Supposed I have some code like so:
// This represents some query that will return type T when executed.
public interface IQuery<T> { }
// An example query that returns bool
public class MyQuery : IQuery<bool> { }
// A separate class that's actually responsible for executing the queries.
public class Executor {
public T Execute<T>(IQuery<T> query)
}
// Usage
executor.Execute(new MyQuery()) // => true/false
No problems so far. But say I want to change my executor class such that it's responsible for instantiation of queries. Ideally I'd like usage to be something like:
// Usage
executor.Execute<MyQuery>() // => true/false
However, I can't seem to find a way to model this method signature. The closest I can get is:
public T Execute<TQuery, T>() where TQuery : IQuery<T>
The problem with this signature is that it requires all of the type parameters to be explicitly specified. There is no way for T to be inferred from TQuery since the generic type constraints are not considered when inferring type parameters.
So far the only workaround I've found is to set up a dummy argument to help with inference from the formal parameters. Basically going back to the original example but instead of passing an actual instance using new MyQuery() I can pass something like default(MyQuery). This is clunky though and the makes the API not at all obvious.
Is there some workaround that I'm missing?
Related
I once again am struggling with reflection. I just give you the piece of code that my debugger does not bite:
public bool HandleCommand(string command)
{
MethodInfo m = methods.FirstOrDefault(t => t.GetCustomAttribute<CommandAttribute>().Name == command);
ICommandSet set =(m.DeclaringType)Activator.CreateInstance(m.DeclaringType);
m?.Invoke(set, null);
return true;
}
Basically, this code is inside a class called CommandHandler. When it's constructed, it loops over all the types in the executing assembly that implement a certain interface, and store all their methods that have a CustomAttribute attached to it in List; For this question's purpose I just assume that everything is working there. The attribute has just one property:
[AttributeUsage(AttributeTargets.Method)]
public class CommandAttribute : Attribute
{
public string Name { get; set; }
public CommandAttribute(string name)
{
Name = name;
}
}
Now in the method you saw above, the HandleCommand() method, I store the method which's name property is equal to the string I passed in in a MethodInfo m. Now my question essentially is, how I properly Invoke this method. m.Invoke needs an object that calls it, and because passing in "this" does not work, and examples online always passed in an instance of the class it was defined in, I figured I needed to create an instance of the class m was defined in and just pass this into the Invoke() method. In practice, this is a lot harder than I thought, and my best guess is what you can see above, with the activator.
ICommandSet set =(m.DeclaringType)Activator.CreateInstance(m.DeclaringType);
First of all, I know for sure that the class that m is declared in implements ICommandSet, because this is a creterium for a type to be inspected for the methods. So this is why I say "ICommandSet set". Then the Activator shall create this instance. But it does not work. The only error message provided states that m is a variable but is used like a type. However, when I pass it in as a param to Activator.CreateInstance() the compiler seems to dig it just fine. I absolutely don't know how I might fix this problem, as I don't really understand what is the problem. Is there anyone out there who can help me?
Also, all the methods are defined in different classes and even projects, so I don't know which class the methods are defined in.
The reason you're receiving this is your syntax is wrong. You cannot perform a cast with a variable dereference as the desire type. As you already know you wish to treat "set" as an "ICommandSet" cast to that instead.
var set = (ICommandSet) Activator.CreateInstance(m.DeclaringType);
You can also do it safe
var set = Activator.CreateInstance(m.DeclaringType) as ICommandSet;
Does anyone have a better way to do the following:
typeof(Service).GetMethod("UpdateData")
.MakeGenericMethod(dataType)
.Invoke(_myService, new object[]{ editData, metaData });
I'd love to do something like:
_myService.UpdateData<dataType>(editData, metaData);
But the <> accessor for generics will not take Type objects. I'm looking for a helper, utility, accessor, or something to make those generic method calls with reflection less cumbersome or at least centralized.
More Specific Example
public class UserService : IUserService
{
async Task<User> UpdateUser<T>(User user, JsonPatchDocument<T> patch){ //do code }
}
Type dtoType = MagicService.getDynamicDtoType();
Type patchType = typeof(JsonPatchDocument<>).MakeGenericType(dtoType);
dynamic patchDoc = _mapper.Map(patch, typeof(JsonPatchDocument<User>), patchType);
User updateUser = await (Task<User>)typeof(UserService).GetMethod("UpdateUser").MakeGenericMethod(dtoType).Invoke(_userService, new object[]{user, patchDoc})
This actually ends up with two issues. One, the dynamic patchDoc isn't the right type to be used in UpdateUser, which I have a separate question in Stackoverflow about (but you can use duck typing here), and the messy calling of that generic method call with reflection. I'm trying to solve both, but for this question I want to clean up the call. If you have ideas on the other piece, which is really a separate issue:
Declaring a type in C# with a Type instance
This answer does not use reflection, but may make your code easier to work with in some circumstances.
