为什么 C# Func<int> 不能分配给 Func<object> ？<div id="text_translate"><p> 我在 C# 研究了协变和逆变。下面我的代码有一个错误。</p><pre> object a = new object(); int b = 10; a = b; // not error Func<object> acO = () => new object(); Func<int> acI = () => 1; acO = acI; // error</pre><p> 错误信息：</p><blockquote><p> 无法将类型“System.Func”隐式转换为类型“System.Func”。</p></blockquote><p> 我认为如果 int -> object 是可能的，那么 Func -> Func 将是可能的。 但事实并非如此。</p><p> 我认为值类型在返回时将被复制以使用，这与引用类型（对象）不同，并且它可能导致意外操作（如异常）。 我的猜测正确吗？</p><p> 我期待着您明智的回答。 感谢您阅读。</p></div></object></int>

Question

I've studied Covariant and Contravariant in C#. and below my code has an error.

object a = new object();
int b = 10;

a = b; // not error


Func<object> acO = () => new object();
Func<int> acI = () => 1;

acO = acI; // error

Error Message:

Cannot implicitly convert type 'System.Func' to type 'System.Func'.

I thought that if int -> object is possible, Func -> Func will be possible. but it is not.

I think value type will be copied to use when it is returned unlike reference type(object) and it can cause unintentional operation(like exception). Is my guess correct?

I'm looking forward to your wise answers. Thank you for reading.

Answer 1

From the Variance in Delegates (C#) docs:

Variance for generic type parameters is supported for reference types only.

A niceblog post diving into the topic by Eric Lippert:

All the built-in reference conversions are identity-preserving. This is why covariant and contravariant conversions of interface and delegate types require that all varying type arguments be of reference types. To ensure that a variant reference conversion is always identity-preserving, all of the conversions involving type arguments must also be identity-preserving. The easiest way to ensure that all the non-trivial conversions on type arguments are identity-preserving is to restrict them to be reference conversions.