I have a generic class that I need to constrain to only value types (int, float, etc.). I have a method that calls the Parse method based on a test of the type. For example:
class MyClass<T>
{
...
private static T ParseEntry(string entry)
{
if(typeof(T) == typeof(float))
{
return (T) float.Parse(entry);
}
if(typeof(T) == typeof(int))
{
.... you get the idea
}
}
}
Constraining T to struct doesn't work and I really want to avoid boxing/unboxing if possible. Any ideas?
EDIT: To give a little more insight into this. I noticed in a library I'm developing that two classes had very similar properties/methods etc. the only difference was the underlying type of data (int or float). THis lead me to a design with generics. The only hangup is because of the call to the specific Parse method depending on if it's a float or int. I could get around it with boxing/unboxing but I really wanted to avoid that if possible.
private static T ParseEntry(string entry)
{
TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));
return (T) conv.ConvertFromString(entry);
}
which has the advantage of working with any type with a type-converter (or you can add your own at runtime). It does have boxing, but really, it isn't so bad. If that is your biggest problem, then you're going about this the wrong way.
Unfortunately, you cannot create constraints of the type of describe. You cannot write, for example:
class MyClass<T> where T : int { ... } // won't compile
You may be better off leaving the constraint to be struct, but add some runtime checks to make sure that T is only one of the supported types when you instantiate the class. Since you haven't said much how you're planning to use your class - it's hard to offer alternative design advice on how to achieve better type safety.
EDIT: You should look at the type converter option that Marc and others recommend. This seems like the way to go.
EDIT: One possible idea that occurs to me, is to make the Parse() method actually be a delegate: Func<string,T> - that you assign during construction. This would make it possible for you to:
Avoid the inefficient and awkward if/else logic.
Improve the future use of your class to other value types (structs, BigDecimal, etc)
Here's a code example of what I mean:
class MyClass<T>
{
private readonly Func<string,T> ParseEntry;
public MyClass( Func<string,T> parser )
{
ParseEntry = parser;
}
}
public static class AvailableParsers
{
public static Func<string,int> ParseInt = s => int.Parse( s );
public static Func<string,float> ParseFloat = s => float.Parse(s);
}
You can read about the available constraints here. The only constraints available are:
struct (optional)
new() (optional)
interface constraint (optional, multiple allows)
base class constraint (optional, only one allowed)
naked constraints ( such as where T : TResult )
Stupid maybe, but - if you are talking about a limited set of types, why don't you just overload the method? Like the *Writer.Write() methods, that have overloads for every "base" type?
In short: Why generic, if you do different things based on the type anyway (that somehow defeats the purpose of doing the same thing for whatever type is passed in)?
Edit: Thinking about the usage I think you might actually want to use Convert.ChangeType.
Something like this:
class MyClass<T> where T: IConvertible
{
private static T ParseEntry(string entry)
{
return (T)Convert.ChangeType(entry, typeof(T));
}
}
I guess I don't see the value here... why not use the Convert class?
Like Mr. Bushkin said, you can't do that. So you're other option would be to simply make several overloads of the method for each primitive type.
Also, imagine that you didn't have to deal with primitive types, but you really wanted to handle all structs - it still wouldn't be possible, since the struct is not guaranteed to implement an (heretofore imaginary) interface called IParsable to be cast into T.
Related
To illustrate it with an example:
??? public class Foo<T> where T:(ClassA || ClassB){}
Is it possible to restrict T for a number of unrelated classes in an or relationship (T is either ClassA or ClassB, but nothing else) or the only way to achieve this is to either
make ClassA and ClassB both implement the same interface
have both classes derive from the same base class
and use these as constraints?
So, to make things clear: my question does not concern whether you can have n number of generic constraints for n number of variables, I want to know if I can have n number of constraints for the very same variable.
I would implement a different type of logic for both and return the appropriate. This, of course, can be done using interfaces.
Even if you can promise that both types have the exact same members, the compiler has no way of verifying this.
