I have two or more variables of class object in c# which has integer values. i want to overload '+' operator so that i won't have to convert these variables when ever i want to add or subtract them. here is my code below:
public static object operator +( object obj1, object obj2)
{
object o = Convert.toint32(obj1) + Convert.toint32(obj2);
return o;
}
no the problem is i am getting an error saying "One of the parameters of a binary operator must be the containing type"
why is this happening? any help is appreciated!
The compiler error tells you exactly what's wrong - if you're going to create a custom binary operator, at least one of the parameter types (for the operands) has to be the same as the type you're declaring the operator in (or a nullable version of it, for value types).
This is mandated in section 10.10.2 of the C# 4 specification:
The following rules apply to binary operator declarations, where T denotes the instance type of the class or struct that contains the operator declaration:
A binary non-shift operator must take two parameters, at least one of which must have type T or T?, and can return any type.
A binary << or >> operator must take two parameters, the first of which must have type T or T? and the second of which must have type int or int?, and can return any type.
Personally I would try to avoid having variables of type object if you know they're actually int values. Why not have int variables instead?
If you're using C# 4, another alternative would be to make them dynamic variables, where the operator overloading would be applied at execution time rather than compile time.
You can't override the operator for existing classes: only your own classes.
This seems silly/ridiculous, the objects are ints, and because you don't want to cast, you'll make your code less readable by adding objects (which seems crazy)? What happens when you want to do myObj + 1? You'll need to make a massive array of operators and overloads and it's not worth the effort.
Use the correct type in the first place, for the sake of whoever has to read or maintain the code in the future.
You're saying that you want to add two objects of type object, you can't do that.
As the error message says, one of the two parameters to the operator method has to be the type where you defined the operator.
ie. if you declared the operator inside a class with the name XYZ, either obj1 or obj2 has to be of that type.
You cannot declare operator overloads for unrelated types.
Basically, you can't do what you want to do.
The compiler error is telling you exactly the problem.
You can't 'inject' an operator overload into somebody else's types - an operator overload must apply to the type in which you declare it.
The term 'apply' in this case is then explained by the compiler error: one of the parameters must be of that type; the other parameter can be of any type.
So, for example:
public class MyType
{
public static object operator +(MyType obj1, object obj2){
//define code for adding the object to 'MyType' and return object,
//or whatever type you want.
}
}
In this example we are creating an operator between MyType and object and it works because it's defined inside MyType. All binary operator overloads must follow the same rule.
You are defining the operator in a class, let's say class Foo. The compiler tells you that at least one of the two operands must have a static type of Foo, for example:
public static object operator +( Foo obj1, object obj2)
{
object o = Convert.toint32(obj1) + Convert.toint32(obj2);
return o;
}
It's not very clear what you are trying to achieve here. If you have objects of class Foo but their static type is object, then you can't do anything such unless you cast them to Foo first.
Related
I have an explicit operator on the class MyVO, which should be non-nullable.
public class MyVO : ValueObject<MyVO>
{
public string Value { get; } // Should never be null
private MyVO(string v) => Value = v;
public static explicit operator MyVO(string vo)
{
if (string.IsNullOrWhiteSpace(vo)) throw new Exception('...');
return new MyVO(vo);
}
However, (MyVO)null will not raise an exception. The body of the method will not be run.
var myVO = (MyVO)null; // myVO will have the null value
How to make sure it's not null?
How to make sure it's not null?
By "it" I assume you mean "the result of the cast from null to MyVO". If that is not what you mean, please clarify the question.
You cannot.
An important rule of C# is a user-defined conversion never "wins" when it conflicts with a built-in conversion. It is legal to convert null to any class type, and so a cast of MyVO on the expression null will always result in a null reference. The compiler does not even consider the user-defined conversions if a built-in conversion works. (Believe me; I wrote that code!)
As D Stanley's answer correctly points out, if the null is the value of any expression of type string then the user-defined conversion is called; there is no built-in conversion from string to MyVO so the compiler looks for an applicable user-defined conversion and finds one.
Since it hurts when you do what you're doing, you should probably stop doing what you are doing. An explicit conversion is probably not the right way to implement the desired behaviour.
