Question

I have the following extension method which asserts that a property (Id) contains a specified attribute (TV):

public static void ShouldHave<T, TV, TT>(this T obj, Expression<Func<T, TT>> exp) {...}

The method can be called like this:

MyDto myDto = new MyDto();
myDto.ShouldHave<MyDto, RequiredAttribute, int>(x => x.Id);

Compiles just fine. I was wondering if it is possible to remove T and TT from the method signature. T because ShouldHave is called on T why it shouldn’t be necessary to specify it explicitly. And TT is the type of the property referenced in the expression (x.Id).

Answer 1

Automatic inference of type arguments only works if no generic arguments are specified in the method call. I.e., this:

myDto.ShouldHave<, RequiredAttribute, >(x => x.Id);

is not valid syntax. You can either have “all or nothing”.

Thus, if you want to infer T and TT, you need to pass the information currently contained in TV in some other way. For example, one option would be to pass the type of the attribute as a parameter:

public static void ShouldHave<T, TT>(this T obj, 
                                     Expression<Func<T, TT>> exp, 
                                     Type attribute) {...}

(Obviously, this will require changes in your implementation of ShouldHave).

Then you should be able to call the method like this:

MyDto myDto = new MyDto();
myDto.ShouldHave(x => x.Id, typeof(RequiredAttribute));