By having both classes implement the same interface, you make that guarantee to the compiler that they do share at least the members of the interface, and those members are available to the generic class to use. This is the reason interfaces exist as well as the whole point of generics.
No, and if you think about it, it wouldn't be very useful.
The purpose of applying generic constraints is to ensure that the code is able to do certain operations on the type. E.g. the new() constraint ensure you can do new T(), which is not possible for an unconstrained type. But applying an "or" constraint does not provide any useful static guarantees for T, so you cant really use it for anything.
You can have multiple constraints for a single type variable though, but they are "and" constraints, i.e. they all have to be fulfilled.
If I get it right, you basically want to evaluate an expression as part of the constraint and to my best knowledge it isn't possible.
Type parameters can have multiple constraints though, just not as an expression. See here from this example:
class EmployeeList<T> where T : Employee, IEmployee, System.IComparable<T>, new()
{
// ...
}
Here T sure has multiple constraints, but not as an expression.
In this case though you might wish to take a look at your design to eliminate the need of your question.
Short answer is: No, you can't. Others already explained why, but in short you would lose the type safety that comes with generics.
If for whatever reason you still needs this, there is a solution to use method overloads and the compiler does the rest.
public void Foo<T>(T instance) where T : ClassA
{
GenericFoo(instance);
}
public void Foo<T>(T instance) where T : ClassB
{
GenericFoo(instance);
}
private void GenericFoo(object instance)
{
// Do stuff
}
I've got an interface structure that looks like this:
At the most basic level is an IDataProducer with this definition:
public interface IDataProducer<out T>
{
IEnumerable<T> GetRecords();
}
and an IDataConsumer that looks like this:
public interface IDataConsumer<out T>
{
IDataProducer<T> Producer { set; }
}
Finally, I've got an IWriter that derives off of IDataConsumer like so:
public interface IWriter<out T> : IDataConsumer<T>
{
String FileToWriteTo { set; }
void Start();
}
I wanted to make IWriter's generic type T covariant so that I could implement a Factory method to create Writers that could handle different objects without having to know what type would be returned ahead of time. This was implemented by marking the generic type "out". The problem is, I'm having a compile error on IDataConsumer because of this:
Invalid variance: The type parameter 'T' must be contravariantly valid on 'IDataConsumer<T>.Producer'. 'T' is covariant.
I'm not really sure how this can be. It looks to me like the generic type is marked as covariant through the whole chain of interfaces, but it is very possible I don't totally understand how covariance works. Can someone explain to me what I am doing wrong?
The problem is that your Producer property is write-only. That is, you are actually using T in a contravariant way, by passing the value that is generic on type T into the implementer of the interface, rather than the implementer passing it out.
One of the things I like best about the way the C# language design team handled the variance feature in generic interfaces is that the keywords used to denote covariant and contravariant type parameters are mnemonic with the way the parameters are used. I always have a hard time remembering what the words "covariant" and "contravariant" mean, but I never have any trouble remembering what out T vs. in T means. The former means that you promise to only return T values from the interface (e.g. method return values or property getters), while the latter means that you promise to only accept T values into the interface (e.g. method parameters or property setters).
You broke that promise by providing a setter for the Producer property.
Depending on how these interfaces are implemented, it's possible what you want is interface IDataConsumer<in T> instead. That would at least compile. :) And as long as the IDataConsumer<T> implementation really is only consuming the T values, that would probably work. Hard to say without a more complete example.
Peter's answer is correct. To add to it: it helps to try out some examples and see what goes wrong. Suppose the code you had originally was allowed by the compiler. We could then say:
class TigerConsumer : IDataConsumer<Tiger>
{
public IDataProducer<Tiger> p;
public IDataProducer<Tiger> Producer { set { p = value; } }
... and so on ...