I guess my question should be how to make MyVO not nullable.
Upgrade to C# 8. C# 8 supports non-nullable annotations on reference types.
Note that the non-nullable annotation should be properly thought of as an annotation. The type system does not guarantee that the value of a variable annotated with a non-nullable annotation will never be observed to be null. Rather, it does its best to warn you when the code looks like it is wrong.
While we are looking at your code, I notice that you are using ValueObject<T>, which I assume you have obtained from something like
https://enterprisecraftsmanship.com/posts/value-object-better-implementation/
Let me take this opportunity to caution you that there are pitfalls to using this pattern; the constraint that you think or want to be applied to T is not the constraint that is applied to T. We often see things like this:
abstract class V<T> where T : V<T>
{
public void M(T t) { ... } // M must take an instance of its own type
}
If we have class Banana : V<Banana> then Banana.M takes as its argument a Banana, which is what we want. But now suppose we have class Giraffe : V<Banana>. In this scenario, Giraffe.M does not take a giraffe; it takes a banana, even though Giraffe has no relationship with Banana at all.
The constraint does not mean that M always takes an instance of its own class. If you are trying to construct a generic type with this kind of constraint in C#, you cannot; the C# type system is not rich enough to express that constraint.
null can be implicitly converted to any reference type, so the compiler is not using your explicit cast operator. try
string s = null;
o = (MyVO)s;
or just inline it
o = (MyVO)((string)s);
(Trying to find a title that sums up a problem can be a very daunting task!)
I have the following classes with some overloaded methods that produce a call ambiguity compiler error:
public class MyClass
{
public static void OverloadedMethod(MyClass l) { }
public static void OverloadedMethod(MyCastableClass l) { }
//Try commenting this out separately from the next implicit operator.
//Comment out the resulting offending casts in Test() as well.
public static implicit operator MyCastableClass(MyClass l)
{
return new MyCastableClass();
}
//Try commenting this out separately from the previous implicit operator.
//Comment out the resulting offending casts in Test() as well.
public static implicit operator MyClass(MyCastableClass l)
{
return new MyClass();
}
static void Test()
{
MyDerivedClass derived = new MyDerivedClass();
MyClass class1 = new MyClass();
MyClass class2 = new MyDerivedClass();
MyClass class3 = new MyCastableClass();
MyCastableClass castableClass1 = new MyCastableClass();
MyCastableClass castableClass2 = new MyClass();
MyCastableClass castableClass3 = new MyDerivedClass();
OverloadedMethod(derived); //Ambiguous call between OverloadedMethod(MyClass l) and OverloadedMethod(MyCastableClass l)
OverloadedMethod(class1);
OverloadedMethod(class2);
OverloadedMethod(class3);
OverloadedMethod(castableClass1);
OverloadedMethod(castableClass2);
OverloadedMethod(castableClass3);
}
public class MyDerivedClass : MyClass { }
public class MyCastableClass { }
There are two very interesting things to note:
Commenting out any of the implicit operator methods removes the ambiguity.
Trying to rename the first method overload in VS will rename the first four calls in the Test() method!
This naturally poses two questions:
What it the logic behind the compiler error (i.e. how did the compiler arrive to an ambiguity)?
Is there anything wrong with this design? Intuitively there should be no ambiguity and the offending call should be resolved in the first method overload (OverloadedMethod(MyClass l, MyClass r)) as MyDerivedClass is more closely related to MyClass rather than the castable but otherwise irrelevant MyCastableClass. Furthermore VS refactoring seems to agree with this intuition.
EDIT:
After playing around with VS refactoring, I saw that VS matches the offending method call with the first overload that is defined in code whichever that is. So if we interchange the two overloads VS matches the offending call to the one with the MyCastableClass parameter. The questions are still valid though.
What it the logic behind the compiler error (i.e. how did the compiler arrive to an ambiguity)?
First we must determine what methods are in the method group. Clearly there are two methods in the method group.
Second, we must determine which of those two methods are applicable. That is, every argument is convertible implicitly to the corresponding parameter type. Clearly both methods are applicable.
