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  • What about optional generic type parameters in C# 5.0?

    - by Lars Corneliussen
    Just a thought. Wouldn't it be useful to have optional type parameters in C#? This would make life simpler. I'm tired of having multiple classes with the same name, but different type parameters. Also VS doesn't support this very vell (file names) :-) This would eliminate the need for a non-generic IEnumerable: interface IEnumerable<out T=object>{ IEnumerator<T> GetEnumerator() } What do you think?

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  • casting Collection<SomeClass> to Collection<SomeSuperClass>

    - by skrebbel
    Hi all, I'm sure this has been answered before, but I really cannot find it. I have a java class SomeClass and an abstract class SomeSuperClass. SomeClass extends SomeSuperClass. Another abstract method has a method that returns a Collection<SomeSuperClass>. In an implementation class, I have a Collection<SomeClass> myCollection I understand that I cannot just return myCollection, because Collection<SomeClass> does not inherit from Collection<SomeSuperClass>. Nevertheless, I know that everything in myCollection is a SomeSuperClass because after all, they're SomeClass objects which extend SomeSuperClass. How can I make this work? I.e. I want public class A { private Collection<SomeClass> myCollection; public Collection<SomeSuperClass> getCollection() { return myCollection; //compile error! } } The only way I've found is casting via a non-generic type and getting unchecked warnings and whatnot. There must be a more elegant way, though? I feel that also using Collections.checkedSet() and friends are not needed, since it is statically certain that the returned collection only contains SomeClass objects (this would not be the case when downcasting instead of upcasting, but that's not what I'm doing). What am I missing? Thanks!

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  • Casting to generic type in Java doesn't raise ClassCastException?

    - by Kip
    I have come across a strange behavior of Java that seems like a bug. Is it? Casting an Object to a generic type (say, K) does not throw a ClassCastException even if the object is not an instance of K. Here is an example: import java.util.*; public final class Test { private static<K,V> void addToMap(Map<K,V> map, Object ... vals) { for(int i = 0; i < vals.length; i += 2) map.put((K)vals[i], (V)vals[i+1]); //Never throws ClassCastException! } public static void main(String[] args) { Map<String,Integer> m = new HashMap<String,Integer>(); addToMap(m, "hello", "world"); //No exception System.out.println(m.get("hello")); //Prints "world", which is NOT an Integer!! } }

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  • How to refactor these generic methods?

    - by Steve Crane
    I have written two nearly identical generic extension methods and am trying to figure out how I might refactor them into a single method. They differ only in that one operates on List and the other on List, and the properties I'm interested in are AssetID for AssetDocument and PersonID for PersonDocument. Although AssetDocument and PersonDocument have the same base class the properties are defined in each class so I don't think that helps. I have tried public static string ToCSVList<T>(this T list) where T : List<PersonDocument>, List<AssetDocument> thinking I might then be able to test the type and act accordingly but this results in the syntax error Type parameter 'T' inherits conflicting constraints These are the methods that I would like to refactor into a single method but perhaps I am simply going overboard and they would besat be left as they are. I'd like to hear what you think. public static string ToCSVList<T>(this T list) where T : List<AssetDocument> { var sb = new StringBuilder(list.Count * 36 + list.Count); string delimiter = String.Empty; foreach (var document in list) { sb.Append(delimiter + document.AssetID.ToString()); delimiter = ","; } return sb.ToString(); } public static string ToCSVList<T>(this T list) where T : List<PersonDocument> { var sb = new StringBuilder(list.Count * 36 + list.Count); string delimiter = String.Empty; foreach (var document in list) { sb.Append(delimiter + document.PersonID.ToString()); delimiter = ","; } return sb.ToString(); }

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  • sOperator as and generic classes

    - by abatishchev
    I'm writing .NET On-the-Fly compiler for CLR scripting and want execution method make generic acceptable: object Execute() { return type.InvokeMember(..); } T Execute<T>() { return Execute() as T; /* doesn't work: The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint */ // also neither typeof(T) not T.GetType(), so on are possible return (T) Execute(); // ok } But I think operator as will be very useful: if result type isn't T method will return null, instead of an exception! Is it possible to do?

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  • Calling a method with an instance of derived class of derived generic type

    - by madsbirk
    Okay, so I have classes class B<T> : A<T> class L : K and a method void Method(A<K> a) {...} What I would like to do is this b = new B<L>(); Method(b); //error But it is not possible to b to the correct type. Indeed it is not possible to make this cast A<K> t = new A<L>(); //error I would really like to not have to change the internals of Method. I have no problems making changes to B and/or L. Do I have any options for making some sort of workaround? I guess it should be possible for Method to execute all of its method calls etc. on b, since B derives from A and L derives from K?

