using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var list1 = new List<string>();

        var list2 = new List<double>();

        var list3 = new List<MyCustomType>();

        var var1 = list1.MakeDefault();

        var var2 = list2.MakeDefault();

        var var3 = list3.MakeDefault();

        Console.WriteLine($"{var1}");

        Console.WriteLine($"{var2}");

        Console.WriteLine($"{var3}");

    }

}

​

public static class GenericListExtensions

{

    public static Tuple<T, Type> MakeDefault<T>(this List<T> list)

    {

        return Tuple.Create<T, Type>(default(T), typeof(T));

    }

}

​

public class MyCustomType {}