}
class GiraffeProducer : IDataProducer<Giraffe>
{
public IEnumerable<Giraffe> GetRecords() {
yield return new Giraffe();
}
TigerConsumer t = new TigerConsumer();
IDataConsumer<Mammal> m = t; // compatible with IDataConsumer<Mammal>
m.Producer = new GiraffeProducer(); // compatible with IDataProducer<Mammal>
foreach(Tiger tiger in t.p.GetRecords())
// And we just cast a giraffe to tiger
Every step on the way here is perfectly typesafe but the program is plainly wrong. Either one of those conversions has to be illegal, or one of the interfaces is not safe for covariance. We wish all those conversions to be legal, and therefore we must detect the lack of type safety in your interface declarations.
EDIT:
I have come to the conclusion that it's impossible to meet all my desired requirements!
Original Question...
I appear to have run into an unfortunate dilemma. It boils down to the fact that structs are not valid generic constraints - which is perfectly sane, but a problem arises non the less...
Why would you ever want to constrain generics to a struct??
Well I don't really want to do that - it is a consequence of interface design.
I will explain my predicament by going through the process that led me there:
Say I have an interface with some (potentially) performance critical generic method, i.e:
interface ISomeInterface
{
Result RelativeComputation(ISomeInterface other);
}
However the ISomeInterface itself is not a sufficiently strong restraint: I want the details that make this computation possible to be an implementation detail - I specifically do not want to expose the required data in the interface itself because it is only needed for that one method, and to make matters worse the required data is generic (of a type not used anywhere else in that or any related interface) so exposing it would litter additional generics into every other interface or class that makes use of ISomeInterface.
So to solve the above the next logical evolution of the code is:
interface ISomeInterface<T> where T : ISomeInterface<T>
{
Result RelativeComputation(T other);
}
Now no implementation details need to be exposed, the constraint is strong, and in most cases this would be the happy ending.
However, situations can arise where T is fulfilled by another interface, i.e:
interface IOtherInterface : ISomeInterface<IOtherInterface>
{
//... *other stuff*
}
Now there are issues again! :(
Firstly, as mentioned in the beginning, this method call could end up in performance critical code, so in those cases it might be desirable to make the other parameter a struct. However with the code above this would inevitably be ruined by boxing.
Secondly the constraint on T in RelativeComputation(T other) that appeared to be strong is actually too weak - unless we choose to expose implementation details again! (Either directly in ISomeInterface as discussed before, or in IOtherInterface.)
The second issue I don't really have a good solution for, other than making IOtherInterface generic and applying the Curiously Repeating Template Pattern on it too, and then add the computation method again. The result being:
interface IOtherInterface<T> : ISomeInterface<IOtherInterface<T>>
where T : IOtherInterface<T>
{
new Result RelativeComputation(T other);
//... *other stuff*
}
...eventually the inheritance hierarchy would reach a concrete type and the implementation details will remain hidden.
This is admittedly not very elegant, but it is also not the real issue so I digress.
The real issue at hand is the "First" issue mentioned above.
At first glance it would appear easy to solve; Make the method itself generic, like so:
interface ISomeInterface<T> where T : ISomeInterface<T>
{
Result RelativeComputation<T2>(T2 other) where T2 : T;
}
Now the parameter can be a struct and there will be no boxing anymore!
(This is true even if T above is an interface, e.g. IOtherInterface.)
Great right!
So lets just make a suitable struct, i.e. implementing ISomeInterface (or a more derived interface, as IOtherInterface) and the problem is, eeh... oh.
If you haven't already figured it out, here is what that code would look like:
struct MyPerformantStruct : ISomeInterface<MyPerformantStruct>
{
Result RelativeComputation<T2>(T2 other) where T2 : MyPerformantStruct; // nope! :(
}
The interface requires that exact signature, but that does not compile!
I'm hoping there is some way to solve this?
(Allow structs with no boxing and hide as much implementation as possible!)
To clarify I don't expect there is a way to use a struct as a generic constraint, but rather I am hoping there is another workaround, like a clever design improvement (and / or hack).
EDIT:
Current answers and comments have given me insight into some additional information that is of relevance:
The interface and the implementing type can reside in different assemblies. (The interface is in a library.)
