I tried to apply operators on Generics (for my example ,multiplication)
public static List<TOutput> Conversion<TOutput>(List<TInput> input)
{
List<TOutput> outList = new List<TOutput>();
foreach(TInput tinput in input)
{
double dbl = tinput *tinput;
outList.Add(dbl);
}
return outList;
}
Any workaround for fixing it?
Not possible without reflecting upon the type. There is some code that does this available as part of MiscUtil.
This is now possible in C# 11 / .NET 7 (or above):
public static List<TOutput> Conversion<TInput, TOutput>(List<TInput> input)
where TInput : IMultiplyOperators<TInput, TInput, TOutput>
{
List<TOutput> outList = new List<TOutput>();
foreach (TInput tinput in input)
{
TOutput product = tinput * tinput;
outList.Add(product);
}
return outList;
}
The compiler shouldn't allow you to assign a double to an unknown type:
outList.Add(dbl);
For all it knows, you could be trying to assign a dbl to a type of FluxCapacitor. Your code is trying to accomplish two incompatible things: to return a list of generic (unknown) type, and 2) to force that type to be a double. This doesn't make sense, which is why you're having difficulty. You can use reflection (as Porges pointed out with an excellent link) to solve this dynamically, but you really need to ask yourself: why are you trying to assign a floating point number to a class that has an unknown type? The calling code could be asking for a result of List<bool>. How much sense would it make to try to assign
double foo = 1.5;
bool bar = foo;
? Zero. You can make the compiler do anything with enough somersaults, but you need to reevaluate the purpose of your routine, why you're trying to put a specific datatype into a generic one, and whether or not this routine needs to return a generic list.
The "MiscUtil" answer (already accepted) would be my first choice ;-p
You might also consider LINQ at the caller:
var prodList = originalList.Select(x=>x*x).ToList();
Since the caller knows the type (assuming it isn't itself generic, this should work.
Just for completeness, another option here (in 4.0) is dynamic:
public static List<TOutput> Conversion<TOutput>(List<TInput> input)
{
List<TOutput> outList = new List<TOutput>();
foreach(TInput tinput in input)
{
TOutput square = (dynamic)tinput * (dynamic)tinput;
outList.Add(square);
}
return outList;
}
You could use a cached lambda to do your calculation (and/or conversion).
This doesn't require the DLR or the dynamic keyword, so it's perfectly usable in C# 3.0
static class Squarer<T>
{
private static readonly Func<T, T> _square;
static Squarer()
{
ParameterExpression x = Expression.Parameter(typeof(T), "x");
_square = Expression.Lambda<Func<T, T>>(
Expression.Multiply(x, x),
x).Compile();
}
public static T Square(T value)
{
return _square.Invoke(value);
}
}
Console.WriteLine(Squarer<double>.Square(1234.5678));
Console.WriteLine(Squarer<decimal>.Square(1234.5678m));
Console.WriteLine(Squarer<int>.Square(1234));
Related
I have a class with a member Predicate which I would like to use in a Linq expression:
using System.Linq;
class MyClass
{
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(m_filterExpression);
}
private IEnumerable<Instrument> m_instrumentList;
private Predicate<Instrument> m_filterExpression;
}
As I read that "Predicate<T> is [...] completely equivalent to Func<T, bool>" (see here), I would expect this to work, since All takes in as argument: Func<Instrument, bool> predicate.
However, I get the error:
Argument 2: cannot convert from 'System.Predicate<MyNamespace.Instrument>' to 'System.Type'
Is there a way to convert the predicate to an argument that this function will swallow?
The two types represent the same logical signature, but that doesn't mean they're just interchangable. A straight assignment won't work, for example - but you can create a new Func<T, bool> from the Predicate<T, bool>. Sample code:
Predicate<string> pred = x => x.Length > 10;
// Func<string, bool> func = pred; // Error
Func<string, bool> func = new Func<string, bool>(pred); // Okay
This is a bit like having two enum types with the same values - you can convert between them, but you have to do so explicitly. They're still separate types.
In your case, this means you could write:
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(new Func<T, bool>(m_filterExpression));
}
The lambda expression approach suggested by other answers will work too, of course.