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  • .NET template class instance - passing a variable data type

    - by FerretallicA
    As the title suggests, I'm tyring to pass a variable data type to a template class. Something like this: frmExample = New LookupForm(Of Models.MyClass) 'Works fine Dim SelectedType As Type = InstanceOfMyClass.GetType() 'Works fine frmExample = New LookupForm(Of SelectedType) 'Ba-bow! frmExample = New LookupForm(Of InstanceOfMyClass.GetType()) 'Ba-bow! LookupForm<Models.MyClass> frmExample; Type SelectedType = InstanceOfMyClass.GetType(); frmExample = new LookupForm<SelectedType.GetType()>(); //Ba-bow frmExample = new LookupForm<(Type)SelectedType>(); //Ba-bow I'm assuming it's something to do with the template being processed at compile time but even if I'm off the mark there, it wouldn't solve my problem anyway. I can't find any relevant information on using Reflection to instance template classes either. (How) can I create an instance of a dynamically typed repository at runtime?

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  • How can I create object in abstract class without having knowledge of implementation?

    - by Greg
    Hi, Is there a way to implement the "CreateNode" method in my library abstract below? Or can this only be done in client code outside the library? I current get the error "Cannot create an instance of the abstract class or interface 'ToplogyLibrary.AbstractNode" public abstract class AbstractTopology<T> { // Properties public Dictionary<T, AbstractNode<T>> Nodes { get; private set; } public List<AbstractRelationship<T>> Relationships { get; private set; } // Constructors protected AbstractTopology() { Nodes = new Dictionary<T, AbstractNode<T>>(); } // Methods public AbstractNode<T> CreateNode() { var node = new AbstractNode<T>(); // ** Does not work ** Nodes.Add(node.Key, node); } } } public abstract class AbstractNode<T> { public T Key { get; set; } } public abstract class AbstractRelationship<T> { public AbstractNode<T> Parent { get; set; } public AbstractNode<T> Child { get; set; } }

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  • how to implement class with collection of string/object pairs so that an object can be returned with

    - by matti
    The values in a file are read as string and can be double, string or int or maybe even lists. An example file: DatabaseName=SomeBase Classes=11;12;13 IntValue=3 //this is required! DoubleValue=4.0 I was thinking something like this: public static T GetConfigValue(string cfgName) { // here we just return for example the value which could // be List[int] if parameter cfgName='Classes' // and LoadConfig was called with Dictionary containing // keyvaluepair 'Classes' / typeof(List[int]) } public static bool LoadConfig(Dictionary reqSettings, Dictionary optSettings) { foreach (KeyValuePair kvPair in reqSettings) { if (ReadCheckAndStore(kVPair, true)) return false; } foreach (KeyValuePair kvPair in reqSettings) { if (ReadCheckAndStore(kVPair, false)) return false; } return true; } private static bool ReadCheckAndStore(KeyValuePair kVPair, bool isRequired) { if (!ReadValue(kVPair.Key, out confValue) && isRequired) //req. IntValue !found return false; //here also have to test if read value is wanted type. //and if yes store to collection. } Thanks a lot & BR! -Matti PS. Additional issue is default values for optional settings. It's not elegant to pass them to LoadConfig in separate Dictionary, but that is an other issue...

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  • How to Pass Parameters to Activator.CreateInstance<T>()

    - by DaveDev
    Hi guys I want to create an instance of a type that I specify in a generic method that I have. This type has a number of overloaded constructors. I'd like to be able to pass arguments to the constructors, but Activator.CreateInstance<T>() doesn't see to have this as an option. Is there another way to do it? Thanks Dave

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  • Convert a generic list to an array

    - by Freewind
    I have searched for this, but unfortunately, I don't get the correct answer. class Helper { public static <T> T[] toArray(List<T> list) { T[] array = (T[]) new Object[list.size()]; for (int i = 0; i < list.size(); i++) { array[i] = list.get(i); } return array; } } Test it: public static void main(String[] args) { List<String> list = new ArrayList<String>(); list.add("abc"); String[] array = toArray(list); System.out.println(array); } But there is an error thrown: Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String; at test.Helper.main(Helper.java:30) How to solve this?

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  • Question concerning SCJP-6 exam

    - by abatishchev
    While preparing for the SCJP-6 exam I faced with a difficult issue. I can’t find answer by myself. Please, answer for the question and give short comments: abstract class A<K> extends Number> { // insert code here } public abstract <K> A<? extends Number> useMe(A<? super K> k); public abstract <K> A<? super Number> useMe(A<? extends K> k); public abstract <K> A<K> useMe(A<K> k); public abstract <V extends K> A<V> useMe(A<V> k); public abstract <V super K> A<V> useMe(A<V> k); public abstract <V extends Character> A<? super V> useMe(A<K> k); public abstract <V super Character> A<? super V> useMe(A<K> k); Which method can be inserted in a placeholder above? P.S. I tried to look on the specification. Those one was not helpful for me.

