c# – 有没有办法指定where T：new()限制但是内部构造函数？

If the constraint is the constructor constraint new(),the type A must not be abstract and must have a public parameterless constructor. This is satisfied if one of the following is true.

• A is a value type,since all value types have a public default constructor
• A is a type parameter having the cosntructor constraint
• A is a type parameter having the value type constraint
• A is a class that is not abstract and contains an explicitly declared public constuctor with no parameters
• A is not abstract and has a default constructor.

如果不满足任何一个条件,则编译器错误.如果您发现自己的类型是公共的但只有内部构造函数,那么它们很可能实际上应该是具有公共构造函数的内部类型.

我建议将内部及其构造函数的类型访问器更改为public,并使其无参数.然后,您的公共无参数构造函数可以调用非参数的私有或内部构造函数来执行任何其他初始化工作.

internal class C<T> where : T new()
{
    public C() : this(new T()) {
    }

    private C(T t) {
        // Do additional initialization
    }
}

请注意,模式是有限的,但没有什么能阻止您使用私有方法.

internal class C<T> where T : new() {
    public C() {
        T t = new T();
        InitializeClass(t);
    }

    private void InitializeClass(T t) {
        throw new NotImplementedException();
    }
}

根据您的更新,这是一个公共单例模式的小例子.

public class Singleton<T> where T : new()
{
    public static Singleton<T> Current {
        get;
        private set;
    }

    internal Singleton() : this(new T()) {
    }

    private Singleton(T t) {
        Current = this;
        // Do whatever you need to with T
    }        

    public String Name {
        get;
        set;
    }
}

用法

// Somewhere in your internal assembly
Singleton<String> singleton = new Singleton<String>();

// In an external assembly
Singleton.Current.Name = "SoMoS";

你甚至不需要以这种方式使用构造函数,你可以轻松地做一些简单的事情.

public class Singleton<T> where T : new()
{
    public static Singleton<T> Current {
        get;
        private set;
    }

    internal Singleton() {
        T t = new T();
        // Do stuff with T
    }

    public String Name {
        get;
        set;
    }
}

如果你不能设计它以满足你的要求,泛型可能不是你要走的路.泛型只能做很多事情并且不能解决所有问题.有工厂模式,注塑等等.