CODEWITHSUNDEEP
c#genericstype-inference
Matěj Pokorný
Matěj Pokorný

Reputation: 17835

The type arguments for method … cannot be inferred from the usage. Try specifying the type arguments explicitly

I'm not sure, why calling Map2 gives me

The type arguments for method 'Program.Map2(object, Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

Here is the code...

void Test()
{
    var test1 = Map1(1, MapIntToString);
    var test2 = Map2(1, MapIntToString);
}

To Map1<From, To>(From value, Func<From, To> mapFunc) => mapFunc(value);
To Map2<From, To>(object value, Func<From, To> mapFunc) => mapFunc((From)value);

string MapIntToString(int value) => Convert.ToString(value);

This is very simplified example. I need to convert some lists of DTOs to Models (and back), but it should be same case...

Upvotes: 1

Views: 12102

Answers (2)

Ehsan Sajjad
Ehsan Sajjad

Reputation: 62488

Because you have defined the parameter of type object while the method MapIntToString has first parameter of type int. So the compiler is not able to figure out that the parameter passed to mapFunc for Map2 i.e object value is currently holding value of type int. Your code will be translated to something like below if we visualize it when will be resolved at run-time but at first place it is not compiling as it's not able to resolve the generic type From:

Map2<Object, String>(object value, Func<object, String> mapFunc) => mapFunc((object)value);

So, obviously this wouldn't work as your method expects parameter of type int not object.

You need to be explicit about the type parameters in this case as Compiler is not smart enough to know that the object value is currently holding value of type int in it.

Upvotes: 2

nvoigt
nvoigt

Reputation: 77285

Being explicit works:

var test2 = Map2<int, string>(1, MapIntToString);

or

var test2 = Map2(1, (Func<int, string>)MapIntToString);

I'm afraid I cannot point to the reason why it does not work implicitly. My personal guess would be that MapIntToString is not a single method, but a Method Group (with one member) that can be converted to a Func<int, string> without problems, but that conversion is not used in resolving generics.

Upvotes: 0

Related Questions