How can you create a default - non indexer - property in C#?
What I mean by this is I can see that I can create indexer default properties as illustrated on this MSDN page.
This allows me to do things like
Widgets widgets = new Widgets();
Widget result = widgets[1];
But what if I want to achieve something like what Nullable<T> does?
Where you can take
Nullable<decimal> nullDec = 1.23m;
decimal result = nullDec.Value;
OR
decimal result = (decimal)nullDec;
Which I assume is simply a default property implementation to nullDec.Value???
Nullable<T> has special handling in the compiler, but you can do most of that by adding implicit or explicit static conversion operators.
For example, for type Foo you can add an operator:
public static implicit operator string(Foo value)
{
return "abc";
}
public static implicit operator Foo(int value)
{
...
}
allowing:
Foo foo = ..
string s = foo; // uses string(Foo value)
and
int i = 123;
Foo foo = i; // uses Foo(int value)
If you inspect the code of Nullable{T} you will see that the explicit cast implementation is like this:
public static explicit operator T(Nullable<T> value)
{
return &value.Value;
}
So yes you are right.
The way Nullable<T> does it is by providing an explicit conversion operator to T.
So perhaps you are looking for something like:
public static explicit operator Widget(Widgets widgets)
{
// Argument checks here.
return widgets[0];
}
which would let you do:
Widgets widgets = ..
Widget firstWidget = (Widget)widgets;
This does look like a really dodgy and unintuitive API to me so I don't recommend doing this at all. Why not just stick to standard indexers?
Not very sure if I correct understant what you're asking for. Wouldn't be enough to implement cast operators on returning type to achieve what you want?
If it's not what you intend, please explain.
Such kind of features are compiler syntactic sugar (in IL code they all are converted to same low level code), so basically it won't be possible to do this without modifying the C# compiler source.
Related
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.
Is there anything wrong with using an implicit operator like the following:
//linqpad c# program example
void Main()
{
var testObject = new MyClass<int>() { Value = 1 };
var add = 10 + testObject; //implicit conversion to int here
add.Dump(); // 11
}
class MyClass<T>
{
public T Value { get; set; }
public static implicit operator T (MyClass<T> myClassToConvert)
{
return myClassToConvert.Value;
}
}
I was thinking I could treat as instance of the object as a value type this way, but seeing as I've never seen an example of this I thought maybe there was a reason not to do something like this that someone could point out?
In my actual code I was thinking of doing this as part of a data abstraction layer, so that I could return objects with information describing the underlying data, but allow the logic code to treat it as a value type when all it needs to know about is the value, and at the same time keep it all nice and type safe with the generics.
If all of the following are true:
all possible values of your MyClass<T> type (including null if it’s not a value type!) map to a valid value of T
the implicit operator never throws (not even for null!)
the implicit conversion makes semantic sense and is not confusing to the client programmer
then there is nothing wrong with this. Of course you could do any of these three things, but it would be bad design. In particular, an implicit operator that throws can be very hard to debug because the place where it is called doesn’t say that it is being called.
For example, consider that T? has no implicit conversion to T (where T is, of course, a value type). If there was such an implicit operator, it would have to throw when the T? is null, as there is no obvious value to convert null to that would make sense for any value type T.
Let me give an example where I had trouble debugging an issue where the implicit operator threw:
public string Foo()
{
return some_condition ? GetSomething() : null;
}
Here, GetSomething returned something of a type I wrote which has a user-defined implicit conversion to string. I made absolutely sure that GetSomething could never return null, and yet I got a NullReferenceException! Why? Because the above code is not equivalent to
return some_condition ? (string)GetSomething() : (string)null;
but to
return (string)(some_condition ? GetSomething() : (Something)null);
Now you can see where the null came from!
That's a great pattern. Just keep in mind that in order to use it as a variable of type T, you have to either explicitly cast it to T, or assign it to a variable of type T. The cast will take place automatically in method calls and other things (such as your addition example) that take a T.
Implicit conversion without assignment?
So I've written a quick bit of code to help quickly convert between business objects and view models. Not to pimp my own blog, but you can find the details here if you're interested or need to know.
One issue I've run in to is that I have a custom collection type, ProductCollection, and I need to turn that in to a string[] in on my model. Obviously, since there is no default implicit cast, I'm getting an exception in my contract converter.
So, I thought I would write the next little bit of code and that should solve the problem:
public static implicit operator string[](ProductCollection collection) {
var list = new List<string>();
foreach (var product in collection)
{
if (product.Id == null)
{
list.Add(null);
}
else
{
list.Add(product.Id.ToString());
}
}
return list.ToArray();
}
However, it still fails with the same cast exception. I'm wondering if it has something to do with being in reflection? If so, is there anything that I can do here?? I'm open to architectural solutions, too!
First, the implicit operator allows for implcit conversions (no casting directive). explicit operators operate on casts.
Nonetheless, that isn't the real issue here. Operators are not polymorphic (they're overloaded, not overridden); that is, in order to take advantage of an overloaded operator, you must be referring to a class in the context of the class that defines it.
For example:
public class Foo
{
public static implicit operator Bar(Foo foo) { return new Bar(); }
}
public class Bar { }
...
void Baz()
{
Foo foo = new Foo();
Bar bar = foo; // OK
object baz = foo;
bar = baz; // won't compile, there's no defined operator at the object level
bar = (Bar)baz; // will compile, but will fail at runtime for the same reason
}
Without knowing what you're doing and how you're doing it, I can't offer much in the way of a meaningful alternative. Some ideas, however:
Override ToString (or use a common base class / interface for your entities with your own version if you don't want to use ToString) on your entities, then enumerate the collections generically to construct your string array
Provide a ToStringArray function on your collections (by way of a common base class or interface) that does something similar
The biggest point is not to use operators for this purpose; they won't help you.
