I have a class that will require a varying number of Func delegates to be passed in the contructor. Each of these delegates will point to a different function, each with a different return type, and with a varying number of parameters (of type double). Each of these functions will then be called accordingly.
Question 1. Right now, to make things easier for those using this class, I am thinking about allowing the user to pass a List<object> of Func delegates. Is this possible, and if so am I able to determine the return type and number of params required for each Func in the method in which the List<object> is passed to (i.e. the constructor) ?
Question 2. If the above is not feasible, will I need to overload the constructor with every different combination of return types/number of params and route each Func accordingly -_- ... if not can someone point me in the right direction, I feel like i'm approaching this in the wrong way ...
note - coming from a python background, I would do something like this (i'm inexperienced in c#):
import inspect
def test(x): return x
inspect.getargspec(test)
returns: ArgSpec(args=['x'], varargs=None, keywords=None, defaults=None)
Many thanks
Question 1. Right now, to make things easier for those using this class, I am thinking about allowing the user to pass a List of Func delegates. Is this possible, and if so am I able to determine the return type and number of params required for each Func in the method in which the List is passed to (i.e. the constructor) ?
Not really. You could allow a List<Delegate> (or some other collection with an element type of Delegate), but nothing Func-specific, for two reasons:
Func is effectively a family of types, with different number of generic type parameters. These types are entirely separate as far as the CLR is concerned; Func<TResult> and Func<T, TResult> are as different as Action<T> and EventHandler.
Even if you were only dealing with several values of the same generic type of delegate, the fact that they could have different type arguments means they're different types to the CLR; there's no way of saying List<Func<>> for "a list of functions with potentially varying type arguments". Again, the CLR treats them as separate types - although this time at least one with a common generic type definition.
Question 2. If the above is not feasible, will I need to overload the constructor with every different combination of return types/number of params and route each Func accordingly
Well, there are several options:
Make all the parameters optional, giving each of them a default value of null, then use named arguments when calling the constructor:
var foo = new Foo(clickHandler: () => ...,
keyHandler: key => ...);
Create a builder, so that the various functions can be set as properties - this works very well with the object initializer syntax:
var foo = new Foo.Builder {
ClickHandler = () => ...,
KeyHandler = () => ...
}.Build();
Both of the latter solutions depends on you really having a specific named purpose, of course.
It would help if you could be clearer about what you're trying to achieve - as dtb says, polymorphism may be a better fit here. You could create an abstract class with no-op implementations of virtual methods, and implementations could choose which ones to override.
Related
For a test, I want to create a generic "helper" method which will take take two arguments, the first argument is a function (or a reference to the function) and the 2nd argument is a list of objects for that function that are to be called as its parameters.
The following does this perfectly:
CallMyFunctionWithParamsPlease(new Func<int, int>(MyMethod), new object[] {1});
public static int CallMyFunctionWithParamsPlease(Delegate func, params object[] args)
{
func.DynamicInvoke(args);
return 3;
}
The thing is, this doesn't look very nice when calling it and I wish to abstract it into another method to act as syntatic sugar.
Ideally I want it to be called like this:
CallMyFunctionWithParamsPlease(myMethod, new Object[] {1});
From what I can gather, there is no elegant solution to do this in C# since I cannot pass myMethod by itself as a reference anywhere, instead I must pass it by declaring a new Func along with the return type of the method. Since I'm not using this return type anywhere, I'm not sure why it's necessary to input this information. My limited understanding is that because C# is statically typed, the compiler must know everything and things like this just aren't possible.
Is this true or not? How would I create syntatic sugar to simply pass a method to another method which can be called there without needing to invoke "new Func"? I would have thought simply passing the function as a reference pointer would allow me to do this, but I'm having difficultly doing this too. I looked into delegates, using "unsafe" with pointers, and a few other options. None of them seem to make this possible, or if they do, they didn't explain it in a manner that I could understand.
I simply want to pass a method to another method, and invoke it with a variable list of object params with variable length whereby I don't need to specify this whilst invoking it. I'm not sure if I'm trying to force C# to do something it's not meant to do here, and instead I'd be better off using a dynamically typed language to do this. The problem is I really enjoy the intellisense that the static typing of C# offers, along with the performance improvements over a language like Python. I'd just like a way to syntactically abstract away the boilerplate with my own helper methods for things like this.
