I was hoping to do something like this, but it appears to be illegal in C#:
public Collection MethodThatFetchesSomething<T>()
where T : SomeBaseClass
{
return T.StaticMethodOnSomeBaseClassThatReturnsCollection();
}
I get a compile-time error:
'T' is a 'type parameter', which is not valid in the given context.
Given a generic type parameter, how can I call a static method on the generic class? The static method has to be available, given the constraint.
In this case you should just call the static method on the constrainted type directly. C# (and the CLR) do not support virtual static methods. So:
T.StaticMethodOnSomeBaseClassThatReturnsCollection
...can be no different than:
SomeBaseClass.StaticMethodOnSomeBaseClassThatReturnsCollection
Going through the generic type parameter is an unneeded indirection and hence not supported.
To elaborate on a previous answer, I think reflection is closer to what you want here. I could give 1001 reasons why you should or should not do something, I'll just answer your question as asked. I think you should call the GetMethod method on the type of the generic parameter and go from there. For example, for a function:
public void doSomething<T>() where T : someParent
{
List<T> items=(List<T>)typeof(T).GetMethod("fetchAll").Invoke(null,new object[]{});
//do something with items
}
Where T is any class that has the static method fetchAll().
Yes, I'm aware this is horrifically slow and may crash if someParent doesn't force all of its child classes to implement fetchAll but it answers the question as asked.
You can do what I call a surrogate singleton, I've been using it as a sort of "static inheritance" for a while
interface IFoo<T> where T : IFoo<T>, new()
{
ICollection<T> ReturnsCollection();
}
static class Foo<T> where T : IFoo<T>, new()
{
private static readonly T value = new();
public static ICollection<T> ReturnsCollection() => value.ReturnsCollection();
}
// Use case
public ICollection<T> DoSomething<T>() where T : IFoo<T>, new()
{
return Foo<T>.ReturnsCollection();
}
The only way of calling such a method would be via reflection, However, it sounds like it might be possible to wrap that functionality in an interface and use an instance-based IoC / factory / etc pattern.
It sounds like you're trying to use generics to work around the fact that there are no "virtual static methods" in C#.
Unfortunately, that's not gonna work.
I just wanted to throw it out there that sometimes delegates solve these problems, depending on context.
If you need to call the static method as some kind of a factory or initialization method, then you could declare a delegate and pass the static method to the relevant generic factory or whatever it is that needs this "generic class with this static method".
For example:
class Factory<TProduct> where TProduct : new()
{
public delegate void ProductInitializationMethod(TProduct newProduct);
private ProductInitializationMethod m_ProductInitializationMethod;
public Factory(ProductInitializationMethod p_ProductInitializationMethod)
{
m_ProductInitializationMethod = p_ProductInitializationMethod;
}
public TProduct CreateProduct()
{
var prod = new TProduct();
m_ProductInitializationMethod(prod);
return prod;
}
}
class ProductA
{
public static void InitializeProduct(ProductA newProduct)
{
// .. Do something with a new ProductA
}
}
class ProductB
{
public static void InitializeProduct(ProductB newProduct)
{
// .. Do something with a new ProductA
}
}
class GenericAndDelegateTest
{
public static void Main()
{
var factoryA = new Factory<ProductA>(ProductA.InitializeProduct);
var factoryB = new Factory<ProductB>(ProductB.InitializeProduct);
ProductA prodA = factoryA.CreateProduct();
ProductB prodB = factoryB.CreateProduct();
}
}
Unfortunately you can't enforce that the class has the right method, but you can at least compile-time-enforce that the resulting factory method has everything it expects (i.e an initialization method with exactly the right signature). This is better than a run time reflection exception.
This approach also has some benefits, i.e you can reuse init methods, have them be instance methods, etc.
You should be able to do this using reflection, as is described here
Due to link being dead, I found the relevant details in the wayback machine:
Assume you have a class with a static generic method:
class ClassWithGenericStaticMethod
{
public static void PrintName<T>(string prefix) where T : class
{
Console.WriteLine(prefix + " " + typeof(T).FullName);
}
}
How can you invoke this method using relection?
