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);
Related
Is it possible to implement a class constrained to two unique generic parameters?
If it is not, is that because it is unimplemented or because it would be impossible given the language structure (inheritance)?
I would like something of the form:
class BidirectionalMap<T1,T2> where T1 != T2
{
...
}
I am implementing a Bidirectional dictionary. This is mostly a question of curiosity, not of need.
Paraphrased from the comments:
Dan: "What are the negative consequence if this constraint is not met?"
Me: "Then the user could index with map[t1] and map[t2]. If they were the same type, there would be no distinction and it wouldn't make any sense."
Dan: The compiler actually allows [two generic type parameters to define distinct method overloads], so I'm curious; does it arbitrarily pick one of the methods to call?
Expanding on the example to highlight the problem:
public class BidirectionalMap<T1,T2>
{
public void Remove(T1 item) {}
public void Remove(T2 item) {}
public static void Test()
{
//This line compiles
var possiblyBad = new BidirectionalMap<string, string>();
//This causes the compiler to fail with an ambiguous invocation
possiblyBad.Remove("Something");
}
}
So the answer is that, even though you can't specify the constraint T1 != T2, it doesn't matter, because the compiler will fail as soon as you try to do something that would violate the implicit constraint. It still catches the failure at compile time, so you can use these overloads with impunity. It's a bit odd, as you can create an instance of the map (and could even write IL code that manipulates the map appropriately), but the C# compiler won't let you wreak havoc by arbitrarily resolving ambiguous overloads.
One side note is that this kind of overloading could cause some odd behaviors if you're not careful. If you have a BidirectionalMap<Animal, Cat> and Cat : Animal, consider what will happen with this code:
Animal animal = new Cat();
map.Remove(animal);
This will call the overload that takes Animal, so it will try to remove a key, even though you might have intended to remove the value Cat. This is a somewhat artificial case, but it's enough to warrant caution when very different behaviors occur as a result of method overloading. In such cases, it's probably easier to read and maintain if you just give the methods different names, reflecting their different behaviors (RemoveKey and RemoveValue, let's say.)
The inequality wouldn't help the compiler to catch errors. When you specify constraints on type parameters, you are telling the compiler that variables of this type will always support a certain interface or will behave in certain ways. Each of those allows the compiler to validate something more like "this method will be present so it can be called on T".
The inequality of type parameters would be more like validating that method arguments are not null. It is part of the logic of the program, not its type safety.
Type constraints seem like a misnomer. While they do contrain what the type parameter is, the purpose is to let the compiler know what operations are available to the type.
If you wanted to, you can have a constraint where T1 and T2 both derive from seperate concrete base classes, but I don't think that's quite what you want.
I'm not entirely sure why this would be a desirable compile time check. It would be possible to essentially by-pass the condition by boxing either the key or the value, thereby rendering the compile-time check useless.
Some consideration needs to be made to determine... what errors am I trying to prevent?
If you are simply stopping a lazy co-worker from not reading the documentation, then add a Debug only check and throw an exception. This way the check can be removed for release code e.g.
#if Debug
if (T1 is T2 || T2 is T1)
{
throw new ArguementException(...);
}
#endif
If you are attempting to prevent a malevolent person from using your library in an unintended way, then perhaps a runtime check is needed, otherwise it would be easy to box the key or value.
No, you cannot use equality (or inequality) as a constraint. Simply put, equality is not a constraint, but a condition. You should test for a condition such as equality or inequality of types in the constructor and throw an appropriate exception.
There is no effective way to do this without imposing any other restrictions on the types themselves. As someone else noted, you could make constraints that the two types derived from two different base classes, but that's probably not very good from a design standpoint.
Edited to add: the reason this is not implemented is most likely because nobody at Microsoft ever considered something like this to be necessary to enforce at compile time, unlike the other constraints which have to do with how you're actually able to use variables of the specified types. And as some commenters have pointed out, you can certainly enforce this at runtime.
Say I have a method that is overloaded such as void PrintInfo(Person) and void PrintInfo(Item), and so on. I try to invoke these methods by passing in an Object.
I'm wondering why it is giving me an error when I do this; aren't all classes inherited from Object? I want to avoid doing an if/switch statement where I check which type the Object is before calling the appropriate method.