UPDATE: Thanks to the comments here it seems I can do this with a lambda expression nice and elegantly. The signature can be simply changed to public static long CallMyFunctionWithParamsPlease<T>(Func<T> func)
If deferred execution is what you want simply pass a Func<TReturnType> to your method (or class). The calling method doesn't need to know how many parameters are involved.
e.g. Assuming MyMethod has a signature int MyMethod(int arg):
CallMyFunctionWithParamsPlease(() => MyMethod(1));
public static int CallMyFunctionWithParamsPlease(Func<int> func)
{
return func();
}
If MyMethod takes two parameters, it's the same call:
CallMyFunctionWithParamsPlease(() => MyMethod(1, 2));
I have a class that will require a varying number of Func delegates to be passed in the contructor. Each of these delegates will point to a different function, each with a different return type, and with a varying number of parameters (of type double). Each of these functions will then be called accordingly.
Question 1. Right now, to make things easier for those using this class, I am thinking about allowing the user to pass a List<object> of Func delegates. Is this possible, and if so am I able to determine the return type and number of params required for each Func in the method in which the List<object> is passed to (i.e. the constructor) ?
Question 2. If the above is not feasible, will I need to overload the constructor with every different combination of return types/number of params and route each Func accordingly -_- ... if not can someone point me in the right direction, I feel like i'm approaching this in the wrong way ...
note - coming from a python background, I would do something like this (i'm inexperienced in c#):
import inspect
def test(x): return x
inspect.getargspec(test)
returns: ArgSpec(args=['x'], varargs=None, keywords=None, defaults=None)
Many thanks
Question 1. Right now, to make things easier for those using this class, I am thinking about allowing the user to pass a List of Func delegates. Is this possible, and if so am I able to determine the return type and number of params required for each Func in the method in which the List is passed to (i.e. the constructor) ?
Not really. You could allow a List<Delegate> (or some other collection with an element type of Delegate), but nothing Func-specific, for two reasons:
Func is effectively a family of types, with different number of generic type parameters. These types are entirely separate as far as the CLR is concerned; Func<TResult> and Func<T, TResult> are as different as Action<T> and EventHandler.
Even if you were only dealing with several values of the same generic type of delegate, the fact that they could have different type arguments means they're different types to the CLR; there's no way of saying List<Func<>> for "a list of functions with potentially varying type arguments". Again, the CLR treats them as separate types - although this time at least one with a common generic type definition.
Question 2. If the above is not feasible, will I need to overload the constructor with every different combination of return types/number of params and route each Func accordingly
Well, there are several options:
Make all the parameters optional, giving each of them a default value of null, then use named arguments when calling the constructor:
var foo = new Foo(clickHandler: () => ...,
keyHandler: key => ...);
Create a builder, so that the various functions can be set as properties - this works very well with the object initializer syntax:
var foo = new Foo.Builder {
ClickHandler = () => ...,
KeyHandler = () => ...
}.Build();
Both of the latter solutions depends on you really having a specific named purpose, of course.
It would help if you could be clearer about what you're trying to achieve - as dtb says, polymorphism may be a better fit here. You could create an abstract class with no-op implementations of virtual methods, and implementations could choose which ones to override.
I have the following extension methods for my MessageBus:
public static class MessageBusMixins
{
public static IDisposable Subscribe<T>(
this IObservable<T> observable,
MessageBus bus)
where T:class
{
...
}
public static IDisposable Subscribe<T>(
this IObservable<Maybe<T>> observable,
MessageBus bus)
{
...
}
}
which compiles fine. However when I try to use it:
IObservable<Maybe<string>> source = ...;
MessageBus bus = ...;
source.Subscribe(bus);
I get the error that neither of the two candidate methods
are most specific. However I thought that Maybe<T> would
be more specific than T or is that not correct?
EDIT
It gets curiouser because if I call the extension method
explicitly then:
MessageBus.SubscribeTo(source, bus);
Then it works and picks the correct method.
Well, you can fix it by specifying the type argument:
source.Subscribe<string>(bus);
... as that's now only the second method is applicable.
