I have a problem with casting a generic class to the interface it is implementing.
My code is like this:
interface foo
{
void foobar();
}
class bar: foo
{
public void foobar()
{
throw new NotImplementedException();
}
}
now I have my factory that creates instances of my classes by the interface, mostly a simple microkernel (service locator). I will simplify it here. Normally it will look up the implementing class from the configs and the factory take the type as T but that doesn't matter for the problem I have.
public static class Factory
{
public static Lazy<foo> CreateLazyInstance()
{
Lazy<foo> instance;
Type type = typeof(bar);
Type lazyType = typeof(Lazy<>);
Type toContruct = lazyType.MakeGenericType(type);
instance = (Lazy<foo>)Activator.CreateInstance(toContruct);
return instance;
}
}
If will fail at:
instance = (Lazy<foo>)Activator.CreateInstance(toContruct);
and claim with an InvalidCastException that it is not possible to cast the type Lazy<bar> to Lazy<foo>.
Is there any way to tell the CLR that this cast will work or to workaround this problem?
No - Lazy<T> is invariant - so a Lazy<string> is not a Lazy<object> for example. (As pointed out in comments, it couldn't be declared as covariant in T, as it's a class, not an interface or delegate.)
However, you can convert one to the other easily enough:
static Lazy<TOutput> CreateLazyProxy<TInput, TOutput>
(Lazy<TInput> input) where TInput : TOutput
{
return new Lazy<TOutput>(() => input.Value);
}
Also, Func<T> is covariant, so this will work too:
static Lazy<TOutput> CreateLazy<TInput, TOutput>(Func<TInput> func)
where TInput : TOutput
{
return new Lazy<TOutput>(func);
}
(Not that you particularly need a method for that - if you've got a Func<TInput>, just construct a Lazy<TOutput> directly.)
An easier way to do this would be to pass in a lambda to the Lazy constructor. So, your code would look like the following:
public static Lazy<foo> CreateLazyInstance()
{
Type type = typeof(bar);
return new Lazy<foo>(() => (foo)Activator.CreateInstance(type));
}
You must do your foo generic parameter : new():
public static Lazy<foo> CreateLazyInstance() where foo : new()
And change your code to find a constructor and call it:
Type t = typeof(foo);
t.GetConstructor(new type[]{});
return (foo)t.Invoke(new object[]{});
Related
I'm attempting to write a generic method based on the following myMethod:
void myMethod(myClassA instanceA, myClassB instanceB)
{
instanceA = new myClassA(instanceB);
}
What I've got so far is something like this (myMethod_new), which has some errors:
void myMethod_new<S, T>(S instanceA, T instanceB) where S : new()
{
instanceA = new S(instanceB);
}
However, since I've not mastered how generics work in C#, how should this be implemented?
You can't create a new instance of a generic type if the type has parameters in the constructor. From the docs (emphasis mine):
The new constraint specifies that a type argument in a generic class declaration must have a public parameterless constructor.
One option is to use Activator.CreateInstance and pass in the parameter(s) required. However, that can lead to runtime exceptions since the caller could pass in any type with any format of constructor.
My advice would be to refactor slightly by removing the constructor parameter and create a method to pass that value in, then use an interface to contstrain it all. For example:
// The interface
public interface IHasThing<T>
{
void SetThing(T value);
}
// An implementation of the interface
public class Foo : IHasThing<string>
{
private string _value;
public void SetThing(string value)
{
_value = value;
}
}
And your updated method that returns the new object:
public S myMethod_new<S, T>(T instanceB) where S : IHasThing<T>, new()
{
var s = new S();
s.SetThing(instanceB);
return s;
}
So now you can call the method like this:
var foo = myMethod_new<Foo, string>("bar");
This might be a silly question and I don't really need this for anything but I was just curious...
The best way to describe it is using a example so here it is:
using System;
namespace GenericExample
{
public interface IFoo { }
public interface IFoo2 { }
public class Foo1: IFoo , IFoo2 { }
public class Foo2 : IFoo, IFoo2 { }
public class MyGeneric<T> where T : IFoo , IFoo2, new() { }
internal class Program
{
public static void Main(string[] args)
{
MyGeneric<Foo1> obj1 = new MyGeneric<Foo1>();
MyMethod(obj1);//I can treat obj1 as MyGeneric<T> in MyMethod
MyGeneric<Foo2> obj2 = new MyGeneric<Foo2>();
//But can I use is as MyGeneric<T> in this method???