I'm not saying it has to be a struct, I'm saying it should be possible to use either a class or a struct (without boxing).
The "implementation details" I'm referring to is some value type (possibly more than one). The method in question is a bool IsLessThan(...) type of method, something I can't seem to evaluate without boxing, unboxing and runtime-checks (no thanks!) - or without knowing implementation details about the underlying type and exposing those in the interface, thus locking the implementation and defeating the whole point of the abstraction - or restricting it on the interface level, which has other issues mentioned above - or also restricting it on the method level, which is what I wanted to do above, but as it turns out that explicitly prevents T from being a struct.
(I prefer to stay on .net 3.5 but this is not critical.)
My problem description is long and it isn't instantly obvious why generics is required on both the interface and the method, so I will provide a code example and illustrate why it doesn't compile (or would require runtime casting):
This code illustrates the need to constrain to the actual type:
interface IExample
{
bool IsLessThan<T>(T other) where T : IExample;
}
class ExampleClass : IExample
{
short someValue;
public bool IsLessThan<T>(T other) where T : IExample
{
return this.someValue < other.???; // <-- what now?
}
}
First of all, you should ask a question about the actual problem you're solving. It's very hard to understand what you're trying to do, and why you're trying to do it, if you use only examples. Perhaps you're approaching the problem wrong, or is there a better solution to your actual problem. There are a myriad of design patterns that solve many problems.
It looks like you're doing micro-optimizations. Have you measured the code in question? Have you found the bottleneck to be really there?
You'd like code that looks like this:
struct MyPerformantStruct : ISomeInterface<MyPerformantStruct>
{
Result RelativeComputation<T2>(T2 other) where T2 : MyPerformantStruct;
}
But... why? If MyPerformantStruct is a struct, then, by definition, there is no class or struct whatsoever that can inherit from it. The only type that could ever satisfy T2 is... MyPerformantStruct. So why not use that directly?
interface ISomeInterface<T>
where T : struct
{
Result RelativeComputation(T other);
}
struct MyPerformantStruct : ISomeInterface<MyPerformantStruct>
{
Result RelativeComputation(MyPerformantStruct other);
}
Is there a reason T is a generic parameter on the type instead of on the member?
interface ISomeInterface
{
Result RelativeComputation<T>(T other)
where T : struct;
}
struct MyPerformantStruct : ISomeInterface
{
Result RelativeComputation<MyPerformantStruct>(MyPerformantStruct other);
}
Would your code not work with things other than structs? Seems unlikely to me, so then you could drop the struct constraint. You'd still get the benefits of unboxed structs when T is a struct.
interface ISomeInterface
{
Result RelativeComputation<T>(T other);
}
struct MyPerformantStruct : ISomeInterface
{
Result RelativeComputation<MyPerformantStruct>(MyPerformantStruct other);
}
And are you sure you can deal with this struct without boxing it, ever? If not, you might as well just abstract over the type and ask for the struct's interface, or object if you really want to. I'm guessing that the only reason you're working with structs is performance. This is the wrong approach. Structs should represent immutable values, such as a date/time pair or a percentage. Structs aren't boxless collections of mutable fields that you use for performance reasons. And you'll run into issues later on if you try to use them as such.
There are two different problems in your question: struct constraints in generic type definitions with regards to boxing, and how to hide implementation details inside an assembly.
