You can assign a method to a delegate with matching type args:
Func<string, DateTime> f = DateTime.Parse;
You can assign a lambda to a delegate with covariant type args:
Func<string, object> f = s => DateTime.Parse(s);
But you can't assign a method to a delegate with covariant type args:
Func<string, object> f = DateTime.Parse; //ERROR: has the wrong return type
Why not?
Variance does not work with value types, since they need to be JITed differently.
Your lambda expression variant does not involve variance; instead, it compiles to a lambda expression with an implicit boxing conversion from DateTime
to object
.
If you use a method that returns a reference type, it work fine:
Func<string, object> f = string.Intern;
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