be used.
Suppose you construct T with a value type X that implements IA.
What does
static void testFunction<T>(T obj) where T : IA
{
IA obj2 = obj;
if (obj == obj2) //ERROR
do when called as testFunction<X>(new X(whatever)) ?
T is X, X implements IA, so the implicit conversion boxes obj to obj2.
The equality operator is now comparing a value type X with a boxed copy of compile-time type IA. That the runtime type is a boxed X the compiler does not care about; that information is ignored.
What comparison semantics should it use?
It cannot use reference comparison semantics because that would mean that obj would also have to be boxed. It won't box to the same reference, so this would always be false, which seems bad.
It cannot use value comparison semantics because the compiler has no basis upon which kind of value semantics it should use! At compile time it does not know whether the type chosen for T in the future will provide an overloaded == operator or not, and even if it does, that operator is unlikely to take an IA as one of its operands.
There are no equality semantics that the compiler can reasonably choose, and therefore this is illegal.
Now, if you constrain T to be a reference type then the first objection goes away, and the compiler can reasonably choose reference equality. If that's your intention then constrain T to be a reference type.
To expand a bit on Reed's answer (which is certainly correct):
Note that the following code results in the same error at compile time:
Guid g = Guid.NewGuid(); // a value type
object o = g;
if (o == g) // ERROR
{
}
The C# language specification says (§7.10.6):
The predefined reference type equality operators are:
bool operator ==(object x, object y);
bool operator !=(object x, object y);
[...]
The predefined reference type equality operators require one of the following:
Both operands are a value of a type known to be a reference-type or the literal null.
Furthermore, an explicit reference conversion (§6.2.4) exists from the type of either operand to the type of the other operand.
One operand is a value of type T where T is a type-parameter and the other operand is the literal null. Furthermore T does not have the value type constraint.
[...]
Unless one of these conditions are true, a binding-time error occurs. Notable implications of these rules are:
[...]
The predefined reference type equality operators do not permit value type operands to be compared. Therefore, unless a struct type declares its own equality operators, it is not possible to compare values of that struct type.
The predefined reference type equality operators never cause boxing operations to occur for their operands. It would be meaningless to perform such boxing operations, since references to the newly allocated boxed instances would necessarily differ from all other references.
Now, in your code example you do not constrain T to be a reference type and hence you get the compile-time error. Your sample can be fixed however by declaring that T must be a reference type:
static void testFunction<T>(T obj) where T : class, IA, IB
{
IA obj2 = obj;
if (obj == obj2) // compiles fine
{
}
}
Try
if (obj.Equals(obj2))
IA doesn't implement any == operator.
Ahh thanks Reed Copsey for that note.
I just also found out that you can put "class" in the where clause like this.
static void testFunction<T>(T obj) where T : class, IA, IB
{
IA obj2 = obj;
if (obj == obj2)
{
}
}
Now its a reference type and it works! :-)
I have a generic struct and two synonyms with defined generic type parameter. I want to define type conversions between these two types like this:
using X = A<int>;
using Y = A<long>;
struct A<T>
{
private T data;
public A(T data)
{
this.data = data;
}
public static explicit operator X(Y value)
{
return new X((int)value.data);
}
public static implicit operator Y(X value)
{
return new Y(value.data);
}
}
and here is an example how I want to use the struct:
class Program
{
static void Main(string[] args)
{
X x = new X(1);
Y y = new Y(2);
x = (X)y; // explicit cast with precision loss
y = x; // implicit cast without precision loss
}
}
Unfortunately each "using" creates new specific definition of the struct and C# compiler treats synonym structures not as a subset of a generic structure, but as an individual structure. That's why compiler reports errors:
"ambigious between X.static implicit operator Y(X) and Y.static implicit operator Y(X)".
"User-defined conversion must convert to or from the enclosing type".
Does anybody knows a way to realize this type conversions without changing type to class?
UPDATE: I'm using .NET 4.
The error
User-defined conversion must convert to or from the enclosing type.
means that your conversion operators need to convert to/from A<T>. Yours are converting to/from A<int/string>. That's not the same thing (much less general at least).
So this cannot work. You have to find some other way to do the conversions. Maybe runtime casting can help here (define the operators as acting on A<T> and do casting inside of them).
I think this problem is unrelated to the type synonyms. In fact they made the question harder to understand.
I have (for example) an object of type A that I want to be able to cast to type B (similar to how you can cast an int to a float)
Data types A and B are my own.
Is it possible to define the rules by which this casting occurs?
Example
int a = 1;
float b = (float)a;
int c = (int)b;
Yes, this is possible using C# operator overloading. There are two versions explicit and implicit.
Here is a full example:
class Program
{
static void Main(string[] args)
{
A a1 = new A(1);
B b1 = a1;
B b2 = new B(1.1);
A a2 = (A)b2;
}
}
class A
{
public int Foo;
public A(int foo)
{
this.Foo = foo;
}
public static implicit operator B(A a)
{
return new B(a.Foo);
}
}
class B
{
public double Bar;
public B(double bar)
{
this.Bar = bar;
}
public static explicit operator A(B b)
{
return new A((int)b.Bar);
}
}
Type A can be cast implicitly to type B but type B must be cast explicitly to type A.
Assuming you want that to be an explcit operation you'll need to write an explicit cast operator like so:
public static explicit operator MyTypeOne(MyTypeTwo i)
{
// code to convert from MyTypeTwo to MyTypeOne
}
You can then use it like so:
MyTypeOne a = new MyTypeOne();
MyTypeTwo b = (MyTypeTwo)a;
I'd question whether you want to actually cast one type to another, or whether you actually want to convert instead. I'd say you should avoid writing cast operators for conversions, if you are just aiming to take advantage of a nice syntax :)
Also, in general it is advised not to use implicit casts, as they allow for unintended type converstions. From MSDN documentation on implicit:
However, because implicit conversions
can occur without the programmer's
specifying them, care must be taken to
prevent unpleasant surprises. In
general, implicit conversion operators
should never throw exceptions and
never lose information so that they
can be used safely without the
programmer's awareness.
You cant overload the cast operator in c# but you can use explicit and implicit conversion operators instead:
"Using Conversion Operators (C# Programming Guide)"