//MyGeneric<?> obj3 = null;
//obj3 = (MyGeneric<?>)obj1;
//obj3 = (MyGeneric<?>)obj2;
Console.ReadLine();
}
public static void MyMethod<T>(MyGeneric<T> arg) where T : IFoo, IFoo2, new()
{
}
}
}
I don't think it is possible to treat obj1 as MyGeneric< T> in Main
but at the same time it feels strange since I can pass it as a MyGeneric< T> argument
You cannot cast it to MyGeneric<T> in Main because in the scope of Main there is no such type as T. Actually it's not really clear what you mean by
to treat obj1 as MyGeneric< T> in Main
When passing obj1 to MyMethod you don't "treat it as MyGeneric<T>". It is the compiler which infers the type of T for you. It knows that T is Foo1 here and translates your call
MyMethod(obj1);
to
MyMethod<Foo1>(obj1);
So the type of the parameter arg inside of MyMethod will at runtime also be MyObject<Foo1>, not an unspecified MyObject<T>.
There is no common base-type for MyGeneric and MyGeneric, so I assume the answer is no. In contrast to Java generics in C# are strongly typed types and not just placeholders, so they don´t have anything in common - except a name. However actually they are different types, think of them as just MyGeneric<T1> being a type Foo and MyGeneric<T2> being Bar.
A way around this is to define a non-generic version of your generic class:
public class Foo1 { }
public class MyNonGeneric { }
public class MyGeneric<T> : MyNonGeneric where T : new() { }
I would like to invoke static members of a class that uses generics without specifying the type and having the compiler infer them.
For example this is the generic class I want to use, with the Static Factory member:
public class GenericClass<T>
{
public T Member;
// Constructor
public GenericClass(T value)
{
Member = value;
}
// static factory method
public static GenericClass<T> StaticFactory(T resultData)
{
return new GenericClass<T>(resultData);
}
}
If I try the following does not compile:
public class Class1
{
public GenericClass<string> returnResult1()
{
return GenericClass.StaticFactory("Won't Compile, but it's clear that T is a string");
}
public GenericClass returnResult2()
{
return GenericClass.StaticFactory("Won't Compile, but it's clear that T is a string");
}
}
Error 1 Using the generic type 'SampleStatic.GenericClass' requires 1 type arguments
Why can't I do like the following with static members?
public void ICanInferTheType<T1>(T1 item);
public void returnResult4()
{
ICanInferTheType("Compiles, recognizes type string");
}
Thanks -
Generic inference only happens for method calls. In your case, you need it for referring to a class. Given class GenericClass<T> { ... }, any reference to GenericClass without a type parameter is a compilation error. The only time generic parameters can be inferred is for generic parameters declared on method signatures.
However, you can achieve your goal anyway. If you declare this additional class, you'll be able to invoke the method the way you want. That's because the generic parameters are moved to the method.
public static class GenericClass {
// static factory method
public static GenericClass<T> StaticFactory<T>(T resultData) {
return new GenericClass<T>(resultData);
}
}
Ok, thanks to #recursive! That answer is almost perfect, it requires one more in the name of the method, like this:
public static class GenericClass
{
// static factory method
public static GenericClass<T> StaticFactory<T>(T resultData)
{
return new GenericClass<T>(resultData);
}
}
The following does not compile on line fm.AddFoo(new StringFoo()); with the error message:
Argument 1: cannot convert from 'ClassLibrary2.StringFoo' to 'ClassLibrary2.IFoo'
This seems logical to me since string inherits from object.
public interface IFoo<T>
{
void Handle(T value);
}
public class StringFoo : IFoo<string>
{
public void Handle(string value)
{ }
}
public class ObjectFoo : IFoo<object>
{
public void Handle(object value)
{ }
}
public class FooManager
{
private readonly List<IFoo<object>> _foos;
public FooManager()
{
_foos = new List<IFoo<object>>();
}
public void AddFoo(IFoo<object> foo)
{
_foos.Add(foo);
}
}
public class Bad
{
public Bad()
{
var fm = new FooManager();
fm.AddFoo(new StringFoo()); \\ This does not compile
}
}
Thanks
Although it may seem like IFoo is a subclass of IFoo it is not. When you close IFoo<> to a specific type is is not creating a subclass of IFoo from IFoo, they are seperate and distinct types with no common hierarchy.