It turns out to be very easy… This is how you Invoke a Static Generic
Method using Reflection:
// Grabbing the type that has the static generic method
Type typeofClassWithGenericStaticMethod = typeof(ClassWithGenericStaticMethod);
// Grabbing the specific static method
MethodInfo methodInfo = typeofClassWithGenericStaticMethod.GetMethod("PrintName", System.Reflection.BindingFlags.Static | BindingFlags.Public);
// Binding the method info to generic arguments
Type[] genericArguments = new Type[] { typeof(Program) };
MethodInfo genericMethodInfo = methodInfo.MakeGenericMethod(genericArguments);
// Simply invoking the method and passing parameters
// The null parameter is the object to call the method from. Since the method is
// static, pass null.
object returnValue = genericMethodInfo.Invoke(null, new object[] { "hello" });
As of now, you can't. You need a way of telling the compiler that T has that method, and presently, there's no way to do that. (Many are pushing Microsoft to expand what can be specified in a generic constraint, so maybe this will be possible in the future).
Here, i post an example that work, it's a workaround
public interface eInterface {
void MethodOnSomeBaseClassThatReturnsCollection();
}
public T:SomeBaseClass, eInterface {
public void MethodOnSomeBaseClassThatReturnsCollection()
{ StaticMethodOnSomeBaseClassThatReturnsCollection() }
}
public Collection MethodThatFetchesSomething<T>() where T : SomeBaseClass, eInterface
{
return ((eInterface)(new T()).StaticMethodOnSomeBaseClassThatReturnsCollection();
}
Related
I want to create a generic class where the type of the class can Parse strings.
I want to use this class for any class that has a static function Parse(string),
like System.Int32, System.Double, but also for classes like System.Guid. They all have a static Parse function
So my class needs a where clause that constraints my generic type to types with a Parse function
I'd like to use it like this:
class MyGenericClass<T> : where T : ??? what to do ???
{
private List<T> addedItems = new List<T>()
public void Add(T item)
{
this.AddedItems.Add(item);
}
public void Add(string itemAsTxt)
{
T item = T.Parse(itemAsTxt);
this.Add(item);
}
}
What to write in the where clause?
I was not happy with the answers that would use reflection to do the Parsing.
I prefer a solution that is type safe, so the compiler would complain about the a missing Parse function.
Normally you would constraint to a class that has an interface. But as others said, there is no common interface. Come to think of it I don't need an interface, I need a function that I can call
So my solution would be to insist that creators would provide a delegate to the Parse Function that would parse a string to type T
class MyGenericClass<T>
{
public MyGenericClass(Func<string, T> parseFunc)
{
this.parseFunc = parseFunc;
}
private readonly Func<string, T> parseFunc;
public void Add(string txt)
{
this.Add(parseFunc(txt));
}
}
Usage:
MyGenericClass<Guid> x = new MyGenericClass<Guid>(txt => Guid.Parse(txt));
MyGenericClass<int> y = new MyGenericClass<int> (txt => System.Int32.Parse(txt));
The answer is simpler than I thought
I might be misunderstanding your question, but will this do the trick?
static void Main(string[] args)
{
Guid g = DoParse<Guid>("33531071-c52b-48f5-b0e4-ea3c554b8d23");
}
public static T DoParse<T>(string value)
{
T result = default(T);
MethodInfo methodInfo = typeof(T).GetMethod("Parse");
if (methodInfo != null)
{
ParameterInfo[] parameters = methodInfo.GetParameters();
object classInstance = Activator.CreateInstance(typeof(T), null);
object[] parametersArray = new object[] { value };
result = (T)methodInfo.Invoke(methodInfo, parametersArray);
}
return result;
}
You can´t do that at compile-time at all. All those methods don´t have anything in common except their names. They neither are defined in any common interface nor have they the same parameters.
Imagine you have your own method that accidently has that name, but does something completely different. In addition it´s also an instance-method:
class MyType
{
void Parse() { ... }
}
As you can see that method don´t even has any argument, making it hard to find any common logic between - say - int.Parse(myString)andmyInstanceOfMyType.Parse()`.
But even if you could do that. What would you do with the item? There´s not much as there´s nothing in common between them. In particular it´s impossible to call the methpd, as they are completely different.
You could loop all types with a similar method at runtime however:
var types = allAssemblies.SelectMany(x => x.GetTypes())
.Where(x => x.GetMethod("Parse") != null);
But be aware that this yields to an AmbiguousMatchException if more than one method per type exists with the name, this is when the method is overloaded.
I am working on rewriting my fluent interface for my IoC class library, and when I refactored some code in order to share some common functionality through a base class, I hit upon a snag.
