C# Error CS1654 – Cannot modify members of ‘{0}’ because it is a ‘{1}’

C# Error

CS1654 – Cannot modify members of ‘{0}’ because it is a ‘{1}’

Reason for the Error & Solution

Cannot modify members of ‘variable’ because it is a ‘read-only variable type’

This error occurs when you try to modify members of a variable which is read-only because it is in a special construct.

One common area that this occurs is within loops. It is a compile-time error to modify the value of the collection elements. Therefore, you cannot make any modifications to elements that are , including . In a collection whose elements are , you can modify accessible members of each element, but any try to add or remove or replace complete elements will generate .


The following example generates error CS1654 because Book is a struct. To fix the error, change the struct to a .

using System.Collections.Generic;  
using System.Text;  
namespace CS1654  
    struct Book  
        public string Title;  
        public string Author;  
        public double Price;  
        public Book(string t, string a, double p)  
    class Program  
        List<Book> list;  
        static void Main(string[] args)  
             //Use a collection initializer to initialize the list  
            Program prog = new Program();  
            prog.list = new List<Book>();  
            prog.list.Add(new Book ("The C# Programming Language",  
                                    "Hejlsberg, Wiltamuth, Golde",  
            prog.list.Add(new Book ("The C++ Programming Language",  
            prog.list.Add(new Book ("The C Programming Language",  
                                    "Kernighan, Ritchie",  
            foreach(Book b in prog.list)  
                //Compile error if Book is a struct  
                //Make Book a class to modify its members  
                b.Price +=9.95; // CS1654