I want to overload division operator in my C# class. So, i wrote:
public string[] operator/ (object obj) {
}
And got error: "Parser error: Overloadable unary operator excepted".
So, i cant overload that operator?
On the MSDN i don't see any example: http://msdn.microsoft.com/en-us/library/3b1ff23f.aspx
Thanks.
//i'm using MonoDevelop on Ubuntu 14.10, if it's needed.
You can overload the division operator, but:
It must always be a binary operator - you've only provider one operand
It must always be static
At least one of the operand types must be the type you're declaring it in
So for example:
using System;
class Program
{
public static string operator/ (Program lhs, int rhs)
{
return "I'm divided!";
}
static void Main(string[] args)
{
Console.WriteLine(new Program() / 10);
}
}
The / operator is a "binary" operator, meaning that it takes two arguments: a / b. The way you've written this, it's trying to overload it as a "unary" operator, such as a++.
Something like this ought to work, assuming your class is called "MyObject".
public static string[] operator/ (MyObject mine, object obj) {
...
}
Usage:
object something = "hi";
string[] result = new MyObject(1) / something;
Related
I'm experimenting with custom integer types and came across an interesting issue involving generics, implicit conversions, and 32 bit integers.
Below is a stripped down example of how to reproduce the problem. If I have two implicit methods that convert int to MyInt and vice versa, I get a compilation error which looks like C# can't resolve which generic type to use. And it only happens with int or uint. All other integer types work fine: sbyte,byte,short,ushort,long,ulong.
If I remove one of the implicit conversion methods, it also works fine. Something to do with circular implicit conversions?
using Xunit;
public class MyInt
{
public int Value;
//If I remove either one of the implicit methods below, it all works fine.
public static implicit operator int(MyInt myInt)
{
return myInt.Value;
}
public static implicit operator MyInt(int i)
{
return new MyInt() { Value = i };
}
public override bool Equals(object obj)
{
if (obj is MyInt myInt)
{
return this.Value == myInt.Value;
}
else
{
int other_int = (int)obj;
return Value == other_int;
}
}
}
Below is the test code showing the compilation errors I get when both implicit methods are defined.
public class Test
{
[Fact]
public void EqualityTest()
{
MyInt myInt = new MyInt();
myInt.Value = 4 ;
Assert.Equal(4, myInt.Value); //Always OK which makes sense
//Compile errors when both implicit methods defined:
// Error CS1503 Argument 1: cannot convert from 'int' to 'string',
// Error CS1503 Argument 2: cannot convert from 'ImplicitConversion.MyInt' to 'string'
Assert.Equal(4, myInt);
}
}
I believe C# is complaining about not being able to convert both types to string as that is the type of the last Xunit.Assert.Equal() overload and all the others failed to match:
//Xunit.Assert.Equal methods:
public static void Equal<T>(T expected, T actual);
public static void Equal(double expected, double actual, int precision);
public static void Equal<T>(T expected, T actual, IEqualityComparer<T> comparer);
public static void Equal(decimal expected, decimal actual, int precision);
public static void Equal(DateTime expected, DateTime actual, TimeSpan precision);
public static void Equal<T>(IEnumerable<T> expected, IEnumerable<T> actual, IEqualityComparer<T> comparer);
public static void Equal<T>(IEnumerable<T> expected, IEnumerable<T> actual);
public static void Equal(string expected, string actual, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false);
public static void Equal(string expected, string actual);
I don't think I've made a mistake with the implicit conversions as I can make other similar examples create the same problem when used with 32 bit ints.
I'm testing in a .NET Core 3.0 project.
Any help would be appreciated. Thanks!