Note: This is something I want to do, not something I have to do. If I have to make do with a different syntax, I will, but if anyone has an idea on how to make my code compile the way I want it, it would be most welcome.
I want some extension methods to be available for a specific base-class, and these methods should be generic, with one generic type, related to an argument to the method, but the methods should also return a specific type related to the particular descendant they're invoked upon.
Better with a code example than the above description methinks.
Here's a simple and complete example of what doesn't work:
using System;
namespace ConsoleApplication16
{
public class ParameterizedRegistrationBase { }
public class ConcreteTypeRegistration : ParameterizedRegistrationBase
{
public void SomethingConcrete() { }
}
public class DelegateRegistration : ParameterizedRegistrationBase
{
public void SomethingDelegated() { }
}
public static class Extensions
{
public static ParameterizedRegistrationBase Parameter<T>(
this ParameterizedRegistrationBase p, string name, T value)
{
return p;
}
}
class Program
{
static void Main(string[] args)
{
ConcreteTypeRegistration ct = new ConcreteTypeRegistration();
ct
.Parameter<int>("age", 20)
.SomethingConcrete(); // <-- this is not available
DelegateRegistration del = new DelegateRegistration();
del
.Parameter<int>("age", 20)
.SomethingDelegated(); // <-- neither is this
}
}
}
If you compile this, you'll get:
'ConsoleApplication16.ParameterizedRegistrationBase' does not contain a definition for 'SomethingConcrete' and no extension method 'SomethingConcrete'...
'ConsoleApplication16.ParameterizedRegistrationBase' does not contain a definition for 'SomethingDelegated' and no extension method 'SomethingDelegated'...
What I want is for the extension method (Parameter<T>) to be able to be invoked on both ConcreteTypeRegistration and DelegateRegistration, and in both cases the return type should match the type the extension was invoked on.
The problem is as follows:
I would like to write:
ct.Parameter<string>("name", "Lasse")
^------^
notice only one generic argument
but also that Parameter<T> returns an object of the same type it was invoked on, which means:
ct.Parameter<string>("name", "Lasse").SomethingConcrete();
^ ^-------+-------^
| |
+---------------------------------------------+
.SomethingConcrete comes from the object in "ct"
which in this case is of type ConcreteTypeRegistration
Is there any way I can trick the compiler into making this leap for me?
If I add two generic type arguments to the Parameter method, type inference forces me to either provide both, or none, which means this:
public static TReg Parameter<TReg, T>(
this TReg p, string name, T value)
where TReg : ParameterizedRegistrationBase
gives me this:
Using the generic method 'ConsoleApplication16.Extensions.Parameter<TReg,T>(TReg, string, T)' requires 2 type arguments
Using the generic method 'ConsoleApplication16.Extensions.Parameter<TReg,T>(TReg, string, T)' requires 2 type arguments
Which is just as bad.
I can easily restructure the classes, or even make the methods non-extension-methods by introducing them into the hierarchy, but my question is if I can avoid having to duplicate the methods for the two descendants, and in some way declare them only once, for the base class.
Let me rephrase that. Is there a way to change the classes in the first code example above, so that the syntax in the Main-method can be kept, without duplicating the methods in question?
The code will have to be compatible with both C# 3.0 and 4.0.
Edit: The reason I'd rather not leave both generic type arguments to inference is that for some services, I want to specify a parameter value for a constructor parameter that is of one type, but pass in a value that is a descendant. For the moment, matching of specified argument values and the correct constructor to call is done using both the name and the type of the argument.
Let me give an example:
ServiceContainerBuilder.Register<ISomeService>(r => r
.From(f => f.ConcreteType<FileService>(ct => ct
.Parameter<Stream>("source", new FileStream(...)))));
^--+---^ ^---+----^
| |
| +- has to be a descendant of Stream
|
+- has to match constructor of FileService
If I leave both to type inference, the parameter type will be FileStream, not Stream.
I wanted to create an extension method that could enumerate over a list of things, and return a list of those things that were of a certain type. It would look like this:
listOfFruits.ThatAre<Banana>().Where(banana => banana.Peel != Color.Black) ...
Sadly, this is not possible. The proposed signature for this extension method would have looked like:
public static IEnumerable<TResult> ThatAre<TSource, TResult>
(this IEnumerable<TSource> source) where TResult : TSource
... and the call to ThatAre<> fails because both type arguments need to be specified, even though TSource may be inferred from the usage.
