I have created a repository method with the following signature:
T Get<TProperty>(int id, Expression<Func<T, TProperty>> include)
This works fine but I would like to extend it to take in an unlimited number of includes. I cannot simply use params because each include will return a different TProperty.
Is there a nice way around this or do I have to create several overloads for 1, 2 or 3 includes for example?
If the TProperty types can be different, and if you want to make them different types, then you'd need several overloads.
T Get<TProperty>(int id, Expression<Func<T, TProperty>> include) { ... }
T Get<TProperty1, TProperty2>(int id, Expression<Func<T, TProperty1>> include1, Expression<Func<T, TProperty2>> include2)
OR you could use:
T Get(int id, params Expression<Func<T, object>>[] includes)
And rely on the covariance of the Func generic delegate, but then you'd have to handle the fact you're losing a bit of type safety.
Because Func is covariance on the return type, this means that if your delegate expects a return of object, you can still pass it a delegate with a narrower return type (though warning, with covariance value types aren't covariantly compatible with object, only reference types).
The code you have posted won't allow you to return a different TProperty but only the type you will specify at runtime as <TProperty>
You could use Params and use Object instead of the generic type
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'.
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'.
Say I have a function such as:
public TProperty Foo<TClass, TProperty>(TClass instance, Expression<Func<TClass, TProperty>> expression)
{
...
}
But then I realize it should only be used for value types, so I add a constraint
public TProperty Foo<TClass, TProperty>(TClass instance, Expression<Func<TClass, TProperty>> expression)
where TProperty : struct
{
...
}
But I then discover that this won't let me pass in expressions that take a nullable TProperty. The only way I can see to handle this is to do the following:
public TProperty Foo<TClass, TProperty>(TClass instance, Expression<Func<TClass, TProperty?>> expression)
where TProperty : struct
{
...
}
But now I am forced to maintain two methods that do exactly the same thing, except that one deals with nullables, and one doesn't.
Further more I can't extract them into a common method, because even though a conversion exists from T to T?, there is apparently no conversion from
Expression<Func<T1, T2>>
to
Expression<Func<T1, T2?>>
If the body of these methods is complicated, I really don't want to have to maintain two separate versions of them, especially when in fact it is a whole family of functions.
Is there any way to avoid this code duplication and still have the compiler enforce that the expression must end in either a value type or a nullable?
(I am currently using C# 3.5, but am open to more recent versions if they provide a clean solution.)
Depending on what you're doing with the expression, you could make both overloads call a private overlaod that takes a (untyped) LambdaExpression.
In cases like these it can be still OK to use method overloading provided the logic is refactored to a private method.
I think you can restrict it on where TProperty : Nullable<TProperty> though I have not checked this
In .NET 4, Func<P, Q> is actually covariant on Q. So this idea might work, but alas, Nullable<T> is not related to T. (Here is an example.), so Func<P, Q> has no chance to be used where Func<P, Q?> is expected.
I am trying to understand specifically why it is neccessary to have this Where<TSource>
What does the type straight after Where tell you?
I understand the 'this' concept which means its an extension method but cannoth understand the type after Where
public static IEnumerable<TSource> Where<TSource>(
this IEnumerable<TSource> source,
Func<TSource, bool> predicate
)
Func<TSource, bool> is a pointer to a function which takes TSource as parameter and returns boolean. For example if you had the following function:
public bool Foo(SomeType abc)
{
return abc.SomeProperty == "123";
}
you could pass it as argument to the Where method if you had a list of SomeType:
SomeType[] values = ...
var result = values.Where(Foo);
You could also use an anonymous function which avoids you the need to declare another function explicitly:
SomeType[] values = ...
var result = values.Where(x => x.SomeProperty == "123");
UPDATE:
I seem to have misunderstood the question. The type after the name of the function Where<TSource> indicates a generic function definition. It indicates that this function has a generic argument which can be of any type. So for example when you write:
SomeType[] values = ...
var result = values.Where(x => x.SomeProperty == "123");
TSource equals SomeType and the compiler is capable of automatically inferring it from the delegate. You could specify it explicitly but it's too much of a writing:
SomeType[] values = ...
IEnumerable<SomeType> result = values.Where<SomeType>(x => x.SomeProperty == "123");
The type in <...> after Where is a declaration of generic type parameter. The Where method is generic which means that some of the types involved in its type declaration can be provided when the method is used. In C#, this is called generics.
The <...> is a place where you declare types that the caller needs to specify when using your method. It is, in some way, similar to declarations of parameters
When using parameters, you say that the caller needs to give you some values and you give them names (e.g. source and predicate).
When writing generic method, you say that the caller needs to give you some type and you give the type a name (e.g. TSource) that you can use in the method declaration and body.
A simple answer to your question would be for type safety and IntelliSense while programming. (All the benifits that Generics provide).
LINQ to Objects works on Extension Methods defined in a type named Enumerable. Mostly, they deal with type IEnumerable<T>.
So, for example, when you want it to operate on a List<String>, the Where<TSource> becomes Where<String>. And C# 3.0 type inferance comes into picture so that you dont have to explicitly specify the String part. Since, the compiler knows you are working with IEnumerable<T>.
To sumarrize, when you use Where method on a List, the method expects an IEnumerable<String> and a Predicate that takes String to filter on input sequence as an input and provides with an IEnumerable<String> as an output.
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.