Clarification:
What I would like to know is why this only fails with 32 bit integers. Implicit conversions are working (confirmed with debugging) when the types are anything else like the example below using a long.
using Xunit;
public class MyLong
{
public long Value;
public static implicit operator long(MyLong myInt)
{
return myInt.Value;
}
public static implicit operator MyLong(long i)
{
return new MyLong() { Value = i };
}
public override bool Equals(object obj)
{
if (obj is MyLong myInt)
{
return this.Value == myInt.Value;
}
else
{
long other_int = (long)obj;
return Value == other_int;
}
}
}
public class Test2
{
[Fact]
public void EqualityTest()
{
MyLong myLong = new MyLong();
myLong.Value = 4 ;
Assert.Equal(4, myLong); //NOTE! `4` is implicitly converted to a MyLong
//object for comparison. Confirmed with debugging.
}
}
Something to do with circular implicit conversions?
Yes (though, you've already demonstrated that much by showing that it works fine when one of the conversions is eliminated).
The reason this is happening with int, and not with other types, is that the type of your literal is int. This means that during overload resolution, the compiler can go either way: convert int to MyInt, or convert MyInt to int. Neither option is clearly "better" than the other, so neither of those conversions survive consideration.
Then, having ruled out the closest possible generic version of the method, of the remaining overloads available the only one left is the Equal(string, string) overload (the only other one left with just two parameters is the Equal<T>(IEnumerable<T>, IEnumerable<T>), which is "worse" than the Equal(string, string) overload according to the overload resolution rules). Having found exactly one method that is clearly "better" than any others, the compiler then tries to use that method with your parameters, which of course don't fit, causing the errors to be emitted.
On the other hand…
When you try to call Equal(4, myLong), you've got two incompatible types. A literal having type int, and the MyLong value. In this case, the compiler tries each parameter one by one and finds that when it uses the MyLong type as the type parameter, it is possible to promote the int literal to a long and then implicitly convert that to MyLong. But it can't go the other way. It's not possible to select int as the generic type parameter, because MyLong can't be implicitly converted to int. So in that case, there is a "better" overload to choose, and so it's chosen.
By explicitly specifying the literal's type, you can try different combinations and see this pattern at work. First, I prefer a simpler wrapper class to test with:
public class Wrapper<T>
{
public T Value;
public static implicit operator T(Wrapper<T> wrapper) => wrapper.Value;
public static implicit operator Wrapper<T>(T value) => new Wrapper<T> { Value = value };
}
Then try these:
Wrapper<int> w1 = new Wrapper<int> { Value = 4 };
Wrapper<long> w2 = new Wrapper<long> { Value = 4 };
Assert.Equal(4, w1); // error
Assert.Equal((short)4, w1); // no error
Assert.Equal(4, w2); // no error
Assert.Equal(4L, w2); // error
The only thing that makes int special is that that's the default type for the numeric literal. Otherwise, a type that wraps int works exactly the same as a type that wraps anything else. As long as a conversion is available only in one direction between the two parameters, everything's fine. But when the conversion is available in both directions, the compiler has no choice but to throw up its hands and give up.
I have a struct with the following operator declared :
public struct myStruct {
public static implicit operator int(Nullable<myStruct> m){
/*...*/
}
}
This operator alone lets me implicitly convert a non-nullable struct to int, but trying to implicitly convert its nullable counterpart still raises a compilation error :
Cannot implicitly convert type myStruct? to int. An explicit
conversion exists (are you missing a cast?)
Apparently the mentioned "explicit" operator is actually the implicit operator I declared, removing this operator altogether also removes the mention of the explicit one.
When it comes to nullable structs, why am I being forced to use this operator explicitly even though it was declared implicit?
EDIT:
So here is the "full code", stripped of everything that doesn't make the compiler error disappear. The struct stays literally the same, all that's new is my testing code :
using System;
public struct boilDown {
public static implicit operator int(Nullable<boilDown> s) { return 0; }
} // END Struct
public class Sandbox {
void Update ()
{
boilDown nonNullable = new boilDown ();
Nullable<boilDown> NullableVersion = new Nullable<boilDown>();
int MyInt;
MyInt = nonNullable; // this work thanks to my operator
MyInt = NullableVersion; // But this line requires an explicit cast
}
}
VERSION :
You all hinted me at a c# version issue.