So a friend was telling me how a game was hacked and how the technique worked. He then asked whats the best way to prevent that kind of attack. The most straight forward way i knew was to A) the shuffle the bits of important value B) hash the values and compare them every time (an int that holds the score or money is likely to be checked rarely).
Then i tried the implementation in C#, i couldnt overload the = operator. How can i do this?
ex code.
class EncryptVal <T>
{
T v;
public T operator = (T v2)
{
//shuffle bits
}
public T operator ()()
{
//return unshuffle bits
}
}
You're looking for the implicit and explicit operator, rather than saying =. This allows you to define how things will work when cast implicitly (ie, just an assignment) and explicitly (ie, there's a casting operator).
public static implicit operator Type1(Type2 p) {}
public static explicit operator Type1(Type2 p) {}
You can encapsulate the value in the class and overload the implicit conversions to and from the class:
public class EncryptVal<T> {
private T _value;
private EncryptVal(T value) {
_value = value;
}
public static implicit operator EncryptVal<T>(T value) {
//shuffle bits
return new EncryptVal<T>(value);
}
public static implicit operator T(EncryptVal<T> value) {
//unshuffle bits
return value._value;
}
}
Usage:
// implicit conversion from int
EncryptVal<int> e = 42;
// implicit conversion to int
int i = e;
You are not allowed to overload the assignment operator in C#. Here's the MSDN documentation on it.
You'll have to create your own function for this behavior.
I assume that you come from C++ where it is very common to write classes that are used like primitive data types. In C# you do things more explicitly.
I would write it as a property or as two methods, eg:
class EncryptVal <T>
{
T v;
public T Value
{
get
{
//return unshuffle bits
}
set
{
//shuffle bits
}
}
}
Dont use = for setting the value. You cant overload assignment.
What you can do is hide it behind a property.
int _encyptedValue;
Public int myInt
{
get
{
return Decrypt(_encryptedValue);
}
set
{
_encryptedValue = Encrypt(value);
}
}
You get to chosse your decryption/encryption
I would go the for implicit/explicit operator overloading for the implementation part.
Probably the explicit one since your conversion does heavy processing, and that it eventually could fail.
I would just add that shuffling bits seems to be only an obfuscation technic that will surely not last long if you have wishfull hackers interested in your game.
You probably need stronger cryptography to protect your data, but more context is needed.
Unlike C++, in C# you can't overload the assignment operator.
I'm doing a custom Number class for arithmetic operations with very large numbers and I want it to have the look-and-feel of the built-in numerical types like int, decimal, etc. I've overloaded the arithmetic operators, but the assignment remains...
Here's an example:
Number a = new Number(55);
Number b = a; //I want to copy the value, not the reference
Is there a workaround for that issue?
you can use the 'implicit' keyword to create an overload for the assignment:
Suppose you have a type like Foo, that you feel is implicitly convertable from a string.
You would write the following static method in your Foo class:
public static implicit operator Foo(string normalString)
{
//write your code here to go from string to Foo and return the new Foo.
}
Having done that, you can then use the following in your code:
Foo x = "whatever";
It's still not at all clear to me that you really need this. Either:
Your Number type should be a struct (which is probable - numbers are the most common example of structs). Note that all the types you want your type to act like (int, decimal etc) are structs.
or:
Your Number type should be immutable, making every mutation operation return a new instance, in which case you don't need the data to be copied on assignment anyway. (In fact, your type should be immutable whether or not it's a struct. Mutable structs are evil, and a number certainly shouldn't be a mutable reference type.)
You won't be able to work around it having the C++ look, since a = b; has other semantics in C++ than in C#. In C#, a = b; makes a point to the same object like b. In C++, a = b changes the content of a. Both has their ups and downs. It's like you do
MyType * a = new MyType();
MyType * b = new MyType();
a = b; /* only exchange pointers. will not change any content */
In C++ (it will lose the reference to the first object, and create a memory leak. But let's ignore that here). You cannot overload the assign operator in C++ for that either.
The workaround is easy:
MyType a = new MyType();
MyType b = new MyType();
// instead of a = b
a.Assign(b);
Disclaimer: I'm not a C# developer
You could create a write-only-property like this. then do a.Self = b; above.
public MyType Self {
set {
/* copy content of value to this */
this.Assign(value);
}
}
Now, this is not good. Since it violates the principle-of-least-surprise (POLS). One wouldn't expect a to change if one does a.Self = b;
Instead of making a copy of the data when passing the reference you could make the class immutable. When the class is immutable having multiple references to it isn't a problem since it can't be changed.
Operations that changes the data would of course return new instances.
An earlier post suggested this:
public static implicit operator Foo(string normalString) { }
I tried this approach... but to make it work you need this:
public static implicit operator Foo(Foo original) { }
and the compiler won't let you have an implicit conversion function from your exact type, nor from any base type of yourself. That makes sense since it would be a backdoor way of overriding the assignment operator, which C# doesn't want to allow.
Here is a solution that worked for myself :
public class MyTestClass
{
private int a;
private string str;
public MyTestClass()
{
a = 0;
str = null;
}
public MyTestClass(int a, string str)
{
this.a = a;
this.str = str;
}
public MyTestClass Clone
{
get
{
return new MyTestClass(this.a, this.str);
}
}
}
Somewhere else in the code :
MyTestClass test1 = new MyTestClass(5, "Cat");
MyTestClass test2 = test1.Clone;
Maybe what you're looking for can be solved using C# accessors.
http://msdn.microsoft.com/en-us/library/aa287786(v=vs.71).aspx