generics

问题 What I want to achieve can be best explained in code: Given shoe and dress class: class Shoe { constructor(public size: number){} } class Dress { constructor(public style: string){} } Have a generic box that can only contain Shoe or Dress. Can't contain both: class Box <T extends Shoe | Dress > { } Then have a utility class that takes care of moving shoes: class ShoeMover { constructor(public size: number[]){} } Also, a utility class for moving Dresses: class DressPacker { constructor(public

问题 I don't think I'm doing anything too esoteric, but I don't see any other questions about this. The following code (I've reduced it to the essentials) generates a compiler error in C# 4. However, it should be obvious what the type argument is - there's a greatest common denominator ("class A") that is also explicitly defined in the return type of the method "Frob". Shouldn't the compiler make a list of all the return types in the lambda expression, create an ancestry tree to find their common

问题 I don't think I'm doing anything too esoteric, but I don't see any other questions about this. The following code (I've reduced it to the essentials) generates a compiler error in C# 4. However, it should be obvious what the type argument is - there's a greatest common denominator ("class A") that is also explicitly defined in the return type of the method "Frob". Shouldn't the compiler make a list of all the return types in the lambda expression, create an ancestry tree to find their common

问题 I don't think I'm doing anything too esoteric, but I don't see any other questions about this. The following code (I've reduced it to the essentials) generates a compiler error in C# 4. However, it should be obvious what the type argument is - there's a greatest common denominator ("class A") that is also explicitly defined in the return type of the method "Frob". Shouldn't the compiler make a list of all the return types in the lambda expression, create an ancestry tree to find their common

问题 I'm using .net framework 4.5 I get the following error Error CS0453 The type 'MyObject' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable' public async Task<Nullable<MyObject>> MyMethod(string myParamter) {} I also tried public async Task<MyObject?> MyMethod(string myParamter) {} If I set the thing as nullable then why do I get the red line below the method name with this error message The stackoverflow answer is easy, make the

问题 I've been using AutoMapper and would like to take generic conversion one step further; instead of saying something like cfg.CreateMap<Container<int>, int>() .ConvertUsing(new ContainerConverter<Container<int>, int>()); I would rather set the AutoMapper to know how to map any Container, such as: cfg.CreateMap<Container<T>, T>() .ConvertUsing(new ContainerConverter<Container<T>, T>()); Since all conversions from Container to T are the same, it would be pointless to re-define this conversion for

问题 I am here today (like yesterday) with another weird interface question. I have a class: public class InputDevice<T> where T : Button { protected List<T> buttons = new List<T>(); protected InputDevice() { //Only allow instanciation from within a derived class } } As you can see, this class cannot be instantiated. A class that derives from it might be able to be instantiated. This is a subclass: public sealed class Keyboard : InputDevice<KeyboardButton> { public Keyboard() { buttons.Add(new

问题 I wonder if it's possible to use generics in the named queries in spring data (using jpa myself), is it possible to do anything like this? @NoRepositoryBean public interface EnumerationRepository<T extends Enumeration> extends JpaRepository<T,Integer> { @Query("Select t.type from T t") public List<String> selectTypes(); } Enumeration class being this @MappedSuperclass public abstract class Enumeration { @Id @GeneratedValue(strategy = GenerationType.AUTO) @Column(name = "id", length = 3)

问题 It is possible to create an array at compile time like; int[] myValues = new int[] { 1, 2, 3 } ; But I would like to do something like this; List<int> myValues = new List<int>() { 1, 2, 3 }; The compiler says No. Is there a way to do this (C# 2.0) without using LINQ (C# 3.0)? 回答1: List<int> myValues = new List<int>(new int[] { 1, 2, 3 } ); This will create an intermediate array however so there may be a more efficient way of doing the same thing. EDIT: John Feminella suggested creating a

问题 I have a class Offer which contains a filed Category. I want all Offers of a specific category to appear on top, followed by all else. I tried this, but to no avail, what would you recommend? Offers = Offers.OrderBy(x => x.Category == "Corporate").ToList(); 回答1: When you order by a boolean value false (0) comes before true (1). To get the elements that match the predicate first you should reverse the sort order by using OrderByDescending : Offers = Offers.OrderByDescending(x => x.Category ==