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  • Select a Dictionary<T1, T2> with LINQ

    - by Rich
    I have used the "select" keyword and extension method to return an IEnumerable<T> with LINQ, but I have a need to return a generic Dictionary<T1, T2> and can't figure it out. The example I learned this from used something in a form similar to the following: IEnumerable<T> coll = from x in y select new SomeClass{ prop1 = value1, prop2 = value2 }; I've also done the same thing with extension methods. I assumed that since the items in a Dictionary<T1, T2> can be iterated as KeyValuePair<T1, T2> that I could just replace "SomeClass" in the above example with "new KeyValuePair<T1, T2> { ...", but that didn't work (Key and Value were marked as readonly, so I could not compile this code). Is this possible, or do I need to do this in multiple steps? Thanks.

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  • Operation Problems in Java Generic

    - by alantheweasel
    I got some problem when i was learning Java Generic : interface calculator<T, R> { public void execute(T t, R r); } class executeAdd<T, R> implements calculator<T, R> { private T first; private R second; public executeAdd(T first, R second) { super(); this.first = first; this.second = second; } @Override public void execute(T t, R r) { // TODO Auto-generated method stub Object o = t + r // ERROR ! What i could do it ? } }

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  • can I get .class from generic type argument?

    - by Mike S
    I have the following class: public abstract class MyClass<T extends Object> { protected T createNewFromData(Reader reader){ GSON.fromJSON(reader,T.class); // T.class isn't allowed :( } } How do I pass a Class<T instance into there? Is there some wierd and wacky work around? Is there a way to get a Class<T reference other than from a pre-instantiated Object of type T? It won't let me do this either: T t = new T(); Class<T> klass = t.class; ANSWER BELOW Thanks to the accepted answer, here is the solution: Type type = new TypeToken<T>(){}.getType(); return gson.fromJson(reader, type);

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  • Changing the Order in a .NET Generic Dictionary

    - by pm_2
    I have a class that inherits from a generic dictionary as follows: Class myClass : System.Collections.Generic.Dictionary<int, Object> I have added a list of values to this in a particular order, but I now wish to change that order. Is there any way (without removing and re-adding) that I could effectively re-index the values; so change the object at index 1 to now be at index 10 for example? For example, this doesn't work: myClass[1].Index = 10;

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  • C# Generic new() constructor problem

    - by LB
    I'm trying to create a new object of type T via its constructor when adding to the list. I'm getting a compile error: The correct error message is: 'T': cannot provide arguments when creating an instance of a variable But it does! Any ideas? public static string GetAllItems<T>(...) where T : new() { ... List<T> tabListItems = new List<T>(); foreach (ListItem listItem in listCollection) { tabListItems.Add(new T(listItem)); // error here. } ... }

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  • Are static members of a generic class tied to the specific instance?

    - by mafutrct
    This is more of a documentation than a real question. I noticed the principle it is not described on SO yet (did I miss it?), so here goes: Imagine a generic class that contains a static member: class Foo<T> { public static int member; } Is there a new instance of the member for each specific class, or is there only a single instance for all Foo-type classes? It can easily be verified by code like this: Foo<int>.member = 1; Foo<string>.member = 2; Console.WriteLine (Foo<int>.member); What is the result, and where is this behavior documented?

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  • C# Improved algorithm

    - by generixs
    I have been asked at interview (C# 3.0) to provide a logic to remove a list of items from a list. I responded int[] items={1,2,3,4}; List<int> newList = new List<int>() { 1, 2, 3, 4, 5, 56, 788, 9 }; newList.RemoveAll((int i) => { return items.Contains(i); }); 1) The interviewer replied that the algorithm i had employed will gradually take time if the items grow and asked me to give even better and faster one.What would be the efficient algorithm ? 2) How can i achieve the same using LINQ? 3) He asked me to provide an example for Two-Way-Closure? (General I am aware of closure, what is Two-Way-Closure?, I replied there is no such term exists,but he did not satisfy).

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  • How can I add to List<? extends Number> data structures?

