从Func <object，string>到Func <string，string>的转换工作但到Func <int，string>失败

Question

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)?

Answer 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

Answer 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.