Third, given that there is more than one applicable method, a unique best method must be determined. In the case where there are only two methods each with only one parameter, the rule is that the conversion from the argument to the parameter type of one must be better than to the other.
The rules for what makes one conversion better than another is in section 7.5.3.5 of the specification, which I quote here for your convenience:
Given a conversion C1 that converts from a type S to a type T1, and a conversion C2 that converts from a type S to a type T2, C1 is a better conversion than C2 if at least one of the following holds:
• An identity conversion exists from S to T1 but not from S to T2
• T1 is a better conversion target than T2
Given two different types T1 and T2, T1 is a better conversion target than T2 if at least one of the following holds:
• An implicit conversion from T1 to T2 exists, and no implicit conversion from T2 to T1 exists
The purpose of this rule is to determine which type is more specific. If every Banana is a Fruit but not every Fruit is a Banana, then Banana is more specific than Fruit.
• T1 is a signed integral type and T2 is an unsigned integral type.
Run down the list. Is there an identity conversion from MyDerivedClass to either MyCastableClass or MyClass? No. Is there an implicit conversion from MyClass to MyCastableClass but not an implicit conversion going the other way? No. There is no reason to suppose that either type is more specific than the other. Are either integral types? No.
Therefore there is nothing upon which to base the decision that one is better than the other, and therefore this is ambiguous.
Is there anything wrong with this design?
The question answers itself. You've found one of the problems.
Intuitively there should be no ambiguity and the offending call should be resolved in the first method overload as MyDerivedClass is more closely related to MyClass
Though that might be intuitive to you, the spec does not make a distinction in this case between a user-defined conversion and any other implicit conversion. However I note that your distiction does count in some rare cases; see my article for details. (Chained user-defined explicit conversions in C#)
What it the logic behind the compiler error?
Well, the compiler determines the signature based on a few things. The number and type of the parameters is next to the name, one of the most important ones. The compiler checks if a method call is ambiguous. It doesn't only use the actual type of the parameter, but also the types it can be implicitly casted to (note that explicit casts are out of the picture, they are not used here).
This gives the issue you describe.
Is there anything wrong with this design?
Yes. Ambiguous methods are a source of a lot of problems. Especially when using variable types, like dynamic. Even in this case, the compiler can't choose which method to call, and that is bad. We want software to be deterministic, and with this code, it can't be.
You didn't ask for it, but I guess the best option is:
To rethink your design. Do you really need the implicit casts? If so, why do you need two methods instead of one?
Use explicit casting instead of implicit casting, to make casting a deliberate choice a compiler can understand.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
C# Implicit/Explicit Type Conversion
I have an int that I am attempting to box, so as to use it in a constructor which takes as a parameter object of CustomType.
An example of what I mean.
int x = 5;
object a = x;
CustomType test = new CustomType(a)
//constructor in question
public CustomType(CustomType a)
{
//set some variables etc
}
Howver I get the following error
The best overloaded method match for X has some invalid arguments.
So obviously I'm way off the mark. Where am I going wrong? Is boxing the right solution or should i be looking at type casting?
That constructor takes a CustomType type as a parameter; you're passing it an int. An int is not a CustomType, and the C# language knows of no implicit conversions from int to CustomType. That's why you're getting an error.
Casting the int to an object doesn't change the fact that it's still not a CustomType.
Looking at that particular constructor, it's a copy constructor. It takes a type of itself as a parameter. There is (one would hope) another constructor that takes another parameter, whether that's an int, or some other type that you haven't mentioned, or possibly no parameters and you need to just set a property after creating a default object.
As for actual solutions, there are many. Here are a few:
Add an additional constructor to CustomType that accepts an int.
Define a function that takes an int as a parameter and returns a CustomObject; this would be a "conversion" function that you could use.
Use a default constructor of CustomType and set a property with your integer (may or may not be applicable, depending on what CustomType does.
Boxing doesn't seem to be related to the issue at hand. It's not something that you should use to pass an integer to a custom type. If you would like to know more about what boxing is, how it works, or when it's useful then consider asking a question that addresses those points because this particular problem would be harmed by using boxing as a solution, not helped.
