带有接口和 class 的类型参数的多重约束

Question

I have void Add<TEntity>(TEntity entity) where TEntity: class, ISpecificEntity; I just want use class of specific interface.

what the point of that? I can use void Add<TEntity>(TEntity entity) where TEntity: ISpecificEntity;

But I do not want to implement structure. I just want use class of some interface.

Yes. I can remove class. But how not to implement structure?

Answer 1

What class, in practice achieves in your example is it makes it harder (but not impossible) to pass in value types.

An example is below. With the class constraint, the (ISpecificEntity) needs to be there. Without the class constraint the (ISpecificEntity) does not need to be there.

It is a common misconception that class does not allow struct s to be passed in. This is not true. It basically means "this type must be a class or an interface ".

If you really must disallow value types, you will need to check the value of entity.GetType().IsValueType inside the method (ie at runtime).

using System;
using System.Threading.Tasks;

namespace ConsoleApp4
{
    public class Bob
    {
        public void Add<TEntity>(TEntity entity) where TEntity : class, ISpecificEntity
        {
            Console.WriteLine(entity.GetType().IsValueType);
            Console.WriteLine("added");
        }
    }

    public interface ISpecificEntity
    {

    }

    public struct SpecificEntity : ISpecificEntity
    {

    }

    class Program
    {
        static async Task Main(string[] args)
        {
            var entity = new SpecificEntity();

            var bob = new Bob();

            bob.Add((ISpecificEntity)entity);
        }
    }
}