I'm not sure if this is doable, but I will just give a shot.
I am calling Assert.AreEqual() method.
For the parameters, I'm passing...
an enum value which has Int32 as the underlying type because I didn't specify the base type
an int (Int32) value
Assert fails because it sees that the enum is not int (which is definitely correct).
However, is there a way to make this Assert pass when the enum has the correct int value as the 2nd parameter?
I can cast the enum to int and have it a quick fix, but it's really ugly.
I was expecting some kind of overriding a method that Assert uses to compare 2 different objects and implicitly make that enum type look like an int. However, I wasn't successful at finding any hint/answer around that so far.
Someone suggested to create a type converter and use the TypeConverterAttribute to get around. If this works for sure and is the only way to do it, I would; however, it does seem a lot of unnecessary work.
By calling Assert.AreEqual(enum, int), you are calling the Assert.AreEqual(object, object) method, as there is no overload that has an enum and an int as parameters.
I do not think that casting the enum to an int is ugly - in fact I think it is perfectly acceptable and a very common technique. If you were doing a normal equality comparison (ie. 1 == MyEnum.Value), you would get a compiler error as there is no implicit conversion, only an explicit one which requires a cast to work.
A type converter may work, but you need to ask yourself if doing that will actually give you any real benefit for the amount of effort involved. Personally, I would just leave the cast to an int.
If you are asserting something like the return value of a method call, why not just assert based on what value you expect?
MyEnum actual = SomeMethod();
Assert.AreEqual(MyEnum.Value, actual);
This does not require the cast as the two types are the same, and you are asserting the expected value.
I was expecting some kind of overriding a method that Assert uses to compare 2 different objects and implicitly make that enum type look like an int.
That won't work, since Assert.AreEqual is a static method, and you cannot override static methods, nor can you add overloads for static methods through extension methods.
Anyway, even if you could, you would violate the contract for Assert.AreEqual. You want to have Assert.AreEqual(myEnum.One, 1) yield true, although myEnum.One.Equals(1) as well as 1.Equals(myEnum.One) return false. That's inconsistent.
Casting to int is not ugly, it's the correct solution to your problem. After all, you want to check if the numerical value of the enum matches some integer. If that's what you want to check, write it down like this.
Related
I'm currently implementing a simple version of algorithmic differentiation with operator overloading in C#. I'm trying to figure out how to design generic math functions that works for with ordinary doubles and my own class "ADouble" that works like a double, but overloads arithmetic operators like +,*,- and so forth.
For instance, i would like to create a function like
Public T MathFunction<T>(T x) where T : "is either double or Adouble"
{
if (x > 0)
return new T(1.0)
else
// something
}
that works for both doubles and ADoubles. In this case it's necessary for me to "new up" a specific value (here 1.0). In other cases i might have to do something like
Public T MathFunction<T>(T x) where T : "is either double or Adouble"
{
T temporaryVar = 2*x;
// .. More calculations
return "some T";
}
I have implemented the necessary interfaces to do a comparison like the above, but i cannot get the rest to work.
I can instantiate my ADouble class with a double by, say
Adouble myADouble = new ADouble(12.3);
but doubles does not have a constructor that works that way, obviously. I've tried different things. First of all i thought of something like
if (typeof(T) == typeof(ADouble)
return new ADouble(1.0)
but this does not work, since the function cannot cast ADouble to a T explicitly (which i understand).
Does anyone have a suggestion on how i could go about implementing generic calculation functions that works with my ADouble class and doubles? Or is the only option to make multiple methods with different signatures? Different suggestions for designs is also greatly appreciated.
Or is the only option to make multiple methods with different signatures?
Called "method overloading".
Yes. This is the correct way to express your "type A or type B" constraint, especially since even if you could successfully express that as a generic constraint, you are still left with the challenge of calling the appropriate constructor.