I don't think boxing or casting is what you're looking for. If you want to pass an integer into your constructor, your constructor should be defined like this:
public CustomType(int a) {
}
Or if you want it to be an object.
public CustomType(object a) {
}
Your constructor looks like a Copy-constructor (to copy an object).
if you want to pass an object variable to your CustomType constructor, then the syntax has to be:
public CustomType(Object a)
{
//Unboxing
int value = (int) a;
}
You should use Explicit Type Conversion and implement some operators for that.
Use Generics.
public CustomType<T>(T a)
{
//set some variables etc
}
You can box int to object but not to an arbitrary type and you cannot take something out of the box that is not in the box. I.e. int i = (int)(object)5; will work, but CustomType x = (CustomType)(object)5; will not work.
You would need a constructor accepting an int
public CustomType(int a) {
....
}
By the way, you created strange recursion. If the only constructor requires a CustomType then you need another CustomType object for the initialization of the first one
CustomType ct = new CustomType(new CustomType(new CustomType(null)));
Boxing is what happens when you convert a (value of a) value type to either
a direct or indirect base class (in the case of int that could be the ValueType class or the object class
an interface that the value type implements (in the case of int that could be either IComparable, IFormattable, IConvertible, IComparable<int>, or IEquatable<int>)
Since you can never make int have other base classes or implement more interfaces, it's not possible to box an int into CustomType.
If you write your own struct then you decide what interfaces it implements, and in that case values of your struct can be boxed into each of those interface types. (You can't change the base classes of a struct, though.)
Inside your class/struct CustomType it is possible to define an implicit conversion from int to CustomType. In this way, it will be legal to say
CustomType ct = 5;
But it's not boxing. It's just an "invisible" call to your implicit operator method. The original object 5 is not put into a box; it is just the argument to your operator method.
Why is this a compile time error?
public TCastTo CastMe<TSource, TCastTo>(TSource i)
{
return (TCastTo)i;
}
Error:
Cannot convert type 'TSource' to 'TCastTo'
And why is this a runtime error?
public TCastTo CastMe<TSource, TCastTo>(TSource i)
{
return (TCastTo)(object)i;
}
int a = 4;
long b = CastMe<int, long>(a); // InvalidCastException
// this contrived example works
int aa = 4;
int bb = CastMe<int, int>(aa);
// this also works, the problem is limited to value types
string s = "foo";
object o = CastMe<string, object>(s);
I've searched SO and the internet for an answer to this and found lots of explanations on similar generic related casting issues, but I can't find anything on this particular simple case.
Why is this a compile time error?
The problem is that every possible combination of value types has different rules for what a cast means. Casting a 64 bit double to a 16 bit int is completely different code from casting a decimal to a float, and so on. The number of possibilities is enormous. So think like the compiler. What code is the compiler supposed to generate for your program?
The compiler would have to generate code that starts the compiler again at runtime, does a fresh analysis of the types, and dynamically emits the appropriate code.
That seems like perhaps more work and less performance than you expected to get with generics, so we simply outlaw it. If what you really want is for the compiler to start up again and do an analysis of the types, use "dynamic" in C# 4; that's what it does.
And why is this a runtime error?
Same reason.
A boxed int may only be unboxed to int (or int?), for the same reason as above; if the CLR tried to do every possible conversion from a boxed value type to every other possible value type then essentially it has to run a compiler again at runtime. That would be unexpectedly slow.
So why is it not an error for reference types?
Because every reference type conversion is the same as every other reference type conversion: you interrogate the object to see if it is derived from or identical to the desired type. If it's not, you throw an exception (if doing a cast) or result in null/false (if using the "as/is" operators). The rules are consistent for reference types in a way that they are not for value types. Remember reference types know their own type. Value types do not; with value types, the variable doing the storage is the only thing that knows the type semantics that apply to those bits. Value types contain their values and no additional information. Reference types contain their values plus lots of extra data.