Following the advice in other answers, I created two functions: one which captures the source, and another which allows callers to express the result:
public static ThatAreWrapper<TSource> That<TSource>
(this IEnumerable<TSource> source)
{
return new ThatAreWrapper<TSource>(source);
}
public class ThatAreWrapper<TSource>
{
private readonly IEnumerable<TSource> SourceCollection;
public ThatAreWrapper(IEnumerable<TSource> source)
{
SourceCollection = source;
}
public IEnumerable<TResult> Are<TResult>() where TResult : TSource
{
foreach (var sourceItem in SourceCollection)
if (sourceItem is TResult) yield return (TResult)sourceItem;
}
}
}
This results in the following calling code:
listOfFruits.That().Are<Banana>().Where(banana => banana.Peel != Color.Black) ...
... which isn't bad.
Notice that because of the generic type constraints, the following code:
listOfFruits.That().Are<Truck>().Where(truck => truck.Horn.IsBroken) ...
will fail to compile at the Are() step, since Trucks are not Fruits. This beats the provided .OfType<> function:
listOfFruits.OfType<Truck>().Where(truck => truck.Horn.IsBroken) ...
This compiles, but always yields zero results and indeed doesn't make any sense to try. It's much nicer to let the compiler help you spot these things.
If you have only two specific types of registration (which seems to be the case in your question), you could simply implement two extension methods:
public static DelegateRegistration Parameter<T>(
this DelegateRegistration p, string name, T value);
public static ConcreteTypeRegistration Parameter<T>(
this ConcreteTypeRegistration p, string name, T value);
Then you wouldn't need to specify the type argument, so the type inference would work in the example you mentioned. Note that you can implement both of the extension methods just by delegation to a single generic extension method with two type parameters (the one in your question).
In general, C# doesn't support anything like o.Foo<int, ?>(..) to infer only the second type parameter (it would be nice feature - F# has it and it's quite useful :-)). You could probably implement a workaround that would allow you to write this (basically, by separating the call into two method calls, to get two places where the type inferrence can be applied):
FooTrick<int>().Apply(); // where Apply is a generic method
Here is a pseudo-code to demonstrate the structure:
// in the original object
FooImmediateWrapper<T> FooTrick<T>() {
return new FooImmediateWrapper<T> { InvokeOn = this; }
}
// in the FooImmediateWrapper<T> class
(...) Apply<R>(arguments) {
this.InvokeOn.Foo<T, R>(arguments);
}
Why don't you specify zero type parameters? Both can be inferred in your sample. If this is not an acceptable solution for you, I'm frequently encountering this problem too and there's no easy way to solve the problem "infer only one type parameter". So I'll go with the duplicate methods.
What about the following:
Use the definition you provide:
public static TReg Parameter<TReg, T>(
this TReg p, string name, T value)
where TReg : ParameterizedRegistrationBase
Then cast the parameter so the inference engine gets the right type:
ServiceContainerBuilder.Register<ISomeService>(r => r
.From(f => f.ConcreteType<FileService>(ct => ct
.Parameter("source", (Stream)new FileStream(...)))));
I think you need to split the two type parameters between two different expressions; make the explicit one be part of the type of a parameter to the extension method, so inference can then pick it up.
Suppose you declared a wrapper class:
public class TypedValue<TValue>
{
public TypedValue(TValue value)
{
Value = value;
}
public TValue Value { get; private set; }
}
Then your extension method as:
public static class Extensions
{
public static TReg Parameter<TValue, TReg>(
this TReg p, string name, TypedValue<TValue> value)
where TReg : ParameterizedRegistrationBase
{
// can get at value.Value
return p;
}
}
Plus a simpler overload (the above could in fact call this one):
public static class Extensions
{
public static TReg Parameter<TValue, TReg>(
this TReg p, string name, TValue value)
where TReg : ParameterizedRegistrationBase
{
return p;
}
}
Now in the simple case where you are happy to infer the parameter value type:
ct.Parameter("name", "Lasse")
But in the case where you need to explicitly state the type, you can do so:
ct.Parameter("list", new TypedValue<IEnumerable<int>>(new List<int>()))
Looks ugly, but hopefully rarer than the simple fully-inferred kind.
Note that you could just have the no-wrapper overload and write:
ct.Parameter("list", (IEnumerable<int>)(new List<int>()))
But that of course has the disadvantage of failing at runtime if you get something wrong. Unfortunately away from my C# compiler right now, so apologies if this is way off.
