Неявный вызов ToString() - C#

namespace пример_2ого
{
    class Material
    {
        private string name {get;set;}
        private double d { get; set; }
 
        public Material() { }
 
        public Material(string newName, double newV)
        {
            d = newV;
            name = newName;
        }
 
        public double D() {return d; }
        public string Name() { return name; }
 
        public override string ToString()
        {
            return string.Format("{0};{1}", name, d);
        }
    }
}

namespace пример_2ого
{
    class Article
    {
        private Material material;
        private double vol { get; set; }
        private string name { get; set; }
 
        public Article(string articleName, double articleVolume, Material articleMaterial)
        {
            Mat = articleMaterial;
            name = articleName;
            vol = articleVolume;
        }
 
        public double Vol()
        { return vol; }
 
        public string Name()
        {return name;}
 
        public Material Mat
        {
            get
            {
                return material;
            }
            set
            {
                material = value;
            }
        }
        public double GetMass()
        {
            return material.D() * vol;
        }
        public override string ToString()
        {
            return string.Format("{0};{1};{2};{3}", name, Mat.ToString(), vol, GetMass());
        }
 
    }
}

using System;
 
namespace пример_2ого
{

    class Program
    {
        static void Main(string[] args)
        {
            Article art = new Article("Wire", 0.03, new Material("Steel", 7850));
            Console.WriteLine(art.ToString());
            Material cu = new Material("Cuprum", 8500);
            art.Mat = cu;
            Console.WriteLine(art.ToString());
            Console.ReadKey();
        }
    }
}