I have a custom class named Optional< T >. This class overrides the implicit operator, something like this:
public static implicit operator Optional<T>(T value)
{
return new Optional<T>(value);
}
public static implicit operator T(Optional<T> value)
{
return value.value;
}
Ok, till now all fine, the direct operations like these work:
Optional<int> val = 33; //ok
int iVal = val; //ok
But, this doesn't work:
Optional<Optional<int>> val = 33; //error, cannot convert implicitly
So I'm wondering how can be supported the previous case.
Cheers.
You cannot chain user-defined implicit operators. If you define an implicit conversion from A to B, and from B to C, there is no implicit conversion from A to C that calls both. You'll either need to create a separate user-defined conversion from A to C, or have an explicit conversion to do half of the conversion for you.
Related
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);
The following operator overload is defined inside my Term class:
public static Term operator *(int c, Term t) {...}
This class also defines an implicit conversion from a Variable to Term:
public static implicit operator Term(Variable var) {...}
I would like to understand why the following does not compile:
static void Main(string[] args)
{
Variable var = ...; // the details don't matter
Console.WriteLine(2 * var); // var isn't implicitly converted to Term...
Console.ReadKey();
}
The compiler says:
Operator '*' cannot be applied to operands of type 'int' and 'OOSnake.Variable'
Why isn't my overload of operator * found?
EDIT: As per the suggestion in the comments, here is a small complete example that re-produces the error:
namespace Temp
{
class A {
}
class B
{
public static implicit operator B(A a) { return new B(); }
public static B operator *(int c, B b) { return new B(); }
}
class Program
{
static void Main(string[] args)
{
Console.WriteLine(2 * new A());
}
}
}
Basically, operator overload resolution doesn't include implicit user-defined conversions in order to find the operators that could be applicable.
From section 7.3.4 of the C# 5 specification:
An operation of the form x op y, where op is an overloadable binary operator, x is an expression of type X, and y is an expression of type Y, is processed as follows:
The set of candidate user-defined operators provided by X and Y for the operation operator op(x, y) is determined. The set consists of the union of the candidate operators provided by X and the candidate operators provided by Y, each determined using the rules of ยง7.3.5. If X and Y are the same type, or if X and Y are derived from a common base type, then shared candidate operators only occur in the combined set once.
And 7.3.5 doesn't include implicit user-defined conversions in its search for a set of operators.
Note that this also wouldn't work if the implicit conversion to Term was declared in the Variable class - although that would be more reasonable to specify and implement, as the compiler could look at the set of conversions from the operand type to other types, and use those for overload resolution.
However, this is only a matter of looking for the operators to start with. The compiler is happy to perform implicit conversions when it considers whether or not an overload is applicable. For example, in your case, if you add:
class A
{
public static B operator *(A a, B b) { return new B(); }
}
Then this is valid:
A a = new A();
Console.WriteLine(a * a);
Console.WriteLine(2 * var) doesn't contain any hint that you would want to convert var to type Term. The compiler sees the int and multiplication operator and a variable of type 'Variable'.
Edit: To clarify, in order for your example to work the compiler would have to go through all the types in scope and see if one just happens to have an implicit conversion from type 'A'.
And if there also happens do be a class C like:
class C
{
public static implicit operator C(A a) { return new A(); }
public static B operator *(int i, C c) { return new C(); }
}
there's no telling what would happen.
Which is why the compiler doesn't do that :)
If you want to make your example to work, you must move the implicit operator that converts A to B to the class A, like this:
namespace Temp
{
class A
{
public static implicit operator B(A a) { return new B(); }
}
class B
{
public static B operator *(int c, B b) { return new B(); }
}
class Program
{
static void Main(string[] args)
{
Console.WriteLine(2 * ((B)new A()));
}
}
}
But you cant use this 2 * new A() because(Jon Skeet answer):
Basically, operator overload resolution doesn't include implicit user-defined conversions in order to find the operators that could be applicable.
https://stackoverflow.com/a/34161798/815590
I have been given a sample statement:
MyClass myclass = 3;
How is it possible to make this a valid statement? What code do I need to include in MyClass to support the implicit conversion from an int?
You need an implicit conversion operator:
public class MyClass
{
private readonly int value;
public MyClass(int value)
{
this.value = value;
}
public static implicit operator MyClass(int value)
{
return new MyClass(value);
}
}
Personally I'm not a huge fan of implicit conversions most of the time. Occasionally they're useful, but think carefully before putting them in your code. They can be pretty confusing when you're reading code.
On the other hand, when used thoughtfully, they can be amazingly handy - I'm thinking particularly of the conversions from string to XName and XNamespace in LINQ to XML.
Here's how:
public class MyClass
{
public static implicit operator MyClass(int i)
{
return new MyClass(i);
}
}
This uses C# feature called implicit conversion operator.
It's possible if MyClass has an implicit conversion from int.
Using Conversion Operators
Just implement an implicit conversion operator.
You need to overload the implicit constructor operator:
public static implicit operator MyClass (int rhs)
{
MyClass c = new MyClass (rhs);
return c;
}
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)"
Let's say you have yourself a class like the following:
public sealed class StringToInt {
private string _myString;
private StringToInt(string value)
{
_myString = value;
} public static implicit operator int(StringToInt obj)
{
return Convert.ToInt32(obj._myString);
}
public static implicit operator string(StringToInt obj)
{
return obj._myString;
}
public static implicit operator StringToInt(string obj)
{
return new StringToInt(obj);
}
public static implicit operator StringToInt(int obj)
{
return new StringToInt(obj.ToString());
}
}
Will you then be able to write code like the following:
MyClass.SomeMethodThatOnlyTakesAnInt(aString);
without it stating that there is no implicit cast from string to int?
[Yes, i could test it myself but i thought i would put it out there and see what all of the gurus have to say]
No C# won't call more than one user defined implicit conversion. From the C# spec section 6.4.3:
Evaluation of a user-defined conversion never involves more than one user-defined or lifted conversion operator. In other words, a conversion from type S to type T will never first execute a user-defined conversion from S to X and then execute a user-defined conversion from X to T.
I am fairly certain this is not possible under C# 3.0. The sections in the reference that covers conversions is 6.4. Namely, 6.4.4 "User-defined implicit conversions".
It only talks about conversions from S->T (and not S->T->U) which covers the cases such as:
StringToInt _t = "foo";
int t = _t;
and
int t = (StringToInt)"foo";
Where both of these cases only involve S->T (twice).
I am pretty sure this is not possible in C# 3.0.
Allowing S->T->U would require much more work to be performed by the type matcher, at least following the algorithm specified.
It does not appear to work. It requires at least one explicit cast. Oh well...
Typos in your snippet:
public StringToInt(string value)
{
_myString = value;
}
public static implicit operator int(StringToInt obj)
{
return Convert.ToInt32(obj._myString);
}
public static implicit operator string(StringToInt obj)
{
return obj._myString;
}
If aString is of type StringToInt, your usage should work.