为什么可以为空<T> ,struct, 允许 &#39;null&#39;

Question

Nullable is an struct. And i know structs cant be assigned 'null'. So how could we assign Nullable's object a null? What is the reason?

Answer 1

It doesn't actually accept a value of null ; it simply has syntactic sugar that allows it to act like it's null . The type actually looks a bit like this:

struct Nullable<T>
{
    private bool hasValue;
    private T value;
}

So when it's initialized hasValue == false , and value == default(T) .

When you write code like

int? i = null;

The C# compiler is actually doing this behind the scenes:

Nullable<T> i = new Nullable<T>();

And similar checks are made when casting null to an int? at run-time as well.

Further Reading

• Why can assign “null” to nullable types