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(i => m_filterExpression(i));
}
You can convert a predicate to a method by calling Invoke. All delegates have this member. Delegates don't have structural identity, but methods can be converted to matching delegates. This fix has a minor performance cost, as it adds an extra layer of indirection. However, most solutions to this problem have that problem. Eric Lippert discusses this in more detail at https://web.archive.org/web/20140625132124/http://blog.coverity.com/2014/06/18/delegates-structural-identity/ .
In your specific case, replace return m_instrumentList.All(m_filterExpression); with return m_instrumentList.All(m_filterExpression.Invoke);
Sample code demonstrating the actual problem.
void Main()
{
Predicate<int> t1 = Foo;
Func<int,bool> t2 = Foo;
Predicate<int> t3 = t2.Invoke; //Legal
Func<int,bool> t4 = t1.Invoke; //Legal
Predicate<int> t5 = t2; //Illegal
Func<int,bool> t6 = t1; //Illegal
}
bool Foo(int x)
{
return x > 20;
}
return m_instrumentList.All(i => m_filterExpression(i));
Since there are a lot of answers i will add one more just for fun.
If you want your code to compile you can use extention methods
//Original Code
class MyClass4
{
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(m_filterExpression);
}
private IEnumerable<Instrument> m_instrumentList;
private Predicate<Instrument> m_filterExpression;
}
Add this class in the same namespace
public static class MyExtentions
{
public static bool All(this IEnumerable<Instrument> enumer, Predicate<Instrument> pred)
{
return enumer.All(e => pred(e));
}
}
As Brian said, you can convert a predicate to a method via Invoke:
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(m_filterExpression.Invoke);
}
I am trying to define a valid input for my method Pairwise. Pairwise takes an argument IEnumerable which I am having trouble figuring out what is exactly. I have tried alot of stuff but can never really get there.
public delegate void PairwiseDel(Type left, Type right);
public static void Pairwise(IEnumerable<Type> col, PairwiseDel del)
{
// stuff happens here which passes pairs from col to del
}
can someone plz tell and illustrate what a valid input for my method would be?
IEnumerable<T> is a very important interface in .NET library. It represents an abstraction describing a sequence of elements of type T.
This generic interface has multiple implementations:
Built-in 1-dimension arrays T[] implement IEnumerable<T>
All generic .NET collections implement IEnumerable<T>
Methods that use yield return produce IEnumerable<T>
Multiple methods in .NET LINQ library both take and return IEnumerable<T>
If you would like to test your method, pass it an array Type[]:
var items = new Type[] { typeof(int), typeof(string), typeof(long) };
Pairwise(items, (a, b) => {
Console.WriteLine("A={0}, B={1}", a.Name, b.Name);
});
This would be a valid input:
var collection = new List<Type>();
collection.Add(typeof(string));
collection.Add(typeof(int));
PairWise(collection, YourDelegateHere);
I'm writing a simple proof of concept for what should be, in essence, a parser generator.
Basically I'm looking for a way that I can write a function that will return a function that converts from a string to some object of a given type - I want to be able to do the following in essence:
Func<string, double> ConvertToDouble = BuildConverter(typeof(0.0));
Obviously this is a pretty contrived example - but if I can do the simple version then I ought to be able to do the more complicated version!
FWIW, what I'm ultimately trying to do is to map a string of values onto a class, but to make it as flexible as possible, I want to do it by having a function that will return a function that does the conversion. In functional terms, I think I want something that looks like this:
a -> (string -> a)
As a first try, I've tried doing this:
public static Func<string, T> BuildParser<T>(T t)
{
if (t is String)
return new Func<string, T>(x => x.ToString());
if (t is double)
return new Func<string, T>(x => double.Parse(x));
}
Which doesn't work at all, but it leaves me feeling a bit stuck as to what approach I ought to be taking - so any help at all would be greatly appreciated!
You cannot mix class with struct types. Beyond that, it will work.
See the code below:
private void Testing() {
var func = BuildParserStruct<double>();
double value = func("5");
}
public static Func<string, T> BuildParserClass<T>() where T : class
{
return x => x as T;
}
public static Func<string, T> BuildParserStruct<T>() where T : struct
{
return (x => (T)Convert.ChangeType(x, typeof(double)));
}
I'm guessing that you want specific behaviors verified at compile time. Why not just call Convert or write individual methods to use? All that your if statements accomplish is to take role of the programmer who should be chosing the appropriate conversion method.