Otherwise, the compiler could call either of:
source.Subscribe<string>(bus);
source.Subscribe<Maybe<string>>(bus);
If you think the first is more specific than the second, you'll have to find the rule in the C# specification which says so :) It's not an unreasonable expectation, but I don't think the normal "more specific" conversions apply to type parameters as well as regular parameters.
So for example, in section 7.5.3.2 of the C# 4 spec ("Better Function Member") there a rule about:
Otherwise if MP has more specific parameter types than MQ, then MP is better than MQ. [... lots of details about less/more specific ...]
... but there's no similar point about type parameters. (The second about normal parameters talks about type arguments, but that's within the parameter types themselves.)
Another alternative is to simply give the methods different names. Do they have subtly different behaviour? If so, why not make that really obvious via the naming? You really don't want someone to get the wrong behaviour just because they were surprised about which overload was called.
I have two overloaded methods, both called FunctionX. One of them returns a Boolean object, and one a predefined class called Logs.
The error I'm getting is: The call is ambiguous between the following methods or properties: 'FunctionX(string)' and 'FunctionX(string)'.
In one of my other methods, I call FunctionX(string), expecting the Log object, but it's throwing this error. I thought the compiler was supposed to look at the return type and figure this out for itself. How can I fix this?
While languages like Perl and Haskell do support overloading by return type, function overloading by return type is not supported by most statically typed languages. So, it is better if you do not make this trivial problem a part of your code.
Added:
You can find more answers in an earlier Stackoverflow discussion here:
Function overloading by return type?
You cannot have more then one function using the same signature e.g.
string Function1(bool t)
int Function1(bool t)
You need to call each function different names, or having different params e.g.
string Function1(bool t)
int Function1(bool t, int g)
You can't overload a method to have different return types. How would the compiler know what to call here?
string Foo() { /* ... */ }
int Foo() { /* ... */ }
object f = Foo();
Language designers need to take all circumstances into account, not only those that are most trivial.
There is no way for the compiler to distinguish between functions with the same method signature except for the return type. And as far as I know no compiler can that is strongly typed. You must change the signature in some way. On option is to use a generic function and provide the return type.
The return type does not partecipating on overloading in C#.
You can, for example:
Declare separate functions FunctionXToLog and FunctionXToBool
FunctionX(string s, out Log logobject), FunctionX(string s, out bool value)
Just to give you a hint.
You cannot have two function with the same signature only differing in return value!
The signature of a method is its name and the types of its parameters - only. Its return type is not part of its signature.
Hence the problem you are having, since the two methods have identical signatures. The compiler does not use the return type, or the type of the object being assigned to, to determine which method to call.
You will have to specify the classname of the method you want (assuming that the two methods are in different classes). Or you will have to provide another parameter for one of the methods, to give it a different signature. Or, finally, you will have to rename one of the methods to make them unambiguous.
I have an overload method - the first implementation always returns a single object, the second implementation always returns an enumeration.
I'd like to make the methods generic and overloaded, and restrict the compiler from attempting to bind to the non-enumeration method when the generic type is enumerable...
class Cache
{
T GetOrAdd<T> (string cachekey, Func<T> fnGetItem)
where T : {is not IEnumerable}
{
}
T[] GetOrAdd<T> (string cachekey, Func<IEnumerable<T>> fnGetItem)
{
}
}
To be used with...
{
// The compile should choose the 1st overload
var customer = Cache.GetOrAdd("FirstCustomer", () => context.Customers.First());
// The compile should choose the 2nd overload
var customers = Cache.GetOrAdd("AllCustomers", () => context.Customers.ToArray());
}
Is this just plain bad code-smell that I'm infringing on here, or is it possible to disambiguate the above methods so that the compiler will always get the calling code right?
Up votes for anyone who can produce any answer other than "rename one of the methods".
Rename one of the methods. You'll notice that List<T> has an Add and and AddRange method; follow that pattern. Doing something to an item and doing something to a sequence of items are logically different tasks, so make the methods have different names.
This is a difficult use case to support because of how the C# compiler performs overload resolution and how it decides which method to bind to.
The first issue is that constraints are not part of the signature of a method and won't be considered for overload resolution.
The second problem you've got to overcome is that the compiler chooses the best match from the available signatures - which, when dealing with generics, generally means that SomeMethod<T>(T) will be considered a better match than SomeMethod<T>( IEnumerable<T> ) ... particularly when you've got parameters like T[] or List<T>.