I would used the solution:
public class JsonDictionary
{
public static readonly Key<int> Foo = new Key<int> { Name = "FOO" };
public static readonly Key<string> Bar = new Key<string> { Name = "BAR" };
IDictionary<string, object> _data;
public JsonDictionary()
{
_data = new Dictionary<string, object>();
}
public void Set<T>(Key<T> key, T obj)
{
_data[key.Name] = obj;
}
public T Get<T>(Key<T> key)
{
return (T)_data[key.Name];
}
public class Key<T>
{
public string Name { get; init; }
}
}
See:
C#: Exposing type safe API over heterogeneous dictionary
this is my code:
bool ch=Type.IsBuiltIn("System.Int32"); // not working-> syntax error
public static class MyExtentions
{
public static bool IsBuiltIn(this Type t, string _type)
{
return (Type.GetType(_type) == null) ? false : true;
}
}
Please I want Extend Type Class by IsBuiltIn new method
You can't have static extension methods. Your extension method works on an instance of the Type class, so to call it you'd have to do something like this:
typeof(Type).IsBuiltIn("System.Int32")
The workaround for this is to just put your extension method in a utility class, e.g. like the following, and call it like a normal static function:
public static class TypeExt
{
public static bool IsBuiltIn(string _type)
{
return Type.GetType(_type) == null;
}
}
// To call it:
TypeExt.IsBuiltIn("System.Int32")
By the way, I don't think this will tell you whether the type is "built-in"; it will merely tell you whether a type with the given name has been loaded into the process.
Extension methods are intended to describe new APIs on instances, not types. In your case, that API would be something like:
Type someType = typeof(string); // for example
bool isBuiltIn = someType.IsBuiltIn("Some.Other.Type");
which... clearly isn't what you wanted; the type here adds nothing and is not related to the IsBuiltIn. There is no compiler trick for adding new static methods to existing types, basically - so you will not be able to use Type.IsBuiltIn("Some.Other.Type").
You can't extend the Type class. You need an instance of the class to create an extension method.
Edit:
See here and here.
I am working on rewriting my fluent interface for my IoC class library, and when I refactored some code in order to share some common functionality through a base class, I hit upon a snag.
Note: This is something I want to do, not something I have to do. If I have to make do with a different syntax, I will, but if anyone has an idea on how to make my code compile the way I want it, it would be most welcome.
I want some extension methods to be available for a specific base-class, and these methods should be generic, with one generic type, related to an argument to the method, but the methods should also return a specific type related to the particular descendant they're invoked upon.
Better with a code example than the above description methinks.
Here's a simple and complete example of what doesn't work:
using System;
namespace ConsoleApplication16
{
public class ParameterizedRegistrationBase { }
public class ConcreteTypeRegistration : ParameterizedRegistrationBase
{
public void SomethingConcrete() { }
}
public class DelegateRegistration : ParameterizedRegistrationBase
{
public void SomethingDelegated() { }
}
public static class Extensions
{
public static ParameterizedRegistrationBase Parameter<T>(
this ParameterizedRegistrationBase p, string name, T value)
{
return p;
}
}
class Program
{
static void Main(string[] args)
{
ConcreteTypeRegistration ct = new ConcreteTypeRegistration();
ct
.Parameter<int>("age", 20)
.SomethingConcrete(); // <-- this is not available
DelegateRegistration del = new DelegateRegistration();
del
.Parameter<int>("age", 20)
.SomethingDelegated(); // <-- neither is this
}
}
}
If you compile this, you'll get:
'ConsoleApplication16.ParameterizedRegistrationBase' does not contain a definition for 'SomethingConcrete' and no extension method 'SomethingConcrete'...
'ConsoleApplication16.ParameterizedRegistrationBase' does not contain a definition for 'SomethingDelegated' and no extension method 'SomethingDelegated'...
What I want is for the extension method (Parameter<T>) to be able to be invoked on both ConcreteTypeRegistration and DelegateRegistration, and in both cases the return type should match the type the extension was invoked on.
The problem is as follows:
I would like to write:
ct.Parameter<string>("name", "Lasse")
^------^
notice only one generic argument
but also that Parameter<T> returns an object of the same type it was invoked on, which means:
ct.Parameter<string>("name", "Lasse").SomethingConcrete();
^ ^-------+-------^
| |
+---------------------------------------------+
.SomethingConcrete comes from the object in "ct"
which in this case is of type ConcreteTypeRegistration
Is there any way I can trick the compiler into making this leap for me?
