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Closed 9 years ago.
I tried to use object expression to extend the IDelegateEvent, but in fsi there was an error FS0039: The value or constructor 'handler' is not defined.
My codes are as follows:
type IDelegateEvent<'Del when 'Del:> Delegate> with
member this.Subscribe hanlder =
do this.AddHandler(handler)
{ new IDisposable with
member x.Dispose() =
this.RemoveHandler(handler) }
And the reference is Matthew Podwysocki's Blog:http://weblogs.asp.net/podwysocki/archive/2009/08/21/f-first-class-events-creating-and-disposing-handlers.aspx
Because of a spelling error hanlder =
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What are the semantics of _internal?
(1 answer)
Closed 4 years ago.
I have the following code in a flutter tutorial that discusses global state:
static GlobalState instance = new GlobalState._();
GlobalState._();
What exactly is the ._ doing? Is this technically just a blank method?
It's a private constructor. This pattern is used to prevent clients from instantiating or subclassing GlobalState, so the only available instance of GlobalState in the program is instance.
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Closed 6 years ago.
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Exception: Object reference not set to an instance of an object.
{System.NullReferenceException: Object reference not set to an instance of an object.
at System.Web.HttpContextExtensions.GetOwinEnvironment(HttpContext context)
at System.Web.HttpContextExtensions.GetOwinContext(HttpContext context)
You did not instantiate UserManager, but trying to use it as a static class which it is not.
Use the new keyword (how do I create an instance of UserManager).
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Closed 7 years ago.
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I am getting an error "Invalid floating point operation" while creating a new form as below:
procedure TfrmForm1.actMyProcedureExecute(Sender: TObject);
var
MyForm2 : TfrmForm2;
begin
MyForm2 := TfrmForm2.Create(Self); //Getting error while executing this statement. I put a breakpoint on Create event of TForm2 form, but before that I am getting this error and breakpoint never comes on OnCreate event of TForm2 form.
end;
The error is raised during execution of the constructor of TfrmForm2. The error is raised before your OnCreate event is executed.
The most likely explanation therefore is that the exception is raised during creation and property setting of the controls specified in the dfm file.
Another possibility, I suppose, is that you have added a constructor for the class and code in there raises the exception. That I suspect is less likely.
Debug this by enabling debug DCUs and then looking at the call stack when the exception is raised. This should give you a pointer to which part of construction is failing. Once you've identified the point of failure you can then attempt to solve the problem.
Finally, in the absence of an MCVE in the question, this is the type of answer you can expect. Broad and general.
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Closed 10 years ago.
I am new to Dependency Injection, using C#, so please forgive me for my lame question. I wanted to post this question anyway before investing time and buying expensive books on this subject.
After going through a few online documents, it appears that using dependency containers along with configuration files one can use this to swith from one type of implementation to another. However this could be easily done by an if/else statement and some config settings.
What is the advantage of using such a cumbersome implementation just to change from one class to another? I see abstract and factory patterns to be much more useful. Maybe I am wrong.
I ha s a real worl case where i needed dependency injection :
2 assembly : 1 responsible for computing a price when you do a search (lets call it Search), and the second responsible for computing a booking price with all the options (let's call it Booking).
The Booking assembly is already referencing Search(because it needs to know the initial price for computing the full price).
But here is the requirement : we need the price from Search to include all mandatories options (yes in the tourism industry you have mandatories options) like "full room cleaning".
So I couldn't had a ref to Booking in Search (because of circular reference). So I decided use dependency injection.
My Search assembly defined an interface
public interface IAddMandatoryOptionService{
void ChangeResultsWithMandatoryOptions(SearchResult[] results);
}
And then my Booking Assembly could implement this interface.
public class AddMandatoryOptionService : IAddMandatoryOptionService{
public void ChangeResultsWithMandatoryOptions(SearchResult[] results){
...
}
}
My SearchService class now would look like
public class SearchService{
public SearchService(IAddMandatoryOptionService optionService){
this.OptionService = optionService;
}
public SearchResult[] Search(Filter filter){
...
this.OptionService.ChangeResultsWithMandatoryOptions(results);
...
return results;
}
}
So my Search Service has no dependency to the AddMandatoryOptionService class (and the Booking assembly), but it's using its functionality.The good IAddMandatoryOptionService will be injected when I create my service (in my Application_Start or with a DI Framework)
The advantages are :
Resolving the circular reference problem.
If I want to unit test my SearchService I'll juste have to mock/fake IAddMandatoryOptionService.
Here the need for injection was more technical than logical, but I think this kind of real world scenario could help you to get the point.
In short, dependency injection is used to be able to losely couple classes. By using an if else statement you introduce a dependency between classes. When adding à new implementation to your if else statement you need to add another else statement.
You've probably read http://en.m.wikipedia.org/wiki/Dependency_injection since they have a pretty good motivation section.
Perhaps complete your q with different code examples.
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Closed 11 years ago.
Google Guice provides some great dependency injection features.
I came across the #Nullable feature recently which allows you to mark constructor arguments as optional (permitting null) since Guice does not permit these by default:
e.g.
public Person(String firstName, String lastName, #Nullable Phone phone) {
this.firstName = checkNotNull(firstName, "firstName");
this.lastName = checkNotNull(lastName, "lastName");
this.phone = phone;
}
https://github.com/google/guice/wiki/UseNullable
What are the other useful features of Guice (particularly the less obvious ones) that people use?
None of 'em are intended to be hidden, but these are my favorite 'bonus features' in Guice:
Guice can inject a TypeLiteral<T>, effectively defeating erasure.
TypeLiteral can do generic type resolution: this tells you that get() on a List<String> returns an Iterator<String>.
Types is a factory for implementations of Java's generic type interfaces.
Grapher visualizes injectors. If your custom provider implements HasDependencies, it can augment this graph.
Modules.override() is incredibly handy in a pinch.
Short syntax for defining parameterized keys: new Key<List<String>>() {}.
Binder.skipSources() lets you to write extensions whose error messages track line numbers properly.
The SPI. Elements.getElements() breaks a module into atoms and Elements.getModule() puts them back together.
If you implement equals() and hashCode() in a Module, you can install that module multiple times without problem.
I like how totally open the Scope interface is: basically, it's just a transformation from Provider to Provider. (Okay, from Key and Provider to Provider)
Want some things to be basically Singleton, but re-read from the database every half hour? It's easy to make a scope for that. Want to run some requests in the background, and have a scope that means "all background requests started from the same HTTP request?" It's relatively easy to write that Scope too.
Want to scope some Key on your server during tests so that it uses a separate instance for each test that you're running from a client? (With the test passing the test id in a Cookie or extra HTTP parameter) That's harder to do, but it's perfectly possible and so someone's already written that for you.
Yes, excessive abuse of Scope will cause Jesse to start hunting around for the stakes and garlic cloves, but its amazing flexibility can be really useful.
One great feature of Guice is how easy it makes implementing method interceptors in any Module, using:
public void bindInterceptor(
Matcher<? super Class<?>> classMatcher,
Matcher<? super Method> methodMatcher,
MethodInterceptor... interceptors);
Now, any method matching methodMatcher within a class matching classMatcher in that Module's scope is intercepted by interceptors.
For example:
bindInterceptor(
Matchers.any(),
Matchers.annotatedWith(Retryable.class),
new RetryableInterceptor());
Now, we can simply annotate any method with #Retryable and our RetryableInterceptor can
retry it if it fails.