如何根据名称从System.Nullable获取默认类型？

using System;

public class Program
{
    public static void Main()
    {
        string typeName = "System.Nullable`1";
        Type nullableType = Type.GetType(typeName);

        if (nullableType != null && nullableType.IsGenericType)
        {
            Type[] genericArguments = nullableType.GetGenericArguments();
            if (genericArguments.Length == 1)
            {
                Type underlyingType = genericArguments[0];
                object defaultValue = Activator.CreateInstance(nullableType);
                Console.WriteLine("默认类型为：" + underlyingType.FullName);
                Console.WriteLine("默认值为：" + defaultValue);
            }
        }
    }
}