If I add two generic type arguments to the Parameter method, type inference forces me to either provide both, or none, which means this:
public static TReg Parameter<TReg, T>(
this TReg p, string name, T value)
where TReg : ParameterizedRegistrationBase
gives me this:
Using the generic method 'ConsoleApplication16.Extensions.Parameter<TReg,T>(TReg, string, T)' requires 2 type arguments
Using the generic method 'ConsoleApplication16.Extensions.Parameter<TReg,T>(TReg, string, T)' requires 2 type arguments
Which is just as bad.
I can easily restructure the classes, or even make the methods non-extension-methods by introducing them into the hierarchy, but my question is if I can avoid having to duplicate the methods for the two descendants, and in some way declare them only once, for the base class.
Let me rephrase that. Is there a way to change the classes in the first code example above, so that the syntax in the Main-method can be kept, without duplicating the methods in question?
The code will have to be compatible with both C# 3.0 and 4.0.
Edit: The reason I'd rather not leave both generic type arguments to inference is that for some services, I want to specify a parameter value for a constructor parameter that is of one type, but pass in a value that is a descendant. For the moment, matching of specified argument values and the correct constructor to call is done using both the name and the type of the argument.
Let me give an example:
ServiceContainerBuilder.Register<ISomeService>(r => r
.From(f => f.ConcreteType<FileService>(ct => ct
.Parameter<Stream>("source", new FileStream(...)))));
^--+---^ ^---+----^
| |
| +- has to be a descendant of Stream
|
+- has to match constructor of FileService
If I leave both to type inference, the parameter type will be FileStream, not Stream.
I wanted to create an extension method that could enumerate over a list of things, and return a list of those things that were of a certain type. It would look like this:
listOfFruits.ThatAre<Banana>().Where(banana => banana.Peel != Color.Black) ...
Sadly, this is not possible. The proposed signature for this extension method would have looked like:
public static IEnumerable<TResult> ThatAre<TSource, TResult>
(this IEnumerable<TSource> source) where TResult : TSource
... and the call to ThatAre<> fails because both type arguments need to be specified, even though TSource may be inferred from the usage.
Following the advice in other answers, I created two functions: one which captures the source, and another which allows callers to express the result:
public static ThatAreWrapper<TSource> That<TSource>
(this IEnumerable<TSource> source)
{
return new ThatAreWrapper<TSource>(source);
}
public class ThatAreWrapper<TSource>
{
private readonly IEnumerable<TSource> SourceCollection;
public ThatAreWrapper(IEnumerable<TSource> source)
{
SourceCollection = source;
}
public IEnumerable<TResult> Are<TResult>() where TResult : TSource
{
foreach (var sourceItem in SourceCollection)
if (sourceItem is TResult) yield return (TResult)sourceItem;
}
}
}
This results in the following calling code:
listOfFruits.That().Are<Banana>().Where(banana => banana.Peel != Color.Black) ...
... which isn't bad.
Notice that because of the generic type constraints, the following code:
listOfFruits.That().Are<Truck>().Where(truck => truck.Horn.IsBroken) ...
will fail to compile at the Are() step, since Trucks are not Fruits. This beats the provided .OfType<> function:
listOfFruits.OfType<Truck>().Where(truck => truck.Horn.IsBroken) ...
This compiles, but always yields zero results and indeed doesn't make any sense to try. It's much nicer to let the compiler help you spot these things.
If you have only two specific types of registration (which seems to be the case in your question), you could simply implement two extension methods:
public static DelegateRegistration Parameter<T>(
this DelegateRegistration p, string name, T value);
public static ConcreteTypeRegistration Parameter<T>(
this ConcreteTypeRegistration p, string name, T value);
Then you wouldn't need to specify the type argument, so the type inference would work in the example you mentioned. Note that you can implement both of the extension methods just by delegation to a single generic extension method with two type parameters (the one in your question).
