问题
I have the following code:
static Func<object, string> s_objToString = (x) => x.ToString();
static Func<string, string> s_stringToString = s_objToString; //compiles
static Func<int, string> s_intToString = s_objToString; //error
The second line compiles but the third line fails to compile with error:
Cannot implicitly convert type '
System.Func<object,string>' to 'System.Func<int,string>'
Why is that?
I understand that with genetics although string is derived from object a List<string> does not derive from List<object>, but here object to string works and object to int fails, why?
OK let's say I understood why; the question now is there a way around it (other then defining MyInt class to box int because Func<object,string> to Func<MyInt,string> works)?
回答1:
It is because Func is defined as Func<in T, out TResult>, MSDN is here, so T is contra-variant with in keyword, that is, you can use either the type you specified or any type that is less derived, but remember that co-variance and contra-variance do not support for value type:
Why covariance and contravariance do not support value type
So, it works for string but does not work out with int. You might need to read more about covariance and contravariance:
http://msdn.microsoft.com/en-us/library/dd233060.aspx
http://msdn.microsoft.com/en-us/library/dd799517.aspx
回答2:
Because co/contra-variance doesn't work for value types.
Please take a look here
Variance is supported only if a type parameter is a reference type. Variance is not supported for value types.
The following doesn’t compile either:
// int is a value type, so the code doesn't compile.
IEnumerable<Object> objects = new List<int>(); // Compiler error here.
来源：https://stackoverflow.com/questions/13025176/conversion-from-funcobject-string-to-funcstring-string-works-but-to-funcint