C# コンパイラエラー CS1656の原因と対策について解説

using System;
using System.Collections.Generic;
namespace CS1656Example
{
    class 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>();
            bookList.Add(new Book("The C# Programming Language", "Author A", 29.95));
            bookList.Add(new Book("The C++ Programming Language", "Author B", 29.95));
            bookList.Add(new Book("The C Programming Language", "Author C", 29.95));
            foreach (Book b in bookList)
            {
                // CS1656エラー発生箇所：反復変数bは読み取り専用のため再代入できません
                if (b.Title == "The C Programming Language")
                    b = new Book("Programming Windows, 5th Ed.", "Author D", 29.95); // エラーCS1656
            }
        }
    }
}

(コンパイル時に「CS1656: 'b' は 'foreach' の反復変数で読み取り専用のため、割り当てることはできません」というエラーメッセージが表示されます)

using System;
namespace CS1656Example
{
    class Resource : IDisposable
    {
        public void Dispose()
        {
            // リソース解放処理をここに記述
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            using (Resource c = new Resource())
            {
                // CS1656エラー発生: using変数は読み取り専用となるため再代入できません
                c = new Resource(); // エラーCS1656
            }
        }
    }
}

(コンパイル時に「CS1656: 'c' は 'using' 変数で読み取り専用のため、割り当てることはできません」というエラーメッセージが表示されます)

using System;
namespace CS1656Example
{
    class Program
    {
        unsafe static void Main(string[] args)
        {
            int[] numbers = new int[] { 1, 2, 3, 4 };
            fixed (int* p = numbers)
            {
                // CS1656エラー発生: fixed変数は固定されているため再代入できません
                p = null; // エラーCS1656
            }
        }
    }
}

(コンパイル時に「CS1656: 'p' は 'fixed' 変数で読み取り専用のため、割り当てることはできません」というエラーメッセージが表示されます)

using System;
using System.Collections.Generic;
namespace CS1656Example
{
    class 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>();
            bookList.Add(new Book("The C# Programming Language", "Author A", 29.95));
            bookList.Add(new Book("The C++ Programming Language", "Author B", 29.95));
            bookList.Add(new Book("The C Programming Language", "Author C", 29.95));
            // forループを使用して各要素にアクセスし、再代入を行う
            for (int i = 0; i < bookList.Count; i++)
            {
                if (bookList[i].Title == "The C Programming Language")
                {
                    bookList[i] = new Book("Programming Windows, 5th Ed.", "Author D", 29.95);
                }
            }
            foreach (Book b in bookList)
            {
                Console.WriteLine(b.Title);
            }
        }
    }
}

The C# Programming Language
The C++ Programming Language
Programming Windows, 5th Ed.

using System;
using System.Collections.Generic;
namespace CS1656Example
{
    class 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>();
            bookList.Add(new Book("The C# Programming Language", "Author A", 29.95));
            bookList.Add(new Book("The C++ Programming Language", "Author B", 29.95));
            bookList.Add(new Book("The C Programming Language", "Author C", 29.95));
            foreach (Book b in bookList)
            {
                // オブジェクト自体の再代入ではなく、メンバーの変更を行う
                if (b.Title == "The C Programming Language")
                {
                    b.Title = "Programming Windows, 5th Ed.";
                    b.Author = "Author D";
                    b.Price = 29.95;
                }
            }
            foreach (Book b in bookList)
            {
                Console.WriteLine(b.Title);
            }
        }
    }
}

The C# Programming Language
The C++ Programming Language
Programming Windows, 5th Ed.