As you pointed out, you cannot set a generic type parameter to be of an explicit struct type. You can, however, enforce a generic type parameter T to both be a struct and implement a specific interface:
interface ISomeInterface<T> where T : struct, ISomeInterface<T>
{
Result RelativeComputation(T other);
}
Now about boxing: generics are meant to prevent boxing from happening (see Benefits of Generics). Consider the following program:
public interface ISomeInterface
{
int GetValue();
}
public struct SomeStruct : ISomeInterface
{
public int GetValue() { return 42; }
}
public interface ISomeOtherInterface1<T> where T : ISomeInterface
{
int ComputeStuff(T other);
}
public interface ISomeOtherInterface2<T> where T : struct, ISomeInterface
{
int ComputeStuff(T other);
}
class ClassImplementingInterface1<T> : ISomeOtherInterface1<T> where T : ISomeInterface
{
public int ComputeStuff(T other) { return other.GetValue(); }
}
class ClassImplementingInterface2<T> : ISomeOtherInterface2<T> where T : struct, ISomeInterface
{
public int ComputeStuff(T other) { return other.GetValue(); }
}
class Program
{
static void Main(string[] args)
{
var value = new SomeStruct();
var obj1 = new ClassImplementingInterface1<SomeStruct>();
var obj2 = new ClassImplementingInterface2<SomeStruct>();
var obj3 = new ClassImplementingInterface1<ISomeInterface>();
var result1 = obj1.ComputeStuff(value);
var result2 = obj2.ComputeStuff(value);
var result3 = obj3.ComputeStuff(value);
}
}
In this exemple I have an interface that is implemented by a struct, and two different generic interfaces: the second one enforces its generic type parameter to be a struct where the first doesn't. In the Main method you see 3 different ways of using those classes. If you look at the assembly code produced by the last 3 lines of code in the Disassembly window in Visual Studio, you will see that the code for the first two calls are identical: no boxing occurs. The assembly code of those two ComputeStuff methods are also identical, because both the code and the effective type parameters are identical. Only the last one causes boxing to happen because it uses the ISomeInterface type as the effective type parameter, and interface types are reference types in the CLR world so any value type passed will indeed be boxed.
About hiding implementation details, it seems unclear to me what you are trying to hide to external code exactly, but maybe what you are missing is that it is possible for public types to implement internal interfaces, if they implement them explicitly:
internal struct PrivateData { ... }
internal interface IMyInterface
{
Result SomePrivateComputation(PrivateData data);
}
public class MyPublicClass : IMyInterface
{
Result IMyInterface.SomePrivateComputation(PrivateData data)
{
...
}
}
Then your code in its assembly (and friend assemblies specified by any InternalsVisibleToAttribute) will be able to do the following:
MyPublicClass myObj;
...
((IMyInterface)myObj).SomePrivateComputation(myData); // No boxing here, no virtual call either: the compiler knows which method to invoke on MyPublicClass.
I've come to the conclusion that what I like to do can not be done.
Something has to give, and in this case it will simply be IOtherInterface.
Simply put, if one wishes to use a struct and not suffer any boxing T in ISomeInterface has to be the struct itself (with the method no longer having its own generics).
Any workaround I can think of would absolutely cost more than a simple boxing instruction.
So I'm closing this as not doable.
Thanks for your replies people.
I have a method with a generic parameter:
internal void DoSomething<T>(T workWithThis)
{
}
I now want to constrain this method to only accept parameters which inherit one of a few interfaces I'd like to specify. However I have not yet found a way to it. What I'd like looks like this:
internal void DoSomething<T>(T workWithThis) where T : ISomething | ISomethingElse
{
}
Obviously this is not working, so I tried it with a static method to check the Type of T:
public static bool CheckType(Type t)
{
return */check here*/
}
internal void DoSomething<T>(T workWithThis) where T : CheckType(typeof(T))
{
}
Obviously this is not going to work either. The question is why? Why is the compiler preventing me from doing that, based on my understanding there is no reason for it not to work
Why is the compiler preventing me from doing that, based on my understanding there is no reason for it not to work
The compiler is preventing you from doing it because you're trying to do something which isn't supported by C# as a language. The syntax you're trying to use does not comply with the productions in section 10.1.5 of the C# spec.
C# as a language simply does not support the scenario you require.
Now as for why the language doesn't allow this sort of flexibility - that comes down to the normal balancing act of:
How many developers would benefit from this and how much
Extra burden on other developers to understand a more complicated language
Resources (primarily within Microsoft) required to design the language feature, implement and test it
Oh, and of course this isn't just C# - the CLR would have to support such a restriction as well, and it would at least encourage other CLR languages to understand it too.