What do you guys think is the best approach in this case?
All Persons are objects , but not all objects are Persons. Because of this you can pass a Person to a method that accepts an object but you can't pass an object to a method that requires a Person.
It sounds like you have some common bit of functionality between various objects that you want to use. Given this, it would be best to find either a common ancestor that has all of the functionality that you need, or an interface that they all implement (that again provides everything that you need).
In the case of printing, you may just need the ToString method. In that case, you can just have the method accept an object and call ToString on it. (That's what many print methods do, such as Console.WriteLine.
You need to understand that because C# is a statically typed language (barring dynamic) the particular overload that is chosen (called overload resolution) is determined at compile time, not run time. That means that the compiler needs to be able to unequivocally determine what type your argument is. Consider:
Object foo;
foo = "String";
foo = 5;
PrintInfo(foo); // Which overload of printinfo should be called? The compiler doesn't know!
There are a few ways to solve this- making foo of type dynamic is one- that will cause the correct overload to be chosen at compile time. The problem with that is that you lose type safety- if you don't have an appropriate overload for that type, your application will still compile but will crash when you try to print the unsupported type's info.
An arguably better approach is to ensure that foo is always of the correct type, rather than just Object.
As #Servy suggests, another approach is to attach the behavior to the type itself. You could, for instance, make an interface IHasPrintInfo:
public interface IHasPrintInfo { String PrintInfo { get; } }
and implement that interface on all items whose info you might print. Then your PrintInfo function can just take an IPrintInfo:
public void PrintInfo(IPrintInfo info) {
Console.WriteLine(info.PrintInfo);
}
here its ambiguate for compiler; compiler can't figure out which version of method (Person/Item) you are intended to call.
I have a method that logs out data, and takes bunch of inputs using the params keyword
public static void LogData<T>(params object[] parameter)
{
// log out the data
}
I'd like to restrict the inputs to strings and numeric types, and my first thought was to use a where clause. The obvious issue with this are that you can't use concrete types in a where clause, but you can get around this by noticing that both string and numeric types are IComparable and IConvertible. I thought this might help:
public static void LogData<T>(params T[] parameter)
where T : IComparable, IConvertible
{
// log out the data
}
This doesn't work because then all the inputs have to be one type.
Is there a way to restrict the inputs to string and numeric types using the params keyword or some other syntax?
I'm not sure why you want to do that, when every object has a ToString method.
Anyway, you can declare a new class - NumberOrString that will support implicit conversions from a string and the numeric types you want to support. Then have your LogData method accept a params NumberOrString[] p
Consiidering you are talking about strings and numeric types, it's not possible. If condition would be or, you can define an optional parameter of type List<T> where T is any chosen numeric type.
I'd create multiple methods which are essentially overloads using the types I want to support.
might be too much code and repetitive but the basic "log out the data" routine would essentially be reused across all overloads.
Here's one potential solution:
public static void LogData(params IConvertible[] parameter)
{
// log out the data
}
Although I wonder if there's actually any reason you need the objects to be IComparable or IConvertible, or if you'd just like to restrict them to known types (i.e. strings or numbers). I think zmbq's answer might be the way to go. Or you might even really just want to see if the object implements a useful ToString method. Here's a way to find that out (not a compile-time restriction, as you might expect, but at runtime using reflection).
Is it in anyway possible ( preferably without using any third party libs), to create a function whose type is determined at runtime in C#?
e.g
public static void myfunc(var x)
{
System.Windows.Forms.MessageBox.Show(x); //just an example
}
NOTE: I want the runtime to determine the type of the parameter and do not want to later cast the parameter to another type, as would be necessary if I use generics. e.g I don't want:
myfunc<T>(T x)
// and then :
MessageBox.Show((string)m);
UPDATE:
I am actually making a function parser for my programming language, which translates to C# code. In my language, I wanted the parameter types to be determined at runtime always. I was looking for some good C# feature for easy translation.
e.g
in my language syntax:
function msg << x
MessageBox.Show x
end
needed to be translated to something that didn't ask for a type at compile time, but would need one at runtime.
e.g
public static void msg(var x)
{
System.Windows.Forms.MessageBox.Show(x);
}
The keyword introduced for runtime binding in C# 4 is dynamic.
public static void myfunc(dynamic x)
This allows you to make assumptions about x that are unchecked at compile time but will fail at runtime if those assumptions prove invalid.
public static void MakeTheDuckQuack(dynamic duck)
{
Console.WriteLine(duck.Quack());
}
The assumption made here is that the parameter will have a method named Quack that accepts no arguments and returns a value that can then be used as the argument to Console.WriteLine. If any of those assumptions are invalid, you will get a runtime failure.