For more information see my article on the subject:
http://ericlippert.com/2009/03/03/representation-and-identity/
C# uses one cast syntax for multiple different underlying operations:
upcast
downcast
boxing
unboxing
numeric conversion
user-defined conversion
In generic context, the compiler has no way of knowing which of those is correct, and they all generate different MSIL, so it bails out.
By writing return (TCastTo)(object)i; instead, you force the compiler to do an upcast to object, followed by a downcast to TCastTo. The compiler will generate code, but if that wasn't the right way to convert the types in question, you'll get a runtime error.
Code Sample:
public static class DefaultConverter<TInput, TOutput>
{
private static Converter<TInput, TOutput> cached;
static DefaultConverter()
{
ParameterExpression p = Expression.Parameter(typeof(TSource));
cached = Expression.Lambda<Converter<TSource, TCastTo>(Expression.Convert(p, typeof(TCastTo), p).Compile();
}
public static Converter<TInput, TOutput> Instance { return cached; }
}
public static class DefaultConverter<TOutput>
{
public static TOutput ConvertBen<TInput>(TInput from) { return DefaultConverter<TInput, TOutput>.Instance.Invoke(from); }
public static TOutput ConvertEric(dynamic from) { return from; }
}
Eric's way sure is shorter, but I think mine should be faster.
The compile error is caused because TSource cannot be implicitly cast to TCastTo. The two types may share a branch on their inheritance tree, but there is no guarantee. If you wanted to call only types that did share an ancestor, you should modify the CastMe() signature to use the ancestor type instead of generics.
The runtime error example avoids the error in your first example by first casting the TSource i to an object, something all objects in C# derive from. While the compiler doesn't complain (because object -> something that derives from it, could be valid), the behaviour of casting via (Type)variable syntax will throw if the cast is invalid. (The same problem that the compiler prevented from happening in example 1).
Another solution, which does something similar to what you're looking for...
public static T2 CastTo<T, T2>(T input, Func<T, T2> convert)
{
return convert(input);
}
You'd call it like this.
int a = 314;
long b = CastTo(a, i=>(long)i);
Hopefully this helps.
Here is my simple code:
T a;
T b;
if (a == b)
// sth.
else
// sth. else
When I try to compile, I get an error saying the == operator is invalid for generic types. So I have to use the object.Equals() method.
Doesn't the == operator actually call the Equals method of object? Why can I use the Equals method of two generic types but not the == operator?
operator == must be overloaded in structs in order to be used, hence a completely unconstrained type parameters cannot use it. You can constrain the function to class to allow default reference comparison:
public void Foo<T>() where T : class {
T a = default(T);
T b = a;
if(a == b)
System.Diagnostics.Debug.WriteLine("");
else
System.Diagnostics.Debug.WriteLine("");
}
The above code works because by default, reference types can be used with operator ==:
For reference types other than string,
== returns true if its two operands refer to the same object.
This is why if (new object() == new object()) {} compiles, even though System.Object doesn't overload operator ==.
The == operator is not defined all possible values of T [thanks Daniel] (or on any constraints you may have placed on T, I assume), so you can't use it. You can only call operators, methods, properties on T that can be called on ALL possible types represented by T.
operator == calls 'Equals' in many cases, but that doesn't mean they are the same thing.
The == token is used to represent two different operators in C#. The first of them is applicable only if the types of the operands fit a particular defined overload of the "equality-check" operator. The second test reference equality and is applicable only if one operand is null, one operand is a class type and the other an interface that instances of that type could implement, both are interfaces, both are the same class types, or both are class types and one is a supertype of the other. The first form will not be usable on a generic unless the generic is constrained to a type for which an equality-check overload is defined; the second form is limited to reference types which are known to satisfy a particular one of the indicated conditions. Note that in some cases, the second operator may be used where the first would be intended, e.g. bool IsSame<T>(T p1, T p2) where T:class { return p1==p2; } will compare two variables of type String by using a reference comparison, even though an overload of == is defined for String. This occurs because T isn't well-enough known to apply a String overload of ==, but both operands of == are known to be the same reference type.
It may be worth noting that some other languages use different tokens for the two operations C# performs using ==. VB.NET, for example, uses = for equality comparison and Is for reference equality.