I suggest you solve this by having two separate methods. Note that they can't just be overloads of generic methods, as overloads cannot differ only by generic type constraints. If you don't mind about boxing for value types implementing the interface, you could overload with:
internal void DoSomething(ISomething something)
{
}
internal void DoSomething(ISomethingElse somethingElse)
{
}
... although then if you pass in a value where the expression is a type implementing both interfaces, you'll end up with overload ambiguity.
Alternatively, just give the two methods different names.
The compiler has to verify all the constraints at compile time, and cannot call a method to do so.
The only things you can specify in the where constraints are:
new() - require a parameterless constructor
class - must be a reference type
struct - must be a value type
SomeBaseClass
ISomeInterface
T : U - must be, inherit from, or implement one of the other generic parameters
See the C# Programming Guide - Constraints on Type Parameters for more information.
As for why, you should never have to answer "I see no reason for this to work". You have to start in the opposite direction, "Why should this work", and then come up with enough plausible and realistic scenarios and requirements to make it worthwhile to implement. See Minus 100 Points by Eric Gunnerson.
To fix this in your code, you should derive both interfaces from a common interface, and add a constraint on that instead.
If the two interfaces have nothing in common, then I question the benefit of actually adding a constraint in the first place.
For instance, if your code is going to call a method on the objects being used with the generic type/method, then obviously both interfaces have to have the same notion about what that method is, and the only way to do that would be for the method to be defined in a common base interface. That the two interfaces happen to have the same method or property declared, with the same signature, does not make it the same method.
Having said that, are you sure you even need generics here?
How about just declaring two methods, each taking one such interface?
internal void DoSomething(ISomething workWithThis)
internal void DoSomething(ISomethingElse workWithThis)
The compiler uses the generic constraint to determine what operations is available on T within the generic method - so allowing an or expression would not be type safe. For example you have two interfaces IFirst and ISecond:
public interface IFirst
{
void First();
}
public interface ISecond
{
void Second();
}
internal void DoSomething<T>(T workWithThis) where T : IFirst or ISecond
{
//How to call this method if the type is ISecond
workWithThis.First();
//How to call this method if the type is IFirst
workWithThis.Second();
}
You can define an empty interface that holds all of them.
Remember, that in C# interfaces can have multiple inheritance.
For example:
public interface IHolder : ISomething, ISomethingElse
{
}
and for generic
internal void DoSomething<T>(T workWithThis) where T : IHolder
{
}
For example I now created a this tiny class:
public static class FileSystemInfoComparers<T> where T : FileSystemInfo
{
public static IEqualityComparer<T> FullName
{
get { return new FullNameComparer(); }
}
private class FullNameComparer : IEqualityComparer<T>
{
public bool Equals(T x, T y) { return x.FullName == y.FullName; }
public int GetHashCode(T obj) { return obj.FullName.GetHashCode(); }
}
}
I would like it if I could just do
var comparer = FileSystemInfoComparers.FullName;
and have an instance of IEqualityComparer<FileSystemInfo>, since I didn't specify any type and FileSystemInfo is the most generic type T can be. With no type constraint the default type could for example be object or something.
Maybe not the best example, but anyways just got curious here :p
Sounds like a recipe for trouble to me.
In particular, you could easily create a non-generic class called FileSystemInfoComparers with a static property of the same name, and suddenly your code would mean something completely different.
I'd rather keep things simple. (Generics are complicated enough already, and type inference in particular is pretty hairy.)
That is an interesting idea and it could definitely work but consider that it would only work in cases where the generic type argument is constrained with a concrete type.
The .NET compilers are very good at type inference but tend to shy away from making any assumptions. I don't see any reason why this couldn't be done except that it would only work in a small number of highly-specific instances. Since it has no general purpose I would imagine that Microsoft would be less inclined to make a change to support it.
You could possible do this by having a helper class that has the default type you want set. But what you are asking for currently cannot be done in C# 3.0.