# Compiler Error CS1656 无法给“variable”赋值，因为它是“read-only variable type” 当变量赋值发生在只读上下文中时，将出现此错误。只读上下文包括 [foreach](https://msdn.microsoft.com/zh-cn/library/ttw7t8t6.aspx) 迭代变量、[using](https://msdn.microsoft.com/zh-cn/library/yh598w02.aspx) 变量和 [fixed](https://msdn.microsoft.com/zh-cn/library/f58wzh21.aspx) 变量。若要解决此错误，请避免在 **using** 块、**foreach** 语句和 **fixed** 语句中给变量赋值。 以下示例生成错误 CS1656，因为它尝试替换 **foreach** 循环内的集合中的所有元素。解决此错误的一种方法是将 **foreach** 循环更改为 [for](https://msdn.microsoft.com/zh-cn/library/ch45axte.aspx) 循环。另一种方法（此处未显示）是修改现有元素的成员；这可能对类起作用，但对结构不起作用。 ``` using System; using System.Collections; using System.Collections.Generic; using System.Text; namespace CS1656_2 { class Book { public string Title; public string Author; public double Price; public Book(string t, string a, double p) { Title=t; Author=a; Price=p; } } class Program { private List<Book> list; static void Main(string[] args) { Program prog = new Program(); prog.list = new List<Book>(); prog.list.Add(new Book ("The C# Programming Language", "Hejlsberg, Wiltamuth, Golde", 29.95)); prog.list.Add(new Book ("The C++ Programming Language", "Stroustrup", 29.95)); prog.list.Add(new Book ("The C Programming Language", "Kernighan, Ritchie", 29.95)); foreach(Book b in prog.list) { // Cannot modify an entire element in a foreach loop // even with reference types. // Use a for or while loop instead if (b.Title == "The C Programming Language") // Cannot assign to 'b' because it is a 'foreach // iteration variable' b = new Book("Programming Windows, 5th Ed.", "Petzold", 29.95); //CS1656 } //With a for loop you can modify elements //for(int x = 0; x < prog.list.Count; x++) //{ // if(prog.list[x].Title== "The C Programming Language") // prog.list[x] = new Book("Programming Windows, 5th Ed.", "Petzold", 29.95); //} //foreach(Book b in prog.list) // Console.WriteLine(b.Title); } } } ``` 下面的示例演示如何在除 **foreach** 循环之外的其他上下文中生成 CS1656： ``` // CS1656.cs // compile with: /unsafe using System; class C : IDisposable { public void Dispose() { } } class CMain { unsafe public static void Main() { using (C c = new C()) { // Cannot assign to 'c' because it is a 'using variable' c = new C(); // CS1656 } int[] ary = new int[] { 1, 2, 3, 4 }; fixed (int* p = ary) { // Cannot assign to 'p' because it is a 'fixed variable' p = null; // CS1656 } } } ```