In general, C# doesn't support anything like o.Foo<int, ?>(..) to infer only the second type parameter (it would be nice feature - F# has it and it's quite useful :-)). You could probably implement a workaround that would allow you to write this (basically, by separating the call into two method calls, to get two places where the type inferrence can be applied):
FooTrick<int>().Apply(); // where Apply is a generic method
Here is a pseudo-code to demonstrate the structure:
// in the original object
FooImmediateWrapper<T> FooTrick<T>() {
return new FooImmediateWrapper<T> { InvokeOn = this; }
}
// in the FooImmediateWrapper<T> class
(...) Apply<R>(arguments) {
this.InvokeOn.Foo<T, R>(arguments);
}
Why don't you specify zero type parameters? Both can be inferred in your sample. If this is not an acceptable solution for you, I'm frequently encountering this problem too and there's no easy way to solve the problem "infer only one type parameter". So I'll go with the duplicate methods.
What about the following:
Use the definition you provide:
public static TReg Parameter<TReg, T>(
this TReg p, string name, T value)
where TReg : ParameterizedRegistrationBase
Then cast the parameter so the inference engine gets the right type:
ServiceContainerBuilder.Register<ISomeService>(r => r
.From(f => f.ConcreteType<FileService>(ct => ct
.Parameter("source", (Stream)new FileStream(...)))));
I think you need to split the two type parameters between two different expressions; make the explicit one be part of the type of a parameter to the extension method, so inference can then pick it up.
Suppose you declared a wrapper class:
public class TypedValue<TValue>
{
public TypedValue(TValue value)
{
Value = value;
}
public TValue Value { get; private set; }
}
Then your extension method as:
public static class Extensions
{
public static TReg Parameter<TValue, TReg>(
this TReg p, string name, TypedValue<TValue> value)
where TReg : ParameterizedRegistrationBase
{
// can get at value.Value
return p;
}
}
Plus a simpler overload (the above could in fact call this one):
public static class Extensions
{
public static TReg Parameter<TValue, TReg>(
this TReg p, string name, TValue value)
where TReg : ParameterizedRegistrationBase
{
return p;
}
}
Now in the simple case where you are happy to infer the parameter value type:
ct.Parameter("name", "Lasse")
But in the case where you need to explicitly state the type, you can do so:
ct.Parameter("list", new TypedValue<IEnumerable<int>>(new List<int>()))
Looks ugly, but hopefully rarer than the simple fully-inferred kind.
Note that you could just have the no-wrapper overload and write:
ct.Parameter("list", (IEnumerable<int>)(new List<int>()))
But that of course has the disadvantage of failing at runtime if you get something wrong. Unfortunately away from my C# compiler right now, so apologies if this is way off.
I would used the solution:
public class JsonDictionary
{
public static readonly Key<int> Foo = new Key<int> { Name = "FOO" };
public static readonly Key<string> Bar = new Key<string> { Name = "BAR" };
IDictionary<string, object> _data;
public JsonDictionary()
{
_data = new Dictionary<string, object>();
}
public void Set<T>(Key<T> key, T obj)
{
_data[key.Name] = obj;
}
public T Get<T>(Key<T> key)
{
return (T)_data[key.Name];
}
public class Key<T>
{
public string Name { get; init; }
}
}
See:
C#: Exposing type safe API over heterogeneous dictionary
I have a class:
class abc <T> {
private T foo;
public string a {
set {
foo = T.parse(value);
}
get{
return foo.toString();
}
}
}
However the T.parse command is giving me an error. Anyone of a way to do what I am trying to do?
I am using this as a base class for some other derived classes.
Edit:
What I ended us doing:
Delegate parse = Delegate.CreateDelegate(typeof(Func<String, T>), typeof(T).GetMethod("Parse", new[] { typeof(string) }));
I do that once in the constructor
and then I do the following in my property:
lock (lockVariable)
{
m_result = (T)parse.DynamicInvoke(value);
dirty = true;
}
C# generic types are not C++ templates. A template lets you do a fancy "search and replace" where you would substitute the name of a type that implements a static parse method for T. C# generics are not a textual search-and-replace mechanism like that. Rather, they describe parameterized polymorphism on types. With a template, all that is required is that the specific arguments you substitute for the parameters are all good. With a generic every possible substitution whether you actually do it or not, has got to be good.
UPDATE:
A commenter asks:
What would be the C# way of doing things when an equivalent to Haskell's Read type class is needed?
Now we come to the deep question underlying the original question.
To clarify for the reader unfamiliar with Haskell: Since C# 2.0, C# has supported "generic" types, which are a "higher" kind of type than regular types. You can say List<int> and a new type is made for you that follows the List<T> pattern, but it is a list specifically of integers.
