如何在 class 声明之外声明泛型类型

Question

I am using this generic type declaration with success throughout my testing application

public class TestController<TSchool>
    where TSchool: IHasStudents, IHasTeachers, IHasAdmin, new()

Beacuse I will create multiple controllers like TestController<University> or TestController<PrimarySchool> and these work great.

However, it gets very repetitive to retype the <TSchool> where TSchool: IHasStudents, IHasTeachers, IHasAdmin, new() in every new class that I need to use TSchool

I have tried:

1. public interface TSchool: IHasStudents, IHasTeachers, IHasAdmin, new()

But the compiler does not like the new() syntax

2. public abstract class TSchool: IHasStudents, IHasTeachers, IHasAdmin, new()

Also to no avail.

Is there a way I can declare the generic type outside of every class declaration?

Answer 1

A generic type is neither an interface nor an abstract class, it's a placeholder with no meaning other than the name its given. It's why you need to tell C# what exactly that placeholder implements to be able to use its functions, at least those exposed through interfaces and/or special things like new() .

You have plenty of other choices, for example inheritance. Instead of accepting generic types, require a base type instead. object is always available (though if you go to that level, laziness has really won over your project). dynamic is also a choice. And so on.

Though I do wonder why you need to be able to instantiate so many of your types that this becomes an issue in the first place. Perhaps dependency injection is the answer to your entire problem in the first place.

Answer 2

One thing you could do is use nested classes

public class TestController<TSchool> where TSchool: IHasStudents, IHasTeachers, IHasAdmin, new()
{
    //don't need to re-declare TSchool, since TSchool is accessable in the nested classes
    public class TestA
    {

    }

    public class TestB
    {

    }
}

With this you will have to think about a way to initialize the nested classes here are some ideas.

using System;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            //initialize controller
            var testController = new TestController<University>();

            //initialize Tests and pass the reference of the controller
            var testA_1 = new TestController<University>.TestA(testController);
            var testB_1 = new TestController<University>.TestB(testController);

            //initialize the Tests without a reference of the controller
            var testA_2 = new TestController<University>.TestA();
            var testB_2 = new TestController<University>.TestB();

            //use create methods, and maybe additionally hide the public constructors
            var testA_3 = testController.CreateTestA();
            var testB_3 = testController.CreateTestB();

            //use a static TestController class, and call the public constructors
            var testA_4 = new StaticTestController<University>.TestA();
            var testB_4 = new StaticTestController<University>.TestB();

            //use a static TestController class but hide the public constructors and use create methods 
            var testA_5 = StaticTestController<University>.CreateTestA();
            var testB_5 = StaticTestController<University>.CreateTestB();
        }
    }
}

public class TestController<TSchool> where TSchool : IHasStudents, IHasTeachers, IHasAdmin, new()
{
    private TSchool _school;

    public TestController()
    {
        _school = new TSchool();
    }
    public class TestA
    {
        public TestA()
        {

        }
        public TestA(TestController<TSchool> controller)
        {
            var school = new TSchool();
            var schoolController = controller._school;
        }
    }
    public class TestB
    {
        public TestB()
        {

        }
        public TestB(TestController<TSchool> controller)
        {
            var school = new TSchool();
            var schoolController = controller._school;
        }
    }
    public TestA CreateTestA()
    {
        return new TestA(this);
    }
    public TestB CreateTestB()
    {
        return new TestB(this);
    }
}
public static class StaticTestController<TSchool> where TSchool : IHasStudents, IHasTeachers, IHasAdmin, new()
{
    private static TSchool _school;

    static StaticTestController()
    {
        _school = new TSchool();
    }

    public class TestA
    {
        public TestA()
        {
            var school = new TSchool();
            var schoolController = _school;
        }
    }

    public class TestB
    {
        public TestB()
        {
            var school = new TSchool();
            var schoolController = _school;
        }
    }

    public static TestA CreateTestA()
    {
        return new TestA();
    }
    public static TestB CreateTestB()
    {
        return new TestB();
    }
}
public class School : IHasAdmin, IHasTeachers, IHasStudents
{

}
public class University : IHasAdmin, IHasTeachers, IHasStudents
{

}
public interface IHasStudents
{

}
public interface IHasTeachers
{

}
public interface IHasAdmin
{

}