If you want behaviors chosen at runtime, you should return Func<string, object>, and make the method non-generic.
The issue with using a generic Type T in the method is that T is fixed for each call to the method, and the logic in the method supposes T to vary in a single call (in one case T is a string, in another case T is a decimal). The compiler cannot sort this out - it would need to allow both returnable instances to have the same type.
I'm not certain of exactly what you're trying to do, but would something like this help?
var stringParser = GetParser<string>();
string s = stringParser("test");
var doubleParser = GetParser<double>();
double d = doubleParser("42");
// ...
public static Func<string, T> GetParser<T>()
{
return (Func<string, T>)_parserCache[typeof(T)];
}
private static readonly Dictionary<Type, Delegate> _parserCache =
new Dictionary<Type, Delegate>
{
{ typeof(string), new Func<string, string>(x => x) },
{ typeof(double), new Func<string, double>(x => double.Parse(x)) }
// etc
};
ADO.Net has an Execute Scalar function that always bothered me because it returns an object. You can write a generic wrapper function to return the appropriate type. Of course this assumes that you know what type will be returned.
Somewhat simplified:
public T ExecuteScalar<T>()
{
return (T)Command.ExecuteScalar();
}
It is possible to create a Func object what references a generic method? like the LINQ OrderBy:
public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)
If I understand you correctly, you're asking if you can reference a generic method from within an anonymous method.
The answer is yes.
For example, suppose you want some Func that returns the elements of an IEnumerable<int> object in sorted order (precisely like OrderBy<int, int>). You could do this:
Func<IEnumerable<int>, Func<int, int>, IOrderedEnumerable<int>> orderByFunc =
System.Linq.Enumerable.OrderBy<int, int>;
Then you could use this Func just like any other:
int[] ints = new int[] { 1, 3, 5, 4, 7, 2, 6, 9, 8 };
// here you're really calling OrderBy<int, int> --
// you've just stored its address in a variable of type Func<...>
foreach (int i in orderByFunc(ints, x => x))
Console.WriteLine(i);
Output:
1
2
3
4
5
6
7
8
9
On the other hand, if you're asking whether it's possible to create a "generic anonymous method," like this:
Func<T> getDefault<T> = () => default(T);
Then it depends on your context. This can be done from within a context where T is already declared as a generic type parameter -- namely, within a generic class or generic method. (See Freddy Rios's answer.) Outside of such a context, unfortunately, it is illegal.
Yes, but it depends on the context - if you are already working with generics, just use the T in the context / if not, then you already know the specific type. In the later, if you need to reuse a bit of logic on a method, u probably already would benefit of moving that into a method, so just do like my second example below.
2 samples:
public T Something<T>() {
Func<T> someFunc = () => { return default(T); };
return someFunc();
}
public Func<T> GetDefaultCreator<T>() {
return () => { return default(T); };
}
Something like this?
Func<Nullable<int>, string> myFunc = c => c.HasValue ? c.ToString() : "null";
That successfully compiles, and you could assign any function to that that takes in a Nullable and returns a string.
I have done something like this:
public static class Helper{
public static IEnumerable<KeyValuePair<string, string>> ToPairs(this NameValueCollection Form)
{
return Form.AllKeys.Cast<string>()
.Select(key => new KeyValuePair<string, string>(key, Form[key]));
}
}
Where this method has become an extension method to the request.form in C# web development.
I think I get it: Given the function static TResult DoSomeStuff<T, TResult>(T obj), can you create a Func<T, TResult> such that it will reference the function above, with no type parameters given at the creation of the reference to it.
I think this could work (You're welcome to test it, I have no C# near me at the moment):
class UselessClass<T, TResult>
{
// If it's a static method, this is fine:
public Func<T, TResult> DaFunc = RelevantClass.DoSomeStuff<T, TResult>;
// If not, something like this is needed:
public UselessClass(SomeClassWhereTheFunctionIs from)
{
DaFunc = from.DoSomeStuff<T, TResult>;
}
}
Also, in OrderBy, it's not actually a generic delegate. It's a declaration of a variable. When the function is given to it, the types are inferred from it.