We could have a class like this:
public class MyService
{
public void UpdateData<T>(Something data, Something otherData)
{
// do stuff
}
}
Later we find that we have to call it using a Type variable instead of a generic argument, which could mean using reflection. Sometimes it's easier to add a non-generic overload to the original class, and call it from the generic method:
public class MyService
{
public void UpdateData(Type dataType, Something data, Something otherData)
{
// do stuff
}
public void UpdateData<T>(Something data, Something otherData)
{
UpdateData(typeof(T), data, otherData);
}
}
We lose the ability to easily impose constraints on the Type argument like we could with the generic method. We can add validation if we need it.
That gets you as close as possible to the code you're trying to write:
_myService.UpdateData(dataType, editData, metaData);
That's assuming that you find yourself still needing the generic methods at all. If you add non-generic overloads and discover that you're not calling the generic ones anymore, you can just delete them.
A significant benefit of this over reflection is that you can identify where your code is called. A method that's only called using reflection will appear unused.
I've read a number of articles including this one on the subject of Empty or 'marker' interfaces. I have concluded that I cannot use custom attributes in my case as I need to be able to include the instance of a class to another method and, as these classes have nothing in common, I have no option but to use a 'marker' interface.
As an example, I might have
public class Foo
{
public int Id {get;set;}
public string Name {get;set;}
}
public class Bar
{
public Guid Identifier {get;set;}
public DateTime DueDate {get;set;}
}
and I need to pass them to a method in another class and because there may be many different types that need to be passed to the method, I've defined it like this...
public void MyMethod(IMyInterface model)
{
// Do something clever here
}
And All I've had to do to make this work is to 'implement' IMyInterface on Foo and Bar.
So to the question. I now find I need to call my MyMethod() method with an anonymous type created from a LINQ statement, so I tried this ...
var data = <Some Complex LINQ>.Select(a=> new { AString = a.Value1, ADecimal = a.Value2});
MyClass.MyMethod(data);
Sadly, I get the following compile-time error:
Error 202 Cannot convert type 'AnonymousType#1' to
'IMyInterface' via a reference
conversion, boxing conversion, unboxing conversion, wrapping
conversion, or null type conversion
Now, I know I could create a local class and use that in the same way as I have my standard classes, but my requirements mean that I'm going to have a lot of these LINQ queries in my up-coming set of work so, if possible, I'd like to find a solution that allows me to use Anonymous Types.
Does anyone know of a solution or workaround for the error I'm getting?
I suspect the answer is no, but I want to know if it is possible to do something like this:
public class MyGenericClass<TSomeClass> {
public void MyGenericMethod<TSomeInterface>()
// This doesn't compile.
where TSomeClass : TSomeInterface
{
//...
}
}
What I mean to indicate in the above (non-working) example is to constrain TSomeInterface such that it can be any base class, implemented interface, or (if you really want to get fancy) implicit conversion of MyGenericClass.
NOTE:
I suspect that the reason why this was never implemented in C# is that generic constraints are not really meant to be code contracts, which is how I am trying to use them here. I really don't care what type TSomeInterface is, so long as it is implemented by TSomeClass.
So far, I have hacked this together:
public class MyGenericClass<TSomeClass> {
public void MyGenericMethod<TIntermediateType, TSomeInterface>()
where TIntermediateType : TSomeClass, TSomeInterface
{
//...
}
}
This more or less enforces the constraint that I want (that TSomeClass must inherit from, or in the case of an interface, implement, TSomeInterface), but calling it is very clumsy, because I have to specify TIntermediateType (even though I really want it to evaluate against TSomeClass):
var myGenericInstance = new MyGenericClass<TSomeClass>();
myGenericInstance.MyGenericMethod(TSomeClass, TSomeInterface);
Additionally, the above hack is broken because a caller could in theory specify a subclass of TSomeClass as the first type parameter, where only the subclass implements TSomeInterface.
The reason that I want to do this is that I am writing a fluent factory pattern for a WCF service, and I would like to prevent the caller (at compile time) from trying to create an endpoint with a contract that the service class doesn't implement. I can obviously check this at runtime (WCF in fact does this for me), but I am a big fan of compile-time checking.
Is there a better/more elegant way to achieve what I am after here?
The way I was able to wrap my head around the reason why this doesn't compile is the following:
Consider this program compiles:
class Program {
class Class1 { }
class Class2 { }
public class MyGenericClass<TSomeClass> {
public void MyGenericMethod<TSomeInterface>() where TSomeClass : TSomeInterface {
}
}
static void Main(string[] args) {
var inst = new MyGenericClass<Class1>();
}
}
Everything is good. The compiler is happy. Now consider I change the Main method:
static void Main(string[] args) {
var inst = new MyGenericClass<Class1>();
inst.MyGenericMethod<Class2>();
}
The compiler will complain that Class1 does not implement Class2. But which line is wrong? The constraint is on the call to MyGenericMethod, but the offending line of code is the creation of MyGenericClass.
In other words, which one gets the red squiggly line?
As discussed in this linked question, you can't use a type parameter that isn't from the current declaration, on the left side of a where clause.
So as suggested by w0lf in that other question, what you can do is provide both types in your interface (rather than method) declaration:
public class MyGenericClass<TSomeClass, TSomeInterface> {
where TSomeClass : TSomeInterface
public void MyGenericMethod() // not so generic anymore :(
{
//...