If you could make your IFoo<> interface covariant it would work, that is if you were allowed to change the declaration of it into:
public interface IFoo<out T>
(note the out). Because with covariance any IFoo<string> would also be an IFoo<object> because string is a reference type and derives from object.
But: A member of IFoo<>, the Handle method, uses the type parameter in a contravariant manner. So your interface cannot be declared covariant (out). (It could be declared contravariant (in) but that goes in the wrong direction for your example above.)
Read up on covariance and contravariance in generics.
The fundamental problem here is that your StringFoo handles only strings. Therefore it can never be used as an IFoo<object> because then you could pass for example a Giraffe instance (Giraffe derives from object, so a Giraffe is an object) into the StringFoo, and that is impossible when its Handle takes a string.
Example 1
Assume a method that decides which method should be used to convert a value.
public static TTarget ConvertValue<TTarget>(object value)
{
Type t = typeof(TTarget);
if (t.IsEnum)
return ParseEnum<TTarget>(value);
else //if ...
return ...;
}
Some methods that handle the value have a generic type parameter with constraints.
public static TEnum ParseEnum<TEnum>(object value)
where TEnum : struct
{
//do something
return ...;
}
The compiler doesn't allow this approach because the type TTarget isn't necessarily a struct and can become NULL and thus cannot be used as TEnum.
Example 2
Assume having a generic method without constraints and a method with additional constraints:
public void DoStuff<T>(T obj)
{
if (obj is IComparable && obj is ICloneable)
DoSpecialStuff<T>(obj);
}
public void DoSpecialStuff<T>(T obj)
where T : IComparable, ICloneable
{
}
This also doesn't work because there is (afaik) no way to cast to multiple interfaces.
Is it possible to reuse a generic type to call a method with additional constraints?
You need to invoke the method using Reflection.
There is no better way.
You should consider calling a non-generic method instead (and passing typeof(TTarget) as a parameter)—ParseEnum shouldn't need to be generic.
As SLaks mentions, the only way to achieve this is by using reflection. Here's one way of doing it. (The delegates are cached in a dictionary so that subsequent calls for the same type don't need reflection.)
public static TTarget ConvertValue<TTarget>(this object value)
{
Type t = typeof(TTarget);
if (t.IsEnum)
{
Delegate del = _delegateCache.GetOrAdd(t, t2 =>
Delegate.CreateDelegate(typeof(Func<object, TTarget>),
_parseEnumInfo.MakeGenericMethod(t2));
return ((Func<object, TTarget>)del)(value);
}
else // if ...
return ...;
}
private static readonly MethodInfo _parseEnumInfo =
typeof(YourParentClass).GetMethod("ParseEnum");
private static readonly ConcurrentDictionary<Type, Delegate> _delegateCache =
new ConcurrentDictionary<Type, Delegate>();
public static TEnum ParseEnum<TEnum>(object value)
where TEnum : struct, IComparable, IConvertible, IFormattable
{
// do something
return ...;
}
Or, to match your second example:
public void DoStuff<T>(T obj)
{
if ((obj is IComparable) && (obj is ICloneable))
{
Delegate del = _delegateCache.GetOrAdd(typeof(T), t =>
Delegate.CreateDelegate(typeof(Action<T>),
this,
_doSpecialStuffInfo.MakeGenericMethod(t));
((Action<T>)del)(obj);
}
}
private static readonly MethodInfo _doSpecialStuffInfo =
typeof(YourParentClass).GetMethod("DoSpecialStuff");
private readonly ConcurrentDictionary<Type, Delegate> _delegateCache =
new ConcurrentDictionary<Type, Delegate>();
public void DoSpecialStuff<T>(T obj)
where T : IComparable, ICloneable
{
}
If for each interface of interest you define a version with an "extra" generic parameter T which inherits both the generic interface and ISelf<out T> (I'd suggest that interface contain a single read-only property "self", of type T), and if each class of interest implements ISelf<ItsOwnType>, then if e.g. IFoo<T> inherits from IFoo and T, a routine that needs something to implement both IFoo and IBar could accept a parameter of type IFoo<IBar>. If the passed-in parameter is called X, X will implement IFoo and X.Self will implement IBar. The beautiful thing about this approach is that any such object implementing any combination of interfaces defined in this way can be typecast to something which will implement any combination of those interfaces, in any order. The one weakness with this approach is that it presumes that objects will define ISelf<T>.Self to return themselves, but there's no real guarantee that such an object won't return something else instead.