c# - Lazy 如何解决需要 new() 约束的问题？

Question

示例 1(不编译):

void Main()
{
    var c = new C<D>();
    c.M.F();
}

class C<T>
{
    T _m = null;
    public T M { get {
        if(_m == null) _m = new T();
        return _m; 
    } }
}

class D
{
    public void F() { Console.WriteLine ("i was created"); }
}

结果:

Cannot create an instance of the variable type 'T' because it does not have the new() constraint

示例 2(有效):

void Main()
{
    var c = new C<D>();
    c.M.F();
}

class C<T>
{
    Lazy<T> _m = new Lazy<T>();
    public T M { get { return _m.Value; } }
}

class D
{
    public void F() { Console.WriteLine ("i was created"); }
}

结果:

i was created

Answer 1

如果深入研究源代码，您会看到 Lazy<T>最终使用 Activator :

return new Lazy<T>.Boxed((T)Activator.CreateInstance(typeof(T)));

这只是使用反射的捷径。由于它不是通过实际的泛型类型参数 ( new T() ) 实例化类型，而是通过反射调用构造函数，所以没有 where T : new()需要约束。