There is no mechanism in C# generics by which the statement return new T(1.0); could be made to compile successfully. Doing so would require some syntax that additionally constrains the types to those with a constructor that has a single parameter of type double, and there is no such feature in C#.
this does not work, since the function cannot cast ADouble to a T explicitly
Actually, it's that it can't cast implicitly. But, close enough. :)
That would be the next hurdle you'd have to clear. Ironically, this is the simplest. The issue there is that, as you wrote the expression, the compiler knows enough about what's going on to know it can't guarantee the cast will succeed. But if you cast the value to object first, you can then cast it to T without the compiler complaining.
Not that I'm suggesting doing that, mind you. The real issue here is that you are trying to use generic syntax for something that really isn't generic. Generic code is for where you can use any type, or at least a broadly constrained type. If you have specific types in mind, and especially if the implementation is different for each specific type, then you should not be using generics.
In that situation, method overloading is much more appropriate.
Aside: assuming your ADouble type is literally equivalent to double, and you can implement a conversion without any loss of data, you should consider writing an implicit conversion, to help make the types interchangeable. That would look something like this:
public static implicit operator ADouble(double value)
{
return new ADouble(value);
}
Then you can initialize ADouble values with simple assignments, like:
ADouble adouble = 1.0;
I have several places where i have Action<Int32>, Action<Boolean>, Action<SomeClassName> being used, but when it comes to passing the action as an argument in a class method, i cannot cast Action<Int32> or Action<Boolean> to Action<Object>.
Why can a cast not be done with a simple type cast? Is this cast possible at all? And lastly, how would i go about this cast if it is possible.
Because it would break type safety.
When you declare Action<Int32>, you say "this is a delegate that requires one Int32 parameter".
If this could be directly cast to Action<object>, then you are now able to call the action with something that is definitely not Int32, for instance a DateTime. In this case, C# prevents you from shooting yourself in the foot.
You can't make a direct cast via covariance (or is it contravariance?) because an Action<int> is fundamentally not an Action<object> - the latter can be passed any object, whereas the former can only be passed an int.
You can work around this by wrapping your action, something like:
Action<int> aInt = ...;
Action<object> aObj = o => aInt((int)o);
This again shows you why the cast might not be a good idea - if you pass aObj anything except an int, you'll get a cast exception.
Conversely, you can in theory just cast an Action<object> to an Action<int>, because if you can pass it any object, you can certainly pass it an int. However, in practice this only works for reference types, not value types - so you could do it with string, but not with int.
Action labors under the restriction that its input is an Int32. This means that the action you use can make assumptions about the input being an int. Eg you can do math with another int n => Console.WriteLine(n + 4). So that action, or function, fits nicely in that box. If you change the box, the assumptions made earlier still have to hold. You basically have a reverse relationship, or contravariance, to the "regular" casting scenario, eg: object a = (object) 2;.
Just remember that anything that fits in the Action box has to make no assumptions about the behaviour of its input. The function n => Console.WriteLine(n) fits snugly inside, because all objects can be printed. Not all objects can act like ints.
I have code where I get a string as input, and I compare it with an integer.
I saw that integer variable also has an Equals function that accepts a string parameter.
I have used it directly thinking it will typecast it.
It did not give any compile time or runtime error, but it always gives a false result.
For example,
int sessionId = 1;
string requestId="1"
return sessionId.Equals(requestId);
sessionId.Equals(requestId) always gives false.
Why is the reason for such behavior? If there is a reason, why are they allowing it run without error?
Integers and strings are always different, and thus "1".Equals(1) returns false.
It compiles because object.Equals(object other) takes an object as the right side, and thus accepts any type.
The reason why this happens is that a string "0" is not the same as 0, so it returns false.
Why is such behavior supported? Because the Equals method allows you to pass an object as a parameter, and a string is in an object, so you are "allowed" to do it. As you have found, it's not very useful in this case.
To solve your problem either get a string representation of the integer, or parse your string to an integer, then compare.
E.g. Try
return (sessionId.ToString() == requestId);
or
return (sessionId == int.Parse(requestId));
If you choose the later you may need to consider if the Parse could fail and how you might handle that.
