LINQのGroupByメソッドについて - Qiita

// 本記事で使うpurchaseList var purchaseList = new List<Purchase> { new("PC", 300000, "id_electronics"), new("乾電池", 600, "id_electronics"), new("靴下", 800, "id_clothing"), new("少年漫画J", 500, "id_books"), new("コーラ", 150, "id_groceries"), new("青年漫画YJ", 600, "id_books"), new("雑誌", 1200, "id_books"), new("技術書", 5000, "id_books"), new("牛乳", 180, "id_groceries"), new("Tシャツ", 2500, "id_clothing"), }; // GroupByメ