I have a struct type in C#. I want to be able to convert null implicitly to this type. For instance, null could be represented by a special value of the struct type and the cast operator should return a struct with this value.
In C++, I could use an implicit cast operator overload of type std::nullptr_t. Is there a comparable type in C#?
I have had the idea to use a special NullType class which has no instances. This works but looks somehow ugly. Is there a better way?
Example:
class NullType
{
private NullType(){} // ensures that no instance will ever be created
}
struct X
{
private static readonly int nullValue = -1;
private int val;
public X(int val){ this.val= val; }
public static implicit operator X(NullType t)
{
return new X(nullValue);
}
}
class MainClass
{
public static void Main(string[] args)
{
X x = null; // Works now!
}
}
No, conversion operators only allow conversion from types, not from a specific value of a type, aka null, so there is no null conversion operator.
The best fitting operator without introducing own types would be this:
public static implicit operator X(object t)
But obviously you don't want to use this. It isn't very safe to use (t can be any value, and the only way to handle non-null cases in an exception).
That said, I think the way you've created it now, using a class that can't be initialized is the best way to do this. In fact the only question is: why do you want to use null, instead of 'just' an default value instance on your struct (X.Null).
Related
Looking Zoran Horvats courses at PluralSight, I'm currently implementing a Maybe type, a bit like Zoran has on its GitHub account: https://github.com/zoran-horvat/option
Generally, a Maybe is a wrapper around objects, which are either set or have a null value, avoiding null reference exceptions.
To make the code a bit shorter, I would like to use implicit conversion to map the values / nulls to their corresponding maybe types. Here an example of my code:
public void Hook(Maybe<Action<Keys>> onKeyDown, Maybe<Action<Keys>> onKeyUp)
{
_keyDownCallback = onKeyDown;
_keyUpCallback = onKeyUp;
_hookService.Hook(HookType.KeyBoardLowLevel, OnHookReceived);
}
As you can see, you can hook and pass two optional callbacks, one for keyDown and one for keyUp. I would like to pass code like this:
nativeKeyboardHookService.Hook(new Action<Keys>(OnNativeKeyDown), null);
The implicit conversion on the Maybe is currently implemented like this:
public static implicit operator Maybe<T>(T value)
{
return ToMaybe(value);
}
public static implicit operator T(Maybe<T> maybe)
{
return ToT(maybe);
}
public static Maybe<T> ToMaybe(T value)
{
if (value == null)
{
return new None<T>();
}
return new Some<T>(value);
}
public static T ToT(Maybe<T> maybe)
{
return maybe.Evaluate(
value => value,
() => default(T));
}
My question: It works fine, if I pass an actual object, mapping it to an Maybe, but if I pass NULL, I still get a NULL object, not a None object. Am I doing here something wrong or is it just not possible? I didn't find any further information regarding such a conversion.
When you pass null to Hook() that's literally all you are doing because your implicit casts aren't being invoked at all. That's because null is a valid value for a reference type, and thus no need to cast.
You can't change Maybe to a struct if you want to keep Some and None because then these would have to be structs too, which means you run into the issue that you can't inherit structs.
You can't implement a common IMaybe<T> interface either because interfaces can't be used with casts.
What I recommend is keep your behavior as is, but don't use null. Instead of passing null, pass something else like Maybe<T>.None:
class Maybe<T>
{
public static Maybe<T> None { get; } = new None<T>();
}
void Hook(..., Maybe<T>.None) { ... }
Or None<T>.Instance:
class None<T>
{
public static None<T> Instance{ get; } = new None<T>();
}
void Hook(..., None<T>.Instance) { ... }
This has the advantage of being more readable and explicit.
Your Maybe<T> is still a reference type, so null is a valid value for it:
Maybe<string> foo = null;
If you want to prevent that, you will need to make it a value type, for example something like this:
public struct Maybe<T>
{
public T Value { get; }
public bool IsEmpty => Value == null;
public Maybe(T value)
{
Value = value;
}
public static implicit operator Maybe<T>(T value)
{
return new Maybe<T>(value);
}
}
Then you can pass null to a method expecting a Maybe<T> and it will properly construct an empty Maybe<T> object.
But note that it being a value type, this now means that it is copied on every method call, so it has a different behavior to a reference type implementation.
In the end, you cannot really implement this nicely in C# simply because there is the null reference in the language. It’s only with C# 8’s nullable reference types that you will be able to prevent nulls altogether.
Assumptions
Let's assume we have a class like so...
public class SomeClass
{
public int Value { get; set; }
public static implicit operator SomeClass(int value)
{
return new SomeClass() { Value = value };
}
}
And then we're simply using an assignment, like so...
SomeClass mySpecialObj = 1; /* mySpecialObj.Value is now 1 */
Scenario
Now, if I assign a value to an object that already has a reference, e.g.
SomeClass mySpecialObj = 1; // 1st assignment
mySpecialObj = 2; // 2nd assignment
Question
On the 2nd assignment, is it possible to check the current value within the assignment operator?
public static implicit operator SomeClass(int value)
{
// can I check mySpecialObj.Value here??
}
You're not overloading the assignment operator, you're creating an implicit conversion from int to SomeClass. These are two very distinct things.
As such, mySpecialObj = 2 effectively means:
Create a new instance of SomeClass using the integer 2
And then assign that instance to the variable mySpecialObj
or,
mySpecialObj = new SomeClass{ Value = 2};
By now it should be clear that the answer is no, the conversion operation cannot know the value of the variable the new instance is going to be assigned to (if it is assigned to a variable).
You cannot do this in C#. The reason for this is that you are not overloading the assignment operator, you are defining an implicit conversion operator, which is not the same thing.
When you do this
mySpecialObj = 2;
C# creates a new, independent, object using the body of your conversion operator. By the time the reference is re-assigned, your code has already finished execution. Your code has no access to the variable being assigned, because that variable does not participate in the conversion defined by your operator. In fact, your operator can be used outside the context of an assignment - for example, if you do this
private static void Foo(SomeClass s) {
Console.WriteLine(s.Value);
}
the presence of the implicit conversion operator lets you do this:
Foo(123);
Now SomeClass produced from 123 is not assigned to any variable at all.
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)"
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.