    - by kunjaan
    I have a List which is declared like this : List<? extends Number> foo3 = new ArrayList<Integer>(); I tried to add 3 to foo3. However I get an error message like this: The method add(capture#1-of ? extends Number) in the type List<capture#1-of ? extends Number> is not applicable for the arguments (ExtendsNumber)

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  • Action(Of T) in Visual Basic in List(Of T).ForEach

    - by Jason
    I have searched high and low for documentation on how to use this feature. While the loop I could write would be simple and take no time, I really would like to learn how to use this. Basically I have a class, say, Widget, with a Save() sub that returns nothing. So: Dim w as New Widget() w.Save() basically saves the widget. Now let's say I have a generic collection List(Of Widget) name widgetList(Of Widget) and I want to run a Save() on each item in that list. It says I can do a widgetList.ForEach([enter Action(Of T) here]) ....but how in the F does this work??? There is no documentation anywhere on the intrablags. Help would be much much appreciated.

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  • How do I access static variables in an enum class without a class instance?

    - by krick
    I have some code that processes fixed length data records. I've defined the record structures using java enums. I've boiled it down the the simplest example possible to illustrate the hoops that I currently have to jump through to get access to a static variable inside the enum. Is there a better way to get at this variable that I'm overlooking? If you compile and run the code, it just prints out "3". Note: the "code" tag doesn't seem to want to format this properly, but it should compile. class EnumTest { private interface RecordLayout { public int length(); } private enum RecordType1 implements RecordLayout { FIELD1 (2), FIELD2 (1), ; private int length; private RecordType1(int length) { this.length = length; } public int length() { return length; } public static int LEN = 3; } private static <E extends Enum<E> & RecordLayout> String parse(String data, Class<E> record) { // ugly hack to get at LEN... try { int len = record.getField("LEN").getInt(record); System.out.println(len); } catch (Exception e) { System.out.println(e); } String results = ""; for (E field: record.getEnumConstants()) { // do some stuff with the fields } return results; } public static void main(String args[]) { parse("ABC", RecordType1.class); } }

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  • Restrict a generic type

    - by Water Cooler v2
    I want to restrict the generic type parameter to: 1) either that of a certain user defined reference type; OR 2) any of the primitive types in the CLR; How do I say something to the effect of: interface IDataManager<T>: IDataManager where T: IDataObject, T: ValueType

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  • Generic list/sublist handling

    - by user628661
    Let's say we have a class class ComplexCls { public int Fld1; public string Fld2; //could be more fields } class Cls { public int SomeField; } and then some code class ComplexClsList: List<ComplexCls>; ComplexClsList myComplexList; // fill myComplexList // same for Cls class ClsList : List<Cls>; ClsList myClsList; We want to populate myClsList from myComplexList, something like (pseudocode): foreach Complexitem in myComplexList { Cls ClsItem = new Cls(); ClsItem.SomeField = ComplexItem.Fld1; } The code to do this is easy and will be put in some method in myClsList. However I'd like to design this as generic as possible, for generic ComplexCls. Note that the exact ComplexCls is known at the moment of using this code, only the algorithm shd be generic. I know it can be done using (direct) reflection but is there other solution? Let me know if the question is not clear enough. (probably isn't). [EDIT] Basically, what I need is this: having myClsList, I need to specify a DataSource (ComplexClsList) and a field from that DataSource (Fld1) that will be used to populate my SomeField

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  • How can Java be improved so that it no longer needs to perform type erasure? [closed]

    - by user63904
    The official Java tutorial on generics explains type erasure and why it was added to the compiler: When a generic type is instantiated, the compiler translates those types by a technique called type erasure — a process where the compiler removes all information related to type parameters and type arguments within a class or method. Type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created before generics. This most likely was a pragmatic approach, or perhaps the least painful one. However, now that generics is widely supported across the industry, what can be done in order for us to not need type erasure? Is it feasible with out needing to break backwards compatibility, or if it is feasible, is it practical? Has the last the last statement in the quote above become self referential? That is: "type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created with Java versions that perform type erasure."

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  • Will there ever be a version of Java which does not perform Type Erasure

    - by user63904
    Type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created before generics Generics were introduced in Java 1.5, so presumably the statement "applications that were created before generics" is referring to Java 1.4? Given that Java 1.4 entered its End Of Life around 2006 and was officially End Of Life'd around 2008. Why is type erasure still being performed in Java 7, etc... Has the statement now become self referential i.e. Type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created with Java versions that perform Type Erasure. Meaning therefore that there will never be a version of Java that doesn't perform Type Erasure.

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  • Interface (contract), Generics (universality), and extension methods (ease of use). Is it a right design?

