Generic method with property expression “type arguments cannot be inferred from useage”

Question

I hope I'm not having a meltdown here - will happily delete this question if I am... but given the following method:

IList<T> ConvertSomething<T, TProperty>(IList<T> original, Expression<Func<T, TProperty>> valuePropertyExpression, Expression<Func<T, TProperty>> namePropertyExpression);

..I can call it like this:

var data = new List<MyType>(); // would be real data of that type
var output = _factory.ConvertSomething<MyType, object>(data, x => x.Id, x => x.Name);

..but not like this:

var output = _factory.ConvertSomething(data, x => x.Id, x => x.Name);

The second version causes the compiler to complain, with the "cannot be inferred from useage" error.

So my questions are:

1. is what I am doing possible when only the method can know about T
2. if not, is there another approach (I need to know the two requested PropertyInfo objects to do my 'conversion' and don't want the user to have to do it by string).

Answer 1

That is because the type of Id and Name are different. since object is compatible with both of them you are able to call it specifying object. You can add a third generic argument like this:

IList<T> ConvertSomething<T, TValue, TName>(
     IList<T> original, 
     Expression<Func<T, TValue>> valuePropertyExpression, 
     Expression<Func<T, TName>> namePropertyExpression);

Then the types should be inferred correctly.