Yes, Equals takes any type on the right side because it is requires an object. But inside the function it requires the same type as the left side. IMHO there's no need to throw an exception for type mismatching because one only wants to know about equality of two types.
Decompiled Equals of int:
public override bool Equals(object obj)
{
return obj is int && this == (int)obj;
}
If someone shows you a car and a banana, and asks whether they are the same thing, would you throw a temper tantrum because a car is a vehicle and a banana is a fruit, or would you simply say "no, they are not the same thing"?
In many languages, trying to compare an integer and a string will yield a compiler error, because the compiler knows that the integer and the string cannot possibly be the same thing and thus any code that tried to compare them would almost certainly be erroneous. On the other hand, when you say sessionId.Equals(requestId), the compiler knows that you are asking that requestId be passed to the Int32 override of Equals. Since that override can accept a reference to any kind of heap object, it has no problem passing the string "1". That method in turn knows that it was given something which isn't the same as an Int32 with the value 1. It doesn't know that the calling code can't possibly supply anything that would match an Int32; all it knows is that the particular value isn't the same, and because the value isn't the same it perfectly happily returns false.
Shouldn’t we be using String.Compare for string comparison and forget about Equals?
I did have the same problem and I believe the function Equals should throw an exception. In my case, I have been comparing a string with a Boolean.
The discussion by now went wrong way. This is my view:
If a comparison between objects belonging to two different classes always returns false, then they do not need to be compared in the first place.
If there is a need to have a function that bypasses type checking, there should be one. However, having the function Equals positioned as a recommended method for string comparison and in the same time introducing the possibility of unneeded bugs (which may sit in your program for eternity) is a kind of irresponsible.
Moreover, it is extremely misleading that the function call String.Equals(string1, string2, StringComparison. xxx) also accepts non-string arguments. Thus not only string1.Equals(string2).
If that is by design, then it is a poor design.
I'm building a HTTP-API wrapper for .NET, which has a bunch of methods to set data in an object, and then it serializes the data and sends it to my server. There are 6 datatypes allowed:
string
int
long
float
double
DateTime
My data attributes use generics:
SetAttribute<T>(string key, T value)
So there is only one generic method to set data. Since I cannot constrain the data types to the 6 mentioned, I use run-time checks and throw an exception when the wrong data type is used.
Now for my problem: I have two versions of SetAttribute, one that takes a single value (of type T) and one that takes multiple values (of type IEnumerable<T>). The problem is that when a programmer uses this wrapper and does not specify the type parameter, the runtime guesses which method to use, for instance:
SetAttribute("testkey","thing,anotherthing,athirdthing".Split(','))
This defaults to the single value method and T is String[] which of course makes my method cast an exception because String[] is not a valid type. If you specify:
SetAttribute<string>("testkey","thing,anotherThing,aThirdThing".Split(','))
The runtime chooses the correct method (multi-value) and no exception is cast because T is then string.
My question: how can I label my methods so that the type parameter is mandatory and must be explicitly defined? Or do I have to detect this at runtime and redirect to the multi-method myself?
Ok, this was originally a comment above since it doesn't necessarily answer your original question but suggests an alternate approach;
I would say using a public generic SetAttribute in this case isn't necessarily a good idea.
Since the types are so constrained, you should probably just write the overloads and move the errors from runtime to compile time. It would also allow you to take IEnumerable<string> etc. with another 6 overloads and eliminate the problem you're having entirely.
You can always implement SetAttribute with a private generic and just call that from each overload, that will remove some duplication.
It will also more or less eliminate the need for runtime checks, since the types are already constrained by the compiler.
Given a parameter type, the compiler finds a best match from your overloads. If you cast your string[] to an IEnumerable<string> you will probably find it works as expected because the best match is a method that has exactly those parameters. But you have no method that takes a string[], so given one as a parameter, the compiler makes the best guess it can.
I would have two separately named methods rather than overloads otherwise it is too easy to run into this problem. Or have 6 separate overloads, as #Joachim suggests.
