C#•2y ago

Declare method using T and where [Answered]

This is some code from a reddit post i saw and there are some thing i dont understand. This is the method:

public static T CreateStudentFromEntity<T>(Student entity) where T : IStudent
        {
            var student = Activator.CreateInstance<T>();

            student.Id = entity.Id;
            student.FirstName = entity.Name;
            student.Surname = entity.Surname;
            student.DateOfBirth = DateTime.Parse(entity.DateOfBirth);
            student.Sex = (Sex)entity.Sex;
            student.Address = CreateAddressFromEntity(entity.Address);

            return student;
        }

public static T CreateStudentFromEntity<T>(Student entity) where T : IStudent
        {
            var student = Activator.CreateInstance<T>();

            student.Id = entity.Id;
            student.FirstName = entity.Name;
            student.Surname = entity.Surname;
            student.DateOfBirth = DateTime.Parse(entity.DateOfBirth);
            student.Sex = (Sex)entity.Sex;
            student.Address = CreateAddressFromEntity(entity.Address);

            return student;
        }

This is the interface that the method uses:

public interface IStudent
{
    long Id { get; set; }
    string? FirstName { get; set; }
    string? Surname { get; set; }
    DateTime DateOfBirth { get; set; }
    Sex Sex { get; set; }
    string? TelephoneNumber { get; set; }
    Address? Address { get; set; }
}

public interface IStudent
{
    long Id { get; set; }
    string? FirstName { get; set; }
    string? Surname { get; set; }
    DateTime DateOfBirth { get; set; }
    Sex Sex { get; set; }
    string? TelephoneNumber { get; set; }
    Address? Address { get; set; }
}

From what i understand, T is used to be able to make the method more "flixble", not restricting it to a specific return type. But when adding "where T : IStudent" - it restricts it to only return a object that inherits the IStudent interface? Could you not just replace T with IStudent and skip the where? Plase let me know if i got in wrong 🙂 Thanks in advance!

15 Replies

Kouhai•2y ago

using a constraint like this is 1 - faster 2 - allows you to have concrete return type Let's say you have an implementation of IStudent like this

public class InternationalStudent : IStudent
{
    long Id { get; set; }
    string? FirstName { get; set; }
    string? Surname { get; set; }
    DateTime DateOfBirth { get; set; }
    string PlaceOfBirth {get; set;}
    Sex Sex { get; set; }
    string? TelephoneNumber { get; set; }
    Address? Address { get; set; }
}

public class InternationalStudent : IStudent
{
    long Id { get; set; }
    string? FirstName { get; set; }
    string? Surname { get; set; }
    DateTime DateOfBirth { get; set; }
    string PlaceOfBirth {get; set;}
    Sex Sex { get; set; }
    string? TelephoneNumber { get; set; }
    Address? Address { get; set; }
}

the returned value from CreateStudentFromEntity<InternationalStudent>(entity) would be of InternationalStudent and you can access PlaceOfBirth without casting

Pobiega•2y ago

Very well explained. This is because of how generics work behind the scenes - the compiler will look at the different types you pass in and actually make a "copy" of the method that works with that specific type.

Unknown User•2y ago

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big OOF•2y ago

Okey, thank you for the answer! Just to get my head around it - Could you say that the use of T in this case is to make a method avalible using multiple classes? And the where to restrict it to specific classes?

Kouhai•2y ago

Yes, where here is used to restrict T to IStudent implementers. In the first place, you can't instantiate an interface, so you have to use a class. If you take a look at the code ToBeClone provided, they added another constraint new() this ensures that the caller has passed a class/struct implementing IStudent

Unknown User•2y ago

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big OOF•2y ago

Okok! Antother question regarding new(), i would like to say that:

where T : IStudent, new()

where T : IStudent, new()

requires a class that implements IStudent interface and new() interface - but new() isnt an interface. Is it instead - it require a the caller to pass an object that can be instantiated? 🙂 Sorry if im asking the same questions over and over, a bit new to this :p

Unknown User•2y ago

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big OOF•2y ago

Okey! So new() set the restriction to only accept an object that includes a parameterless contructor? 🙂

Kouhai•2y ago

Yes, it accepts a type that has a parameterless constructor

Unknown User•2y ago

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Kouhai•2y ago

they didn't say redirections, they said restriction 😅

Unknown User•2y ago

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big OOF•2y ago

Haha no worries! Thank you guys for the great help, really appriciated 🙂

Accord•2y ago

✅ This post has been marked as answered!

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Declare method using T and where [Answered]