I'm working on Unity 2017.1.0f3, which rather than .Net, uses Mono 2.0.50727.1433. (This apparently is a NET3.5 Equivalent, but even their experimental NET4.6 equivalent has this issue.)
I'll ask this question to them and see what they say.
You could explicitly cast the NullableVersion to int like below.
using System;
public struct boilDown {
public static implicit operator int(Nullable<boilDown> s) { return 0; }
} // END Struct
public class Sandbox {
static void Main()
{
}
void Update ()
{
boilDown nonNullable = new boilDown ();
Nullable<boilDown> NullableVersion = new Nullable<boilDown>();
int MyInt;
MyInt = nonNullable; // this work thanks to my operator
MyInt = (int)NullableVersion; // works now
}
}
Thanks all who told me this code should compile.
Unity confirmed this error to be a bug on their end.
I'm trying to write a tool for serialization of values. And I was hoping to get some nice syntax working.....
float f = 9999.0f;
ByteSerializer s = new ByteSerializer ();
s.Write(f);
Where the params of Write() could be anything and any number:
public void Write (params dynamic[] objects)
{
for (int i =0;i<objects.Length;i++) {
byteList.AddRange (GetBytes (objects[i]));
}
}
GetBytes() should now be called according to the type of the object that was passed:
public byte[] GetBytes ( object v)
{
//Shouldn't actually do anything since it's a dummy
return new byte[0];
}
public byte[] GetBytes ( System.Single v)
{
//Why is this not called?
return BitConverter.GetBytes (v);
}
But it will always go straight for the method that takes an object as parameter.
I initially tried (params object[]) as method parameters and found this behaviour to be rather obvious. But why does dynamic[] behave the same?
objects[i].GetType() reports a System.Single so what's going on here? Is what I am trying to do just not possible?
There is no reason this shouldn't work with dynamic exactly as you described. One of the primary features of the DLR is to do at runtime what would have been done at compile time if the types had been known then. For example, the following program:
public class Program
{
public static void Main(string[] args)
{
WriteItems("a string", (byte)1, 3f, new object());
}
private static void WriteItems(params dynamic[] items)
{
foreach(dynamic item in items)
{
Write(item);
}
}
private static void Write(byte b)
{
Console.WriteLine("Write byte: {0}", b);
}
private static void Write(float f)
{
Console.WriteLine("Write Single: {0}", f);
}
private static void Write(string s)
{
Console.WriteLine("Write string: {0}", s);
}
private static void Write(object o)
{
Console.WriteLine("Write object: {0}", o);
}
}
Produces the output:
Write string: a string
Write byte: 1
Write Single: 3
Write object: System.Object
Method overloading relies on knowing at compile time the types of all the arguments. Here, you can't know the argument types at compile time - because it's dynamic - so the compiler uses the lowest common type - which is object. The difference between object and dynamic is that dynamic allows late binding syntax - but that doesn't help you here.
If you want to call GetBytes for the correct types, you need to use reflection. Specifically, you want to use GetMethod to find an overload of GetBytes that matches the type of each argument, and then call it. with Invoke. Something like this(not tested):
byteList.AddRange (GetType()
.GetMethod("GetBytes", new Type[objects[i].GetType()])
.Invoke(null, new object[objects[i]]));
The dynamic type behaves like the object type. The only difference is that for dynamic the compiler does no type checking (so you can avoid compiler errors when working e.g. with COM).
What you intend, can be done anyway. But you must take the long road and do an explicit typechecking/cast in your for loop (a long but simple switch/case construct).
HTH Thomas
I have the following signature for overloaded +:
public static double operator +(MyClass x, MyEnum e)
and an expression of the form:
x.Value = someMyClassValue + MyEnum.X;
The behavior the debugger shows is as if the expression had been:
x.Value = MyEnum.X;
The overload never gets called.