}
}
That, however, greatly limits your MyGenericMethod and forces your class to declare before-hand what base interface you with to allow.
So another option is to use a static method with more type parameters:
public class MyGenericClass<TSomeClass> {
public static void MyGenericMethod<TSomeClass, TSomeInterface>
(MyGenericClass<TSomeClass> that)
where TSomeClass : TSomeInterface
{
// use "that" instead of this
}
}
Possibly you could make it an extension method to make it appear to the user like an actual method.
Neither of these is exactly what you wanted, but probably better than the intermediate type solution.
As for the reason for why not?, my guess is that it would complicate the compiler without adding enough value. Here's a discussion by Angelika Langer of the same subject but about Java. Although there are significant differences between C# and Java, I think her conclusion might apply here as well:
The bottom line is that the usefulness of lower bounds on type
parameters is somewhat debatable. They would be confusing and perhaps
even misleading when used as type parameters of a generic class. On
the other hand, generic methods would occasionally profit from a type
parameter with a lower bound. For methods, a work-around for the
lack of a lower bound type parameter can often be found. Such a
work-around typically involves a static generic method or a lower
bound wildcard.
She also gives a nice use case, see the link above.
An extension method provides the best solution, though it doesn't totally solve all your concerns.
public class MyGenericClass<TSomeClass>
{
}
public static class MyGenericClassExtensions
{
public static void MyGenericMethod<TSomeClass, TSomeInterface>(this MyGenericClass<TSomeClass> self)
where TSomeClass : TSomeInterface
{
//...
}
}
It is still necessary to specify both types when calling MyGenericMethod, but it prevents the caller from specifying an incorrect type for TSomeClass as is possible with the approach you came up with. With this approach, the method can be called like so:
var myGenericInstance = new MyGenericClass<TSomeClass>();
myGenericInstance.MyGenericMethod<TSomeClass, TSomeInterface>();
It will be a compile error if the type parameter MyGenericClass is declared with does not match the first type parameter to MyGenericMethod.
Since the first type parameter can be inferred by the this argument, it is often possible for the compiler to infer both type parameters if their additional parameters to the method.
I have a method like the following:
public IEnumerable<T> GetControls<T>()
: where T : ControlBase
{
// removed.
}
I then created a class:
public class HandleBase<TOwner> : ControlBase
: TOwner
{
// Removed
}
I'd like to be able to call
GetControls<HandleBase<this.GetType()>>;
where it would use the type of THIS class to pass to the HandleBase. This would in essentially get all HandleBase that have an owner of THIS type.
How can I achieve this?
EDIT:
I'm using .NET 2.0 so solutions greater than 2.0 will not work.
The idea is to have ControlBase have a collection of other ControlBase for "children". Then they can be queried based on their type with GetControls<T>(). This would allow me to, for example, get all HandleBase for a Shape. Then I can take all of these and set Visible=false or do something else with them. Thus I can manipulate children of a specific type for a collection.
HandleBase<TOwner> requires the TOwner since it has a reference to the "owning type". So you can only add anything that extends HandleBase to a Shape. Make sense?
Thanks for all the help!
You can do this either by specifying a type at compile-time or by using reflection.
You can do it with reflection like this:
typeof(SomeClass).GetMethod("GetControls")
.MakeGenericMethod(typeof(HandleBase<>).MakeGenericType(GetType()))
.Invoke(someObject, null);
Note that it would return an object; you would not be able to cast it to IEnumerable<T> (Unless you know what T is at compile-time, in which case there's no point). You would be able to cast it to IEnumerable.
However, this is a bad idea.
There is probably a better solution for you; please provide more detail.
You can't. Generics is a compile-time feature. You would need to include the type as a non-generic parameter to the method, and pass it in there.
Note that type parameters are not variables. Therefore, you cannot use a variable in place of a type parameter.
You could, however, do this through reflection, or by using a special construct which is pretty limited but may solve your case:
public class MyClass<TSelf> where TSelf: MyClass<TSelf> {
public IEnumerable<T> GetControls<T>() where T: ControlBase {
// removed.
}
public void MyCall() {
GetControls<HandleBase<TSelf>>();
}
}
public class MyConcreteClass: MyClass<MyConcreteClass> {
}
Probably there is no way to do what you ask for. Extending your example:
X x = GetControls<HandleBase<this.GetType()>>;
What should the type X be here? However from other background information it seems that you need to get list of all controls of given type. You could do this for example in such way:
public IEnumerable<ControlBase> GetControls(Type type) {
//your logic here
}
Depending on your other usages and goals you might also want to return non-generic IEnumerable.
Since GetControls() returns an enumeration, you might find a way to filter the resulting enumeration with .OfType<T>, something like
List<T2> list = controlList.GetControls<T>().OfType<T2>().ToList();
You would need a generic constraint somehwere along the lines of
where T2 : T
This is specific to my implementation of this but I was able to solve this by creating a non-generic HandleBase first and then a generic HandleBase<TOwner> since the only place TOwner was being used was the property Owner.
Then when I can call GetControls<HandleBase> and get all HandleBase regardless of the Owner.
Thanks all for answers!