    - by Saeed Neamati
    I'm trying to design a simple conversion framework based on these requirements: All developers should follow a predefined set of rules to convert from the source entity to the target entity Some overall policies should be able to be applied in a central place, without interference with developers' code Both the creation of converters and usage of converter classes should be easy To solve these problems in C# language, A thought came to my mind. I'm writing it here, though it doesn't compile at all. But let's assume that C# compiles this code: I'll create a generic interface called IConverter public interface IConverter<TSource, TTarget> where TSource : class, new() where TTarget : class, new() { TTarget Convert(TSource source); List<TTarget> Convert(List<TSource> sourceItems); } Developers would implement this interface to create converters. For example: public class PhoneToCommunicationChannelConverter : IConverter<Phone, CommunicationChannle> { public CommunicationChannel Convert(Phone phone) { // conversion logic } public List<CommunicationChannel> Convert(List<Phone> phones) { // conversion logic } } And to make the usage of this conversion class easier, imagine that we add static and this keywords to methods to turn them into Extension Methods, and use them this way: List<Phone> phones = GetPhones(); List<CommunicationChannel> channels = phones.Convert(); However, this doesn't even compile. With those requirements, I can think of some other designs, but they each lack an aspect. Either the implementation would become more difficult or chaotic and out of control, or the usage would become truly hard. Is this design right at all? What alternatives I might have to achieve those requirements?

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  • Did the Unity Team fix that "generics handling" bug back in 2008?

    - by rasx
    At my level of experience with Unity it might be faster to ask whether the "generics handling" bug acknowledged by ctavares back in 2008 was fixed in a public release. Here was the problem (which might be my problem today): Hi, I get an exception when using .... container.RegisterType(typeof(IDictionary<,), typeof(Dictionary<,)); The exception is... "Resolution of the dependency failed, type = \"IDictionary2\", name = \"\". Exception message is: The current build operation (build key Build Key[System.Collections.Generic.Dictionary2[System.String,System.String], null]) failed: The current build operation (build key Build Key[System.Collections.Generic.Dictionary2[System.String,System.String], null]) failed: The type Dictionary2 has multiple constructors of length 2. Unable to disambiguate. When I attempt... IDictionary myExampleDictionary = container.Resolve(); Here was the moderated response: There are no books that'll help, Unity is a little too new for publishers to have caught up yet. Unfortunately, you've run into a bug in our generics handling. This is currently fixed in our internal version, but it'll be a little while before we can get the bits out. In the meantime, as a workaround you could do something like this instead: public class WorkaroundDictionary : Dictionary { public WorkaroundDictionary() { } } container.RegisterType(typeof(IDictionary<,),typeof(WorkaroundDictionary<,)); The WorkaroundDictionary only has the default constructor so it'll inject no problem. Since the rest of your app is written in terms of IDictionary, when we get the fixed version done you can just replace the registration with the real Dictionary class, throw out the workaround, and everything will still just work. Sorry about the bug, it'll be fixed soon!

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  • First languages with generic programming support

    - by oluies
    Which was the first language with generic programming support, and what was the first major staticly typed language (widely used) with generics support. Generics implement the concept of parameterized types to allow for multiple types. The term generic means "pertaining to or appropriate to large groups of classes." I have seen the following mentions of "first": First-order parametric polymorphism is now a standard element of statically typed programming languages. Starting with System F [20,42] and functional programming lan- guages, the constructs have found their way into mainstream languages such as Java and C#. In these languages, first-order parametric polymorphism is usually called generics. From "Generics of a Higher Kind", Adriaan Moors, Frank Piessens, and Martin Odersky Generic programming is a style of computer programming in which algorithms are written in terms of to-be-specified-later types that are then instantiated when needed for specific types provided as parameters. This approach, pioneered by Ada in 1983 From Wikipedia Generic Programming

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  • First languages with generic programming support

    - by oluies
    Which was the first language with generic programming support, and what was the first major staticly typed language (widely used) with generics support. Generics implement the concept of parameterized types to allow for multiple types. The term generic means "pertaining to or appropriate to large groups of classes." I have seen the following mentions of "first": First-order parametric polymorphism is now a standard element of statically typed programming languages. Starting with System F [20,42] and functional programming lan- guages, the constructs have found their way into mainstream languages such as Java and C#. In these languages, first-order parametric polymorphism is usually called generics. From "Generics of a Higher Kind", Adriaan Moors, Frank Piessens, and Martin Odersky Generic programming is a style of computer programming in which algorithms are written in terms of to-be-specified-later types that are then instantiated when needed for specific types provided as parameters. This approach, pioneered by Ada in 1983 From Wikipedia Generic Programming

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