I also have:
public static double operator +(MyClass x, object o)
but that doesn't get called either for enums, though it does for other cases.
I also have overloads for string, int, float, double, and they all work perfectly. Why is enum a special case, and why the odd behavior? Could this be a bug in the Mono compiler?
I'm using Mono 2.10.8.1 on Ubuntu 13.04.
Afternote
The problem was that I had also defined an implicit cast to int. See my answer for details.
The problem was that I had also defined:
public static implicit operator int(MyClass o)
The implicit cast takes precedence over the overloaded operator, and the whole addition expression takes the type of the enum.
Since I wanted to keep the implicit cast to int, I adopted this solution:
public enum MyEnum : ulong
With that, the cast to int no longer takes place.
The following program demonstrates the problem I was having. It's output is "SECOND" instead of "OK".
using System;
public class EnumPlus
{
public enum Constant
{
FIRST,
SECOND
};
// if this implicit cast is removed the result is what I expected
public static implicit operator int(EnumPlus f)
{
return 1;
}
public static string operator+(EnumPlus o, int i)
{
Console.WriteLine("operator + called for int");
return "BAD";
}
public static string operator+(EnumPlus o, Constant Constant)
{
Console.WriteLine("operator + called for enum");
return "OK";
}
public static void Main()
{
EnumPlus o = new EnumPlus();
Console.WriteLine(o + Constant.FIRST);
}
}
is it possible to define an extension method that at the same time is an operator?
I want for a fixed class add the possibility to use a known operator that actually can't be applied.
For this particular case i want to do this:
somestring++; //i really know that this string contains a numeric value
And i don't want to spread types conversions for all the code.
I know that i could create wrapper class over an string and define that operator but i want to know if this kind of thing is possible to avoid search-and-replace every string declaration with MySpecialString.
Edited: as most have say string is sealed, so derivation isn't possible, so i modify "derived" to "wrapper", my mistake.
That is not possible in C#, but why not a standard extension method?
public static class StringExtensions {
public static string Increment(this string s) {
....
}
}
I think somestring.Increment() is even more readable, as you're not confusing people who really dont expect to see ++ applied to a string.
A clear example of where this would be useful is to be able to extend the TimeSpan class to include * and / operators.
This is what would ideally work...
public static class TimeSpanHelper
{
public static TimeSpan operator *(TimeSpan span, double factor)
{
return TimeSpan.FromMilliseconds(span.TotalMilliseconds * factor);
}
public static TimeSpan operator *(double factor, TimeSpan span) // * is commutative
{
return TimeSpan.FromMilliseconds(span.TotalMilliseconds * factor);
}
public static TimeSpan operator /(TimeSpan span, double sections)
{
return TimeSpan.FromMilliseconds(span.TotalMilliseconds / factor);
}
public static double operator /(TimeSpan span, TimeSpan period)
{
return span.TotalMilliseconds / period.TotalMilliseconds);
}
}
No, it is not possible to do from outside of the class. ++ operator should be defined inside class which is being incremented. You can either create your own class which will be convertible from string and will have ++ overload or you can forget about this idea and use regular methods.
No, you can't have an extension method which is also an operator. Extension methods can only be declared in static classes, which can't have instances and according to the C# spec,
User-defined operator declarations always require at least one of the parameters to be of the class or struct type that contains the operator declaration. [7.3.2]
Therefore, it is impossible for an extension method to also be an overloaded operator.
Additionally, you can't override System.String since it is a sealed class.
The string class is sealed in C#, so creating a string-derived class actually isn't possible.
That being said, an extension method will of course work just fine (as will a standard static method in a helper class) but it won't be an operator, just ordinarily-named method.
Currently this is not supported because Extension methods are defined in separate static class and static classes cannot have operator overloading definitions.