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  • Converting non-generic List type to Generic List type in Java 1.5

    - by Shaun F
    I have a List that is guaranteed to contain just one type object. This is created by some underlying code in a library that I cannot update. I want to create a List<ObjectType> based on the incoming List object so that my calling code is talking to List<ObjectType>. What's the best way to convert the List (or any other object collection) to a List<ObjectType>.

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  • Java class object from type variable

    - by Alexander Temerev
    Is there a way to get Class object from the type variable in Java generic class? Something like that: public class Bar extends Foo<T> { public Class getParameterClass() { return T.class; // doesn't compile } } This type information is available at compile time and therefore should not be affected by type erasure, so, theoretically, there should be a way to accomplish this. Does it exist?

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  • Creating circular generic references

    - by M. Jessup
    I am writing an application to do some distributed calculations in a peer to peer network. In defining the network I have two class the P2PNetwork and P2PClient. I want these to be generic and so have the definitions of: P2PNetwork<T extends P2PClient<? extends P2PNetwork<T>>> P2PClient<T extends P2PNetwork<? extends T>> with P2PClient defining a method of setNetwork(T network). What I am hoping to describe with this code is: A P2PNetwork is constituted of clients of a certain type A P2PClient may only belong to a network whose clients consist of the same type as this client (the circular-reference) This seems correct to me but if I try to create a non-generic version such as MyP2PClient<MyP2PNetwork<? extends MyP2PClient>> myClient; and other variants I receive numerous errors from the compiler. So my questions are as follows: Is a generic circular reference even possible (I have never seen anything explicitly forbidding it)? Is the above generic definition a correct definition of such a circular relationship? If it is valid, is it the "correct" way to describe such a relationship (i.e. is there another valid definition, which is stylistically preferred)? How would I properly define a non-generic instance of a Client and Server given the proper generic definition?

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  • What would be the best .NET 2.0 type to represent .NET 3.5 HashSet<T>?

    - by Will Marcouiller
    I'm writing myself a class library to manage Active Directory. I have an interface: Public Interface ISourceAnnuaire(Of T as {IGroupe, ITop, IUniteOrganisation, IUtilisateur}) Readonly Property Changements As Dictionary(Of T, HashSet(Of String)) End Interface This Changements property is used to save in memory the changes that occur on a particular element that is part of the source. However, I am stuck with .NET Framework 2.0. What would be the closest .NET 2.0 for HashSet(Of String)?

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  • Compilers behave differently with a null parameter of a generic method

    - by Eyal Schneider
    The following code compiles perfectly with Eclipse, but fails to compile with javac: public class HowBizarre { public static <P extends Number, T extends P> void doIt(P value) { } public static void main(String[] args) { doIt(null); } } I simplified the code, so T is not used at all now. Still, I don't see a reason for the error. For some reason javac decides that T stands for Object, and then complains that Object does not conform to the bounds of T (which is true): HowBizarre.java:6: incompatible types; inferred type argument(s) java.lang.Number,java.lang.Object do not conform to bounds of type variable (s) P,T found : <P,T>void required: void doIt(null); ^ Note that if I replace the null parameter with a non-null value, it compiles fine. Which of the compilers behaves correctly and why? Is this a bug of one of them?

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  • Ambiguous Generic restriction T:class vs T:struct

    - by Maslow
    This code generates a compiler error that the member is already defined with the same parameter types. private T GetProperty<T>(Func<Settings, T> GetFunc) where T:class { try { return GetFunc(Properties.Settings.Default); } catch (Exception exception) { SettingReadException(this,exception); return null; } } private TNullable? GetProperty<TNullable>(Func<Settings, TNullable> GetFunc) where TNullable : struct { try { return GetFunc(Properties.Settings.Default); } catch (Exception ex) { SettingReadException(this, ex); return new Nullable<TNullable>(); } } Is there a clean work around?

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  • Generic Factorial function in C#

    - by mqpasta
    I want to write a generic function to calculate factorial in C# ... like: static T Factorial<T>(T n) { if (n <= 1) return 1; return Factorial<T>(n - 1); } but obviously having restriction that we can't perform operations on type 'T'. any alternative?

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  • Auto convert java source to use generic rather than raw types

    - by Sam
    Is there a way/tool to auto convert Java source code from using raw types to using generic types? I have some legacy code with 677 references to raw types: ArrayList 47 Vector 420 Hashtable 61 Enumeration 64 Class 7 Iterator 78 TOTAL 677 Now I could manually look through the code to infer the generic types and replace, but that is going to take a long time.

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