声明泛型类时 new() 的目的是什么？

Question

What is the purpose of new() while declaration of BaseEntityCollection class?
If I'm going to remove it, I got an error with the following message "T must be a non-abstract type with a public parameterless constructor in order to use it as parameter ..."

public abstract partial class BaseEntityCollection<T> : 
       List<T> where T : BaseEntity, new()

Answer 1

It means that whatever class you specify for T , it has a default (no parameters) constructor.

Therefore, in the generic class, you can do new T() and it will create a new object of type T.

Answer 2

Writing new() forces the parameter to have a default constructor.

Without it, you can't write new T() .
Your error happens when you try to pass a non- new() type as a new() 'd parameter.

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Also, do not inherit List<T> .
Instead, you should inherit Collection<T> , which is designed for inheritance.

Answer 3

The type T has to have a parameterless constructor. This enables you to create new instances by doing var t = new T() which would be impossible otherwise.

Answer 4

它是通用约束的符号：必须具有（公共）无参数构造函数。

Answer 5

This means that your generic type has to have parameterless constructor.

BaseEntityCollection<T> : List<T>

I am not sure what are you doing here, but I think it is against Liskov's rule. Check your hierarchy.

Constraints on Type Parameters

Answer 6

That is one of the possible "generic type constraints" you can associate with your generic type. Using the constraint "new()" will only allow you to use a generic type if it has a parameterless constructor. This can be useful for things like serialization, or factory-type methods, where you need to create an object of type T.

Here are some other generic type constraints: http://msdn.microsoft.com/en-us/library/d5x73970(v=vs.80).aspx

Answer 7

It's a generic constraint. In this case, it's a the new Constraint .