无法从“方法组”转换；到“System.Action<对象>”错误
我创建了以下函数：
public void DelegatedCall(Action<Object> delegatedMethod)
并定义了以下方法
public void foo1(String str) { }
但是，当我尝试使用 foo1 调用 DelegateCall 时：
DelegatedCall(foo1);
...我收到以下编译器错误：
参数 1：无法从“方法组”转换为“System.Action
此错误的原因是什么以及如何更正它？不幸的是，不能将 foo1 转换为 Action。
I have created the following function:
public void DelegatedCall(Action<Object> delegatedMethod)
And defined the following method
public void foo1(String str) { }
However, when I try to call DelegateCall with foo1:
DelegatedCall(foo1);
...I get the following compiler error:
Argument 1: cannot convert from 'method group' to 'System.Action<object>'
What is the reason for this error and how can I correct it? Unfortunately, casting foo1 to Action is not an option.
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DeleatedCall需要一个接受任何对象作为参数的委托。但是您传递给DelegateCall的函数foo1只能处理string参数。因此，转换不是类型安全的，因此是不可能的。输入参数是逆变，但您的代码需要协方差。 （请参阅协方差与反方差之间的差异。）
您可以使
DeleatedCall 通用：...或者让它接受任何委托：
但是实现它很丑陋并且需要反射。它还不会在编译时验证该函数是否只有一个参数。
DelegatedCallexpects a delegate that takes anyobjectas an argument. But your functionfoo1that you are passing toDelegatedCallcan only cope with astringargument. So, the conversion isn't type-safe and thus is not possible.Input parameters are contra-variant, but your code needs covariance. (See Difference between Covariance & Contra-variance.)
You can make
DelegatedCallgeneric:...or have it take any delegate:
But then implementing it is ugly and requires reflection. It also doesn't verify that the function has only one parameter at compile-time.
方差并非如此。 即使在 4.0 中，它也不会
相信每个对象都是字符串。
请注意，如果它是
foo1(object)和Action（即相反），它可能会起作用（在 4.0 中），因为每个字符串 是一个对象。Variance doesn't work that way around; you would need
As even in 4.0 it won't believe that every object is a string.
Note that if it was
foo1(object)andAction<string>(i.e. the other way around) it probably would work (in 4.0), since every string is an object.