Given classes defined as
class Duck
{
public string Quack()
{
return "Quack!";
}
}
class FakeDuck
{
public string Quack()
{
return "Moo!";
}
}
And method calls
MakeTheDuckQuack(new Duck());
MakeTheDuckQuack(new FakeDuck());
MakeTheDuckQuack(42);
The first two succeed, as runtime binding succeeds, and the third results in an exception, as System.Int32 does not have a method named Quack.
Generally speaking, you would want to avoid this if possible, as you're essentially stipulating that an argument fulfill an interface of some sort without strictly defining it. If you are working in an interop scenario, then perhaps this is what you have to do. If you are working with types that you control, then you would be better served trying to achieve compile time safety via interfaces and/or base classes. You can even use different strategies (such as the Adapter Pattern) to make types you do not control (or cannot change) conform to a given interface.
If you need to know the type... then you need to know the type. You can't have your cake and eat it too.
First off, the cast in your example is unnecessary as all objects implement ToString(). Instead of telling us what you think you need, tell us what problem you are trying to solve. There is almost certainly a solution either via generics or the use of the dynamic keyword (though dynamic is rarely needed), but we need more info. If you add more I'll update this answer.
You could use a type of object or, if you don't know how many items are available, you could use a params object array, i.e. params object[] cParams.
Just out of curiosity:
Many LINQ extension methods exist as both generic and non-generic variants, for example Any and Any<>, Where and Where<> etc. Writing my queries I usually use the non-generic variants and it works fine.
What would be the cases when one has to use generic methods?
--- edit ---
P.S.: I am aware of the fact that internally only generic methods are called and the compiler tries to resolve the content of the generic brackets <> during compilation.
My question is rather what are the cases then one has to provide the type explicitly and not to rely on the compiler's intuition?
Always. The C# compiler is smart enough to infer what the type of the method is based on the parameters. This is important when the type is anonymous, and thus has no name.
obj.SomeMethod(123); //these calls are the same
obj.SomeMethod<int>(123);
obj.SomeMethod(new { foo = 123 }); //what type would I write here?!
Edit: To be clear, you are always calling the generic method. It just looks like a non-generic method, since the compiler and Intellisense are smart.
Edit: To your updated question, you would want to be specific if you want to use a type that is not the type of the object you are passing. There are two such cases:
If the parameter implements an interface, and you want to operate on that interface, not the concrete type, then you should specify the interface:
obj.DoSomething<IEnumerable<Foo>>( new List<Foo>() );
If the parameter is implicitly convertible to another type, and you want to use the second type, then you should specify it:
obj.DoSomethingElse<long> ( 123 ); //123 is actually an int, but convertible to long
On the other hand, if you need a cast to do the conversion (or you insert one anyway), then you don't need to specify:
obj.DoYetAnotherThing( (Transformed)new MyThing() ); // calls DoYetAnotherThing<Transformed>
One example I ran into today:
ObjectSet<User> users = context.Users;
var usersThatMatch = criteria.Aggregate(users, (u, c) => u.Where(c));
The above code won't work because the .Where method doesn't return an ObjectSet<User>. You could get around this one of two ways. I could call .AsQueryable() on users, to make sure it's strongly typed as an IQueryable, or I could pass specific type arguments into the Aggregate method:
criteria.Aggregate<Func<User, bool>, IEnumerable<User>>(
PersonSet, (u, c) => u.Where(c));
Another couple of more common examples are the Cast and OfType methods, which have no way to infer what type you want, and in many cases are being called on a non-generic collection in the first place.
In general, the folks that designed the LINQ methods went out of their way to avoid the need to use explicit types in these generic methods, and for the most part you don't need to. I'd say it's best to know it's an option, but avoid doing it unless you find it necessary.