Yes it's possible but you'll need to specify the type argument(s)
func<int> f = myClass.returnsT<int>;
where
class myClass
{
T returnsT<T>()
{...}
}
it Will not work without the type arguments
Is there any way that I can generalise the type definitions here?
Ideally, I'd like to be able to change the type of 'testInput' and have test correctly infer the type at compile time.
public static void Run()
{
var testInput = 3;
var test = ((Func<int, int>) Identity).Compose<int,int,int>(n => n)(testInput);
Console.WriteLine(test);
}
public static Func<T, V> Compose<T, U, V>(this Func<U, V> f, Func<T, U> g)
{
return x => f(g(x));
}
public static T Identity<T> (this T value)
{
return value;
}
Update:
I can specify the type of the function passed into Compose but this still specifies the type in the line.
public static void Run()
{
var testInput = 3;
var identity = (Func<int, int>) Identity;
var test = identity.Compose((int n) => n)(testInput);
Console.WriteLine(test);
}
A little context; I'm working through Wes Dyer's The Marvel of Monads
Well seeing as I'm on a roll for spewing out text tonight I'll have my own stab at it. I should note that I am no expert on the C# compiler, I haven't read the specification (any of them... for anything), and although that article you linked was really interesting, I'd be lying if I said I was any sort of expert on that either (or even understood it all 100%).
Caveats aside, my take on your question is this:
Is there any way that I can generalise
the type definitions here?
I think the short answer is no. With the information provided, there is simply not enough information for the type inference part of C#'s compiler to infer enough information from the usage of the various variables.
As the other answers here demonstrate, it can be simplified. You can use #Lee's IdentityFunc to allow for type inference with var identity. However, even with this addition, it is still not possible with your sample code to infer all the type variables of Compose.
Imagine the following situation:
public static Func<T, V> Compose<T, U, V>(this Func<U, V> f, Func<T, U> g)
{
return x => f(g(x));
}
public static T Identity<T> (this T value)
{
return value;
}
public static Func<T, T> IdentityFunc<T>(this T value)
{
return (Func<T, T>)Identity;
}
and
public static void Run()
{
var a = 3; // a is int
var i = a.IdentityFunc(); // i is Func<int, int>;
var test = i.Compose(n => n)(a) // test is expected to be int
}
Initially this might appear as if test should easily be inferred to int. However, the return type of i.Compose can only be inferred after the fact, from its usage. The C# compiler obviously won't allow this.
public static void Run()
{
var a = 3; // a is int
var i = a.IdentityFunc(); // i is Func<int, int>;
var c = i.Compose(n => n) // c is Func<T, int> - T cannot be resolved without knowledge of n
var test = c(a); // ideally have type inference infer c (Func<T, int>) as Func<int, int>
}
In that example, upon usage of c with a the compiler would have to retrospectively infer the return type of the call to i.Compose<T, U, V>(n => n) to be Func<int, int>. This is obviously not possible in the C# compiler. Take away the call c(a) and the compiler would have no knowledge of the usage of c, which would remove any possibility of inferring T (not that it can anyway). It is possible a more advanced type inference system would be able to do this sort of inferring based on the usage of a generic return (possibly F# - another topic I'm no expert on).
As Wes Dyer doesn't provide specific usage of that particular example, it's unknown whether there is some other magic he uses to allow for the degree of type inference you're trying to achieve.
More qualified people like Eric Lippert would be able to provide you with a much greater level of detail (and technical accuracy / acuity). I read a great response he wrote on here to a question on type inference, but I can't find it. His blog has lots of great information. You could try contacting him if you're interested. Also, his answer to this question here discusses monads (and ultimately links back to Wes Dyer's article) buy you might be interested in reading it: Monad in plain English? (For the OOP programmer with no FP background)
You could write an extension method to return the Identity function for a type:
public static Func<T, T> IdentityFunc<T>(this T value)
{
return (Func<T, T>)Identity;
}
(or just return v => value;)
Your test then becomes
var testInput = 3;
var identity = testInput.IdentityFunc();
test = identity.Compose((int n) => n)(testInput);
The closest I can get is to explicitly type the parameter for the n => n lambda:
var test = ((Func<int, int>)Identity).Compose((int n) => n)(testInput);
I don't think you can achieve your ideal; C# type inference does not work like that.
You might enjoy F#.