This is all true, but it would be nice for M$ to add this functionality in the future. Sometimes the framework is just missing things and an extension can help plug the gap (or fix the issue) this can sometimes be operators.
An example. To compare IP Addresses, you must use the Equals method to directly compare (of course parts of the struct could also be compared as could the address bytes individually - but that's another story). However, using the == operator always returns false at the object level (i.e. without converting them to strings etc). How hard is it to put the Equals method call inside the == operator call (that's rhetorical), but we can't do it. This is inconsistant and a place for bugs to creep in (note it does not fail, just always equates to false - whereas Equals does not).
I would argue that you should use a wrapper class, even if you could write an extension operator.
//i really know that this string contains a numeric value
is exactly the sort of situation that type-safety was invented for.
Another way of looking at it is that by writing that operator, you have broken many other functions and operators that work with the string class, since they don't necessarily preserve the property of containing a numeric value. By using a wrapper class, not a derived class, you only re-implement those features of string that make sense for numeric strings.
i was in a very similar situation as you described: i needed to increase the text (containing a numeric value for sure) in a Windows Forms textbox.
I understand your need as you described
somestring++; //i really know that this string contains a numeric value
My soultion is something like that which i believe is close to your description
somestring = (incrementable)somestring + 1
All i needed to do was
creating class called incrementable
defining an explicit operator in it (to aid converting string to incrementable )
defining an implicit operator in it (to aid converting incrementable back to string )
operator for + (plus sign)
Here's how my class looks in complete
public class incrementable
{
public string s; // For storing string value that holds the number
public incrementable(string _s)
{
s = _s;
}
public static explicit operator incrementable(string tmp)
{
return new incrementable(tmp);
}
public static implicit operator string(incrementable tmp)
{
return tmp.s;
}
public static incrementable operator +(incrementable str, int inc) // This will work flawlessly like `somestring = (incrementable)somestring + 1`
=> new incrementable((Convert.ToInt32(str.s) + inc).ToString());
public static incrementable operator ++(incrementable str) // Unfortunately won't work, see below
=> new incrementable((Convert.ToInt32(str.s) + 1).ToString());
}
Unfortunately i just couldn't get managed to improve my class by the usage of unary ++ operator. The reason against of usage of implicit conversion like ((incrementable)somestring)++ is that it is going to result in error saying The operand of an increment or decrement operator must be a variable, property or indexer hence can not be result of that casting.
Anyway, hope this helps!
As shown in the other answers, it cannot be done directly. But what if you need it, say you want to improve StringBuilder like
void Main()
{
var log = (StringBuilder)"Hello ";
log += "World!";
log += "\nThis example shows how to extend StringBuilder";
log.ToString().Dump();
}
how can you achieve this (i.e. use + operator instead of sb.Append(str);) ?
Answer:
In this case, you can't do it directly, but what you can do is:
Run it in DotNetFiddle
void Main()
{
var log = (StrBuilder)"Hello "; // same as: "Hello ".ToStrBuilder();
log += "World!";
log += "\nThis example shows how to extend StringBuilder";
log.ToString().Dump();
}
public static class Extensions
{
public static StrBuilder ToStrBuilder(this string str)
{
return new StrBuilder(str);
}
}
public class StrBuilder
{
private StringBuilder sb;
public StrBuilder()
{
sb = new StringBuilder();
}
public StrBuilder(string strB)
{
sb = new StringBuilder(strB);
}
public static implicit operator StrBuilder(string self)
{
return new StrBuilder(self);
}
public static StrBuilder operator +(StrBuilder sbA, string strB)
{
return sbA.Append(strB);
}
public StrBuilder Append(string strB)
{
sb.Append(strB);
return this;
}
public override string ToString()
{
return sb.ToString();
}
}
Note: You can't inherit from StringBuilder because it is a sealed class, but you can write a class that "boxes" a StringBuilder - which is, what is done here (thanks to IanNorton's answer regarding implicit conversion).