This is just a curiosity question I was wondering if anyone had a good answer to:
In the .NET Framework Class Library we have for example these two methods:
public static IQueryable<TSource> Where<TSource>(
this IQueryable<TSource> source,
Expression<Func<TSource, bool>> predicate
)
public static IEnumerable<TSource> Where<TSource>(
this IEnumerable<TSource> source,
Func<TSource, bool> predicate
)
Why do they use Func<TSource, bool> instead of Predicate<TSource>? Seems like the Predicate<TSource> is only used by List<T> and Array<T>, while Func<TSource, bool> is used by pretty much all Queryable and Enumerable methods and extension methods... what's up with that?
While Predicate has been introduced at the same time that List<T> and Array<T>, in .net 2.0, the different Func and Action variants come from .net 3.5.
So those Func predicates are used mainly for consistency in the LINQ operators. As of .net 3.5, about using Func<T> and Action<T> the guideline states:
Do use the new LINQ types Func<> and
Expression<> instead of custom
delegates and predicates
I've wondered this before. I like the Predicate<T> delegate - it's nice and descriptive. However, you need to consider the overloads of Where:
Where<T>(IEnumerable<T>, Func<T, bool>)
Where<T>(IEnumerable<T>, Func<T, int, bool>)
That allows you to filter based on the index of the entry as well. That's nice and consistent, whereas:
Where<T>(IEnumerable<T>, Predicate<T>)
Where<T>(IEnumerable<T>, Func<T, int, bool>)
wouldn't be.
Surely the actual reason for using Func instead of a specific delegate is that C# treats separately declared delegates as totally different types.
Even though Func<int, bool> and Predicate<int> both have identical argument and return types, they are not assignment-compatible. So if every library declared its own delegate type for each delegate pattern, those libraries would not be able to interoperate unless the user inserts "bridging" delegates to perform conversions.
// declare two delegate types, completely identical but different names:
public delegate void ExceptionHandler1(Exception x);
public delegate void ExceptionHandler2(Exception x);
// a method that is compatible with either of them:
public static void MyExceptionHandler(Exception x)
{
Console.WriteLine(x.Message);
}
static void Main(string[] args)
{
// can assign any method having the right pattern
ExceptionHandler1 x1 = MyExceptionHandler;
// and yet cannot assign a delegate with identical declaration!
ExceptionHandler2 x2 = x1; // error at compile time
}
By encouraging everyone to use Func, Microsoft is hoping that this will alleviate the problem of incompatible delegate types. Everyone's delegates will play nicely together, because they will just be matched up based on their parameter/return types.
It doesn't solve all problems, because Func (and Action) can't have out or ref parameters, but those are less commonly used.
Update: in the comments Svish says:
Still, switching a parameter type from
Func to Predicate and
back, doesn't seem to make any
difference? At least it still compiles
without any problems.
Yes, as long as your program only assigns methods to delegates, as in the first line of my Main function. The compiler silently generates code to new a delegate object that forwards on to the method. So in my Main function, I could change x1 to be of type ExceptionHandler2 without causing a problem.
However, on the second line I try to assign the first delegate to another delegate. Even thought that 2nd delegate type has exactly the same parameter and return types, the compiler gives error CS0029: Cannot implicitly convert type 'ExceptionHandler1' to 'ExceptionHandler2'.
Maybe this will make it clearer:
public static bool IsNegative(int x)
{
return x < 0;
}
static void Main(string[] args)
{
Predicate<int> p = IsNegative;
Func<int, bool> f = IsNegative;
p = f; // Not allowed
}
My method IsNegative is a perfectly good thing to assign to the p and f variables, as long as I do so directly. But then I can't assign one of those variables to the other.
The advice (in 3.5 and above) is to use the Action<...> and Func<...> - for the "why?" - one advantage is that "Predicate<T>" is only meaningful if you know what "predicate" means - otherwise you need to look at object-browser (etc) to find the signatute.
Conversely Func<T,bool> follows a standard pattern; I can immediately tell that this is a function that takes a T and returns a bool - don't need to understand any terminology - just apply my truth test.
For "predicate" this might have been OK, but I appreciate the attempt to standardise. It also allows a lot of parity with the related methods in that area.
Related
So, I ran across an answer by Servy ( https://stackoverflow.com/a/15098242/496680 ) and some of his code does this:
public static int BinarySearch<TSource, TKey>(...)
for an extension method, but he calls it like this:
arr.BinarySearch(...)
I asked around and somebody metioned that its an implied generic type parameter.
I googled them but found no information on them.
I understand how generics work but I'm failing to understand how/when to use these.
Why does servy use them in his extention method?
Is there a more official name for these that I can search for?
Well, you left out the most important part that makes it all work. The type parameters can be inferred by the actual object parameters passed in.
For instance:
static class Extensions {
internal static IEnumerable<U> Test<T, U>(
this IEnumerable<T> items,
Func<T, U> converter) {
foreach (T item in items) {
yield return converter(item);
}
}
}
This extension method works on any IEnumerable class and will convert each item in the enumeration to another type based on the converter you provided. This is standard generics.
