你把协变和逆变弄错了.让我们考虑 Action<object> 和 Action<string>.删除实际的泛型，您正在尝试执行以下操作:

You've got the covariance and contravariance the wrong way round. Let's consider Action<object> and Action<string>. Removing the actual generics, you're trying to do something like this:

private Action<object> _foo;

public void Foo(Action<string> bar)
{
    // This won't compile...
    _foo = bar;
}

现在假设我们接着写:

_foo(new Button());

这很好，因为 Action<object> 可以被传递 any 对象...但是我们已经用一个 必须 的委托对其进行了初始化> 接受一个字符串参数.哎哟.

That's fine, because Action<object> can be passed any object... but we've initialized it with a delegate which must take a string argument. Ouch.

这不是类型安全的，因此无法编译.

This isn't type safe, so doesn't compile.

不过， 的另一种方式也可以:

The other way would work though:

private Action<string> _foo;

public void Foo(Action<object> bar)
{
    // This is fine...
    _foo = bar;
}

现在当我们调用 _foo 时，我们必须 传入一个字符串 - 但这很好，因为我们已经用一个可以接受任何 object 引用作为参数，所以我们可以给它一个字符串.

Now when we invoke _foo, we have to pass in a string - but that's fine, because we've initialized it with a delegate which can take any object reference as a parameter, so it's fine that we happen to be giving it a string.

所以基本上 Action<T> 是 逆变 - 而 Func 是 协变:

So basically Action<T> is contravariant - whereas Func<T> is covariant:

Func<string> bar = ...;
Func<object> foo = bar; // This is fine
object x = foo(); // This is guaranteed to be okay

不清楚你想用这个动作做什么，所以很遗憾，我无法就如何解决这个问题提供任何建议......

It's not clear what you're trying to do with the action, so unfortunately I can't really give any advice on how to get around this...