【C#】CS1654エラーの原因と対処法：foreachループ内での読み取り専用struct変更問題を解説

using System;
using System.Collections.Generic;
namespace CS1654Sample
{
    // Bookをクラスに変更しているサンプル
    class Book
    {
        public string Title;
        public string Author;
        public double Price;
        public Book(string title, string author, double price)
        {
            // 変数名は英語表記、コメントは日本語で記述
            Title = title;   // 書籍のタイトル
            Author = author; // 著者名
            Price = price;   // 価格
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            List<Book> bookList = new List<Book>
            {
                new Book("The C# Programming Language", "Hejlsberg, Wiltamuth, Golde", 29.95),
                new Book("The C++ Programming Language", "Stroustrup", 29.95),
                new Book("The C Programming Language", "Kernighan, Ritchie", 29.95)
            };
            // foreachループ内でクラスのメンバー変更が可能な例
            foreach (Book book in bookList)
            {
                // Priceプロパティを直接変更する
                book.Price += 9.95;
            }
            // 結果を出力する
            foreach (Book book in bookList)
            {
                Console.WriteLine($"{book.Title}: {book.Price}");
            }
        }
    }
}

The C# Programming Language: 39.9
The C++ Programming Language: 39.9
The C Programming Language: 39.9

using System;
using System.Collections.Generic;
namespace CS1654Sample
{
    struct Book
    {
        public string Title;
        public string Author;
        public double Price;
        public Book(string title, string author, double price)
        {
            Title = title;
            Author = author;
            Price = price;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            List<Book> bookList = new List<Book>
            {
                new Book("The C# Programming Language", "Hejlsberg, Wiltamuth, Golde", 29.95),
                new Book("The C++ Programming Language", "Stroustrup", 29.95),
                new Book("The C Programming Language", "Kernighan, Ritchie", 29.95)
            };
            // forループを使用し、リストの要素を直接変更する方法
            for (int i = 0; i < bookList.Count; i++)
            {
                Book tempBook = bookList[i];
                tempBook.Price += 9.95;
                bookList[i] = tempBook; // 変更した要素を再代入する必要があります
            }
            // 結果の出力
            foreach (Book book in bookList)
            {
                Console.WriteLine($"{book.Title}: {book.Price}");
            }
        }
    }
}

The C# Programming Language: 39.9
The C++ Programming Language: 39.9
The C Programming Language: 39.9