Now, there are many ways to call this method:
IEnumerable<int> values = Enumerable.Range<int>(1, 10);
Func<int, string> converter = i => i.ToString("0.00");
// Variation 1, explicit calling
IEnumerable<string> results1 = Extensions.Test<int, string>(values, converter);
// Variation 2, explicit calling with type inference
IEnumerable<string> results2 = Extensions.Test(values, converter);
// Variation 3, extension method calling, still providing explicit types
IEnumerable<string> results3 = values.Test<int, string>(converter);
// Variation 4, extension method with type inference
IEnumerable<string> results4 = values.Test(converter);
All four variations call the same method and return the same result. Type inference works by looking at the parameters passed and automatically inferring their types based on what's being provided. In our examples above, it's able to determine that type T is of type int because we passed in an IEnumerable<int> into the parameter for IEnumerable<T>. It is also able to infer that type U is of type string because we passed in a Func matching the initial type of T with int and returning a string. So the Func<T, U> is filled in with our converter function of Func<int, string>.
From the inference above, it's a standard generic method at that point. Type inference and extension methods are nothing more than convenience/syntactic sugar. In fact, if you decompile the output you can see that extension methods are replaced with static calls and are usually defined with the type parameters explicitly filled out. (This varies based on your decompiler and the set options).
He uses a generic method in this case because it allows his method to work with any type contained within a Collection<T>. The generic method makes this very flexible, and usable for any type. He uses the type inferrence when calling the method because it simplifies the code at the call site.
The automatic handling is called Type Inferrence, and is covered, in detail, in the C# Language Specification, section 7.5.2: Type Inferrence. If you want to understand it in detail, I would recommend downloading the C# language specification.
The term I usually hear is 'type inference'.
I am writing a simple Memoize helper that allows caching method results instead of computing them every time. However, when I try to pass a method into Memoize, the compiler can't determine the type arguments. Aren't they obvious from my method signature? Is there a way around this?
Sample code:
using System;
using System.Collections.Concurrent;
public static class Program
{
public static Func<T, V> Memoize<T, V>(Func<T, V> f)
{
var cache = new ConcurrentDictionary<T, V>();
return a => cache.GetOrAdd(a, f);
}
// This is the method I wish to memoize
public static int DoIt(string a) => a.Length;
static void Main()
{
// This line fails to compile (see later for error message)
var cached1 = Memoize(DoIt);
// This works, but is ugly (and doesn't scale to lots of type parameters)
var cached2 = Memoize<string, int>(DoIt);
}
}
Error message:
error CS0411: The type arguments for method 'Program.Memoize<T, V>(Func<T, V>)'
cannot be inferred from the usage. Try specifying the type arguments explicitly.
Isn't DoIt() signature compatible with Func<string, int>?
Yes it is. It's fine to convert it to that specific type, like this for example:
Func<string, int> func = DoIt;
var cachedDoit = Memoize(func);
The problem you're running into is that type inference basically doesn't work particularly well with method group conversions. When you pass DoIt as an argument, that's a method group. In your case it only refers to a single method, but it could refer to multiple methods, with different signatures... and that complicates things.
I often see this come up with LINQ, where I'd like to call foo.Select(SomeMethodGroup), but type inference fails. There's some support for method groups within type inference, but it's not everything we might want it to be.
This isn't a matter of the C# team being lazy... type inference is hugely complicated, and any change is really fraught with danger in terms of backward compatibility. It's in section 7.5.2 of the C# 5 specification if you want to have a look - but frankly that's part of the spec where I get lost really quickly.
So, I ran across an answer by Servy ( https://stackoverflow.com/a/15098242/496680 ) and some of his code does this:
public static int BinarySearch<TSource, TKey>(...)
for an extension method, but he calls it like this:
arr.BinarySearch(...)
I asked around and somebody metioned that its an implied generic type parameter.
I googled them but found no information on them.
I understand how generics work but I'm failing to understand how/when to use these.
Why does servy use them in his extention method?
Is there a more official name for these that I can search for?
Well, you left out the most important part that makes it all work. The type parameters can be inferred by the actual object parameters passed in.
For instance:
static class Extensions {
internal static IEnumerable<U> Test<T, U>(
this IEnumerable<T> items,
Func<T, U> converter) {
foreach (T item in items) {
yield return converter(item);
}
}
}
This extension method works on any IEnumerable class and will convert each item in the enumeration to another type based on the converter you provided. This is standard generics.