But more fundamentally, you have to consider whether operating on a single value vs. a collection of values is really the same operation. If they are logically different, then you probably want to use different names just for clarity. Perhaps there are some use cases where you could argue that the semantic differences between single objects and collections of objects are not meaningful ... but in that case, why implement two different methods at all? It's unclear that method overloading is the best way to express the differences. Let's look at an example that lends to the confusion:
Cache.GetOrAdd("abc", () => context.Customers.Frobble() );
First, note that in the example above we are choosing to ignore the return parameter. Second, notice that we call some method Frobble() on the Customers collection. Now can you tell me which overload of GetOrAdd() will be called? Clearly without knowing the type that Frobble() returns it's not possible. Personally I believe that code whose semantics can't be readily inferred from the syntax should be avoided when possible. If we choose better names, this issue is alleviated:
Cache.Add( "abc", () => context.Customers.Frobble() );
Cache.AddRange( "xyz", () => context.Customers.Frobble() );
Ultimately, there are only three options to disambiguate the methods in your example:
Change the name of one of the methods.
Cast to IEnumerable<T> wherever you call the second overload.
Change the signature of one of the methods in a way that the compiler can differentiate.
Option 1 is self-evident, so I'll say no more about it.
Options 2 is also easy to understand:
var customers = Cache.GetOrAdd("All",
() => (IEnumerable<Customer>)context.Customers.ToArray());
Option 3 is more complicated. Let's look at ways we can be achieve it.
On approach is by changing the signature of the Func<> delegate, for instance:
T GetOrAdd<T> (string cachekey, Func<object,T> fnGetItem)
T[] GetOrAdd<T> (string cachekey, Func<IEnumerable<T>> fnGetItem)
// now we can do:
var customer = Cache.GetOrAdd("First", _ => context.Customers.First());
var customers = Cache.GetOrAdd("All", () => context.Customers.ToArray());
Personally, I find this option terribly ugly, unintuitive, and confusing. Introducing an unused parameter is terrible ... but, sadly it will work.
An alternative way of changing the signature (which is somewhat less terrible) is to make the return value an out parameter:
void GetOrAdd<T> (string cachekey, Func<object,T> fnGetItem, out T);
void GetOrAdd<T> (string cachekey, Func<IEnumerable<T>> fnGetItem, out T[])
// now we can write:
Customer customer;
Cache.GetOrAdd("First", _ => context.Customers.First(), out customer);
Customer[] customers;
var customers = Cache.GetOrAdd("All",
() => context.Customers.ToArray(), out customers);
But is this really better? It prevents us from using these methods as parameters of other method calls. It also makes the code less clear and less understandable, IMO.
A final alternative I'll present is to add another generic parameter to the methods which identifies the type of the return value:
T GetOrAdd<T> (string cachekey, Func<T> fnGetItem);
R[] GetOrAdd<T,R> (string cachekey, Func<IEnumerable<T>> fnGetItem);
// now we can do:
var customer = Cache.GetOrAdd("First", _ => context.Customers.First());
var customers = Cache.GetOrAdd<Customer,Customer>("All", () => context.Customers.ToArray());
So can use hints to help the compiler to choose an overload for us ... sure. But look at all of the extra work we have to do as the developer to get there (not to mention the introduced ugliness and opportunity for mistakes). Is it really worth the effort? Particularly when an easy and reliable technique (naming the methods differently) already exists to help us?
Use only one method and have it detect the IEnumerable<T> case dynamically rather than attempting the impossible via generic constraints. It would be "code smell" to have to deal with two different cache methods depending on if the object to store/retrieve is something enumerable or not. Also, just because it implements IEnumerable<T> does not mean it is necessarily a collection.
constraints don't support exclusion, which may seem frustrating at first, but is consistent and makes sense (consider, for example, that interfaces don't dictate what implementations can't do).
That being said, you could play around with the constraints of your IEnumerable overload...maybe change your method to have two generic typings <X, T> with a constraint like "where X : IEnumerable<T>" ?
ETA the following code sample:
void T[] GetOrAdd<X,T> (string cachekey, Func<X> fnGetItem)
where X : IEnumerable<T>
{
}