Genericsの制約

cspractice.cs

    class Program
    {
        static void Main(string[] args)
        {

            var foo = SingletonClass<List<string>>.GetInstance();
            foo.Add("bbb");
            foo.Add("aaa");

            var anotherFoo = SingletonClass<List<string>>.GetInstance();
            Console.WriteLine(String.Join(",", anotherFoo));

            var bar = SingletonClass<List<int>>.GetInstance();
            Console.WriteLine(bar.Count);

            var baz = SingletonClass<FooClass>.GetInstance(); // Error

        }
    }


    public class FooClass
    {

        public string Foo { get; set; }
        public FooClass(string foo)
        {
            this.Foo = foo;
        }
    }

    public class SingletonClass<T>
        where T : new()
    {

        private static T instance = default(T);

        public static T GetInstance()
        {
            if (instance == null)
            {
                instance = new T();
            }

            return instance;
        }

        private SingletonClass()
        {
            // Initialize here.
        }
    }