Now, there are many ways to call this method:
IEnumerable<int> values = Enumerable.Range<int>(1, 10);
Func<int, string> converter = i => i.ToString("0.00");
// Variation 1, explicit calling
IEnumerable<string> results1 = Extensions.Test<int, string>(values, converter);
// Variation 2, explicit calling with type inference
IEnumerable<string> results2 = Extensions.Test(values, converter);
// Variation 3, extension method calling, still providing explicit types
IEnumerable<string> results3 = values.Test<int, string>(converter);
// Variation 4, extension method with type inference
IEnumerable<string> results4 = values.Test(converter);
All four variations call the same method and return the same result. Type inference works by looking at the parameters passed and automatically inferring their types based on what's being provided. In our examples above, it's able to determine that type T is of type int because we passed in an IEnumerable<int> into the parameter for IEnumerable<T>. It is also able to infer that type U is of type string because we passed in a Func matching the initial type of T with int and returning a string. So the Func<T, U> is filled in with our converter function of Func<int, string>.
From the inference above, it's a standard generic method at that point. Type inference and extension methods are nothing more than convenience/syntactic sugar. In fact, if you decompile the output you can see that extension methods are replaced with static calls and are usually defined with the type parameters explicitly filled out. (This varies based on your decompiler and the set options).
He uses a generic method in this case because it allows his method to work with any type contained within a Collection<T>. The generic method makes this very flexible, and usable for any type. He uses the type inferrence when calling the method because it simplifies the code at the call site.
The automatic handling is called Type Inferrence, and is covered, in detail, in the C# Language Specification, section 7.5.2: Type Inferrence. If you want to understand it in detail, I would recommend downloading the C# language specification.
The term I usually hear is 'type inference'.
I've got legacy code that defined the following helper
public delegate R Function<T, R>(T t);
But I want to supply a Func<T,TResult>
casting attempts fail to compile
Cannot convert type 'System.Func<T,TResult>' to 'Rhino.Mocks.Function<T,TResult>'
Is there a way that not only compiles, but functions at runtime?
The problem is you are trying to combine two different delegate types: Func<T, TResult> and Function<T, TResult>. Even though they have the same signature they are different, and hence incompatible, types.
Use a lambda to create a conversion layer between the two.
Func<T, TResult> func = ...;
TheMethod(x => func(x));
Jared and phoog are both correct. A third way to do the same thing, just to round it out, is:
Func<int, string> func = i => i.ToString();
Function<int, string> function = func.Invoke;
That is, the new delegate is a delegate to the invoke method of the first delegate, which has the correct signature.
That one has to do this is vexing in the extreme. Were we designing the runtime from scratch today, knowing what we now known about how people use delegate types, I think it likely that there would either be one built in delegate type for each signature (much as there is one "single dimensional array of integers" type), or that there would be "structural identity" amongst delegate types. Next time you design the type system for a virtual machine, keep that in mind.
You can create a lambda as JaredPar suggests, or pass one to the constructor of the other:
Func<int, string> f1 = i => i.ToString();
Function<int, string> f2 = new Function<int, string>(f1);
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Why is Func<T> ambiguous with Func<IEnumerable<T>>?
I noticed a very weird overload resolution issue with generics...
Consider the following methods:
static void Foo<TSource>(TSource element, Func<TSource, int> selector)
{
"int".Dump();
}
static void Foo<TSource>(TSource element, Func<TSource, double> selector)
{
"double".Dump();
}
static T Identity<T>(T value)
{
return value;
}
(C# 4, tested in LINQPad)
If I try to call Foo with a lambda expression as the selector, everything works fine:
Foo(42, x => x); // prints "int"
But if I replace x => x with Identity, the compiler can't decide between the 2 Foo overloads:
Foo(42, Identity);
// The call is ambiguous between the following methods or properties:
// 'UserQuery.Foo<int>(int, System.Func<int,int>)' and
// 'UserQuery.Foo<int>(int, System.Func<int,double>)'
How can the second overload be a valid candidate ? Type inference correctly determines that TSource is int, so the T parameter for the Identity method has to be int as well, so the return type has to be int too... Identity could be a Func<int,int> or a Func<double,double>, but not a Func<int,double>!
And it gets worse! Even if I specify all type parameters explicitly, I still get the same error:
Foo<int>(42, Identity<int>); // The call is ambiguous...
How can there be any ambiguity here? As far as I can tell, there is no way the overload that takes a Func<int,double> can be a candidate. I guess the explanation must be somewhere in the specifications, but I can't find the relevant bit... or it might be a bug in the compiler, but I guess it's unlikely.
Note that it does work if I explicitly create the delegate:
Foo(42, new Func<int, int>(Identity)); // prints "int"
So, could someone explain what's going on here? Also, why does it work with a lambda but not with a method group?
Isn't it simply because the return type isn't part of the method's signature?
The fact that the Identity<T> method's argument type and return type are guaranteed to be the same isn't taken into account by the compiler when attempting to decide which overload of Foo<TSource> is required. If the return type isn't considered then Identity<int> could equally be convertible to Func<int, int>, Func<int, double> or Func<int, anything>.
I think that LukeH is correct. However, to answer the second bit of your question: the delegate of the lambda will already have all types filled in (e.g. always be a Func<int, int> if TSource is an int), which is why there is no ambiguity in that case. It's not like a function signature where the return type is ignored.