问题
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.
回答1:
DelegatedCall expects a delegate that takes any object as an argument. But your function foo1 that you are passing to DelegatedCall can only cope with a string argument. 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 DelegatedCall generic:
DelegatedCall<T>(Action<T> action)
...or have it take any delegate:
DelegatedCall(Delegate action)
But then implementing it is ugly and requires reflection. It also doesn't verify that the function has only one parameter at compile-time.
回答2:
Variance doesn't work that way around; you would need
DelegatedCall(obj => foo1((string)obj));
As even in 4.0 it won't believe that every object is a string.
Note that if it was foo1(object) and Action<string> (i.e. the other way around) it probably would work (in 4.0), since every string is an object.
来源：https://stackoverflow.com/questions/4704734/cannot-convert-from-method-group-to-system-actionobject-error