I wrote an extension method for IQueryable that returns the same type of IQueryable, just a little filtered. So to speak:
public static IEnumerable<T> Foo<T>(this IEnumerable<T> source, int? howmany = null) { if (howmany.HasValue) return source.Take(howmany.Value); return source; }
calling the above method is as simple as someLinqResult.Foo(2)
I need another method that will return an instance of another general class, but with a different base class (not the same as the original T above). So, here is the code, suppose that this method should return a list of the given type (except for the input type that has IQueryable!), But the same length [the actual problem is converting the results of the NHibernate query, but this is not matter):
public static List<TTarget> Bar<TTarget,TSource>(this IEnumerable<TSource> source) { return new List<TTarget>(source.Count()); }
now, given that strLinqResult is an IQueryable<string> , I need to call it strLinqResult.Bar<int,string>(); .
Point I need to pass both types, even if the first is already known , as I call the method on an already defined IQuerable .
Since this was enough to call Foo(2) and not Foo<string>(2) , I thought that the compiler could automatically "pass / guess" the type.
So why do I need to call the second method Bar<int,string>(); , not just Bar<int>() ?
actual code:
public static ListResponse<TResponse> BuildListResponse<T,TResponse>(this IQueryable<T> iq, ListRequest request) where TResponse: new() { var result = iq.ApplyListRequestParams(request).ToList().ConvertAll(x => x.TranslateTo<TResponse>()); var tcount = iq.Count(); return new ListResponse<TResponse> { Items = result, _TotalCount = tcount, _PageNumber = request._PageNumber ?? 1, }; }
ApplyListRequestParams is a kind of Foo method from the sample code - it just applies the pagination and ordering options available in the ListRequest object.
Items is a public List<T> Items in class ListResponse<T> .
TranslateTo is a method from ServiceStack.
The aforementioned method, called IQueryable<T> , returned by NHibernate (T is the domain model), accepts query parameters (ordering, pagination), applies them, and then converts the list of results from DomainModel to a DTO object of type TResponse . Then this list is wrapped in a common response class (general, so it is reused for many types of DTO)