I would suggest a better solution would be to test whether the value passed in is IEnumerable after it fails everything else and treat it as such if it is. (I imagine that you're handling IEnumerable as a seventh case already).
One solution would be to break your original method into 6 non-generic overloads, and add another generic overload for collections:
void SetAttribute(string key, int value);
void SetAttribute(string key, string value);
// etc
// abd this takes care of collections:
void SetAttribute<T>(string key, IEnumerable<T> value);
(OK, I'll expose the depths of my ignorance here, please be gentle)
Background
I've got a method which looks (a bit) like this:
public void AddLink(Enum enumVal)
{
string identifier = m_EnumInterpreter(enumVal);
AddLink(identifier);
}
The EnumInterpreter is a Func<Enum, string> that is passed in when the parent class is created.
I'm using Enum because at this level it is 'none of my business'- I don't care which specific enum it is. The calling code just uses a (generated) enum to avoid magic strings.
Question
If the EnumInterpreter sends back an empty string, I'd like to throw an exception with the actual value of enumVal. I thought I would just be able to cast to int, but it the compiler won't have it. What am I doing wrong? (Please don't say 'everything').
System.Enum cannot be directly cast to Integer, but it does explicitly implement IConvertible, meaning you can use the following:
public void AddLink(Enum enumVal)
{
string identifier = m_EnumInterpreter(Convert.ToInt32(enumVal));
AddLink(identifier);
}
Keep in mind that if your Enum is actually using something other than an Integer (such as a float), you'll lose the non-integer data on conversion. Or obviously replace the Convert call with whatever you are converting from (if it's known)
No, you aren't able to cast it to an int because System.Enum is not an enum, it's just the base class for enums.
EDIT:
You can get the value as follows, but it is ugly:
int intVar = (int)enuYourEnum.GetType().GetField("value__").GetValue(objYourEnum);
try this..
m_EnumInterpreter((int) (object) enumVal);
Various things here:
1) the answer of Ryan looks ok, but... I would rather pass the Enum down to the enum interpreter, so that you can do the whole Convert.To... there. If you know that you are using ONLY integer based enums, the Convert.ToInt32() is just fine. Otherwise you may want to add by using either reflection or try/catch other conversions.
2) You may also consider using members of the Enum class, like .GetName(), .GetValue(), etc. since they deal directly with the defined names and values independent of the enum type.
3) technically I would not throw the exception outside the enum interpreter. If that condition is generally true, throw the exception from inside the enum interpreter, so that all uses of the class will benefit of the validation. Or you might end up duplicating code.
4) you seem to have an C++/MFC background judging from your variable naming. You might want to get into C# style naming conventions, it will ease your life when using/reading other peoples code and libraries. Check out MS's StyleCop for a good addin to help with naming.
I don't know whether to include this in my question, or as an answer. The problem is that it isn't THE answer, but it is the answer that works for me.
What I discovered, by chance while trying something else, is that if I just wodge it onto the end of a string, I get what I want:
throw new Exception("EnumInterpreter returns empty string for enumVal=" + enumVal);
//EnumInterpreter returns empty string for enumVal=3720116125
I actually simplified to int in my question, the real data type is uint (in this particular instance). Fortunately, given that I only actually wanted the string, I don't have to worry about that.
I'm not sure which of the three other answers is 'right', so vote away...
For me it was enough to cast to object first, since it's just a compilation error.
public static int AsInt(this Enum #this)
{
return (int)(object)#this;
}
I understand that this is probably not the solution to your exact problem, but I just want to post how I solved this for a particular API I was using.
int result = (int) (ActualEnumType) MethodThatReturnsSystemEnumType( arg1, arg2 );
Hopefully that will be of help to someone. Double cast FTW.
Why not parse the enum to a string and return the actual enum value?
public enum MyEnum { Flower = 1, Tree = 2, Animal = 3 };
string name = MyEnum.Flower.ToString(); // returns "Flower"
I think .ToString() is deprecated and I'm not sure about the new way to do it. I would've thought the actual enum representation would be more useful than the int?