Haskell supports an even higher kind of type in its type system. With generic types you can say "every MyCollection<T> has a method GetValue that takes an int and returns a T, for any T you care to name". With generic types you can put constraints on T and say "and furthermore, T is guaranteed to implement IComparable<T>..." With Haskell typeclasses you can go even further and say the moral equivalent of "...and moreover, T is guaranteed to have a static method Parse that takes a string and returns a T".
The "Read" typeclass is specifically that typeclass that declares the moral equivalent of "a class C that obeys the Read typeclass pattern is one that has a method Parse that takes a string and returns a C".
C# does not support that kind of higher type. If it did then we could typecheck patterns in the language itself such as monads, which today can only be typechecked by baking them into the compiler (in the form of query comprehensions, for example.) (See Why there is no something like IMonad<T> in upcoming .NET 4.0 for some more thoughts.)
Since there is no way to represent that idea in the type system, you're pretty much stuck with not using generics to solve this problem. The type system simply doesn't support that level of genericity.
People sometimes do horrid things like:
static T Read<T>(string s)
{
if (typeof(T) == typeof(int)) return (T)(object)int.Parse(s);
if ...
but that is in my opinion a bit abusive; it really is not generic.
You could use reflection. You cannot access static members through a generic parameter.
class Abc<T> {
private T foo;
public string a {
set {
foo = Parse<T>(value);
}
get {
return foo.ToString();
}
}
static T Parse<T>(string s)
{
var type = typeof(T);
var method = type.GetMethod("Parse", new[] { typeof(string) });
return (T)method.Invoke(null, new[] { s });
}
}
C# doesn't have templates. .NET generics don't work like C++ templates.
With an appropriate constraint, you can use instance methods on parameters with generic type, but there's no way to constrain static members.
However, you could use reflection, something along the lines of typeof(T).GetMethod("Parse"), to make a Func<string,T> delegate.
T.parse is not known in the generic parameter. You have to make it known.
dont use reflection. Its slow and generally a bad solution in this case.
use generics in a correct way.
You have to specify that T can be only classes which implement an interface which contains a parse method:
class abs<T> where T : IParsable<T>
{
//your implementation here
}
interface IParsable<T>
{
T Parse(string value);
}
public class Specific : IParsable<Specific>
{
public Specific Parse(string value)
{
throw new NotImplementedException();
}
}
You can't call a static method on a generic class.
Look at this post: Calling a static method on a generic type parameter
But here is a little workaround:
public interface iExample
{
iExample Parse(string value);
}
class abc<T> where T : iExample, new()
{
private T foo;
public string a
{
set
{
foo = (T)(new T().Parse(value));
}
get
{
return foo.ToString();
}
}
}
So if you have an class that implements iExample
public class SelfParser : iExample
{
public iExample Parse(string value)
{
return new SelfParser();
}
}
You will be able to use it like this:
abc<SelfParser> abcInstance = new abc<SelfParser>();
abcInstance.a = "useless text";
string unParsed = abcInstance.a; // Will return "SelfParser"
While you can't do exactly that with Generics (there are no type constraints for that enforce a specific method signature, only struct/object/interface constraints).
You can create a base class whose constructor takes the Parse method. See my Int32 implementation at the bottom.
class MyParseBase <T>
{
public MyBase (Func<string,T> parseMethod)
{
if (parseMethod == null)
throw new ArgumentNullException("parseMethod");
m_parseMethod = parseMethod;
}
private T foo;
public string a {
set
{
foo = m_parseMethod(value);
}
get
{
return foo.toString();
}
}
}
class IntParse : MyParseBase<Int32>
{
public IntParse()
: base (Int32.Parse)
{}
}
This is a variation on Oleg G's answer which removes the need for the new() type constraint. The idea is you make a Parser for each type you want to be contained in an abs, and inject it - this is a formalization of the Func<string, T> approach as well.
interface IParser<T>
{
T Parse(string value);
}
class abs<T>
{
private readonly IParser<T> _parser;
private T foo;
public abs(IParser<T> parser)
{
_parser = parser;
}
public string a {
set
{
foo = _parser.Parse(value);
}
get
{
return foo.ToString();
}
}
class abc<T> {
private T foo;
public string a {
set {
var x_type = typeof(T);
foo = (T)x_type.InvokeMember("Parse", System.Reflection.BindingFlags.InvokeMethod, null, value, new []{value});
}
get{
return foo.ToString();
}
}
}