Foreach结构奇怪的编译错误在C＃ [英] Foreach struct weird compile error in C#

问题描述

 命名空间MyNamespace 
 {
 public struct MyStruct 
 {
 public string MyString; 
 public int MyInt; 
 public bool MyBool; 
} 
 
 public class MyClass 
 {
 private List< MyStruct> MyPrivateVariable; 
 
 public List< MyStruct> MyVariable 
 {
 get 
 {
 if（MyPrivateVariable == null）
 {
 MyPrivateVariable = new List< MyStruct>（）; 
 
 MyPrivateVariable.Add（new MyStruct（））; 
 MyPrivateVariable.Add（new MyStruct（））; 
} 
 
 return MyPrivateVariable; 
} 
} 
 
 public void MyLoop（）
 {
 foreach（MyVariable中的MyStruct ms）
 {
 //不编译，但它的工作原理，如果你通过立即窗口或Quickwatch执行它
 ms.MyBool = false; 
 
 //编译，工作
 MyFunction（ms）; 
} 
} 
 
 public void MyFunction（MyStruct ms）
 {
 ms.MyBool = false; 
} 
} 
} 
  

任何合理的解释？

编译器返回：

错误：
无法修改成员'ms'，因为它是'foreach迭代
变量'

EDIT： p>

额外的问题：

我刚刚尝试更改 MyFunction ，它实际上不会更新 ms 。但是：如果我去quickwatch并在那里分配相同的值，它会更新 ms 。为什么会发生这种情况，如果它不应该在第一时间编译，不应该quickwatch抛出异常？

EDIT2： / p>

好的，快速观察也适用于 ms 的副本，所以这就是为什么我可以编辑它的值，实际上改变了 MyPrivateVariable 的内容。

解决方案

使用它们作为可变结构。避免这样做：

为什么是可变structsevil？

namespace MyNamespace
{
    public struct MyStruct
    {
        public string MyString;
        public int MyInt;
        public bool MyBool;
    }

    public class MyClass
    {
        private List<MyStruct> MyPrivateVariable;

        public List<MyStruct> MyVariable
        {
            get
            {
                if (MyPrivateVariable == null)
                {
                    MyPrivateVariable = new List<MyStruct>();

                    MyPrivateVariable.Add(new MyStruct());
                    MyPrivateVariable.Add(new MyStruct());
                }

                return MyPrivateVariable;
            }
        }

        public void MyLoop()
        {
            foreach (MyStruct ms in MyVariable)
            {
                // Doesn't compile, but it works if you execute it through the Immediate window, or in Quickwatch
                ms.MyBool = false;

                // Compiles, works
                MyFunction(ms);
            }
        }

        public void MyFunction(MyStruct ms)
        {
            ms.MyBool = false;
        }
    }
}

Any reasonable explanations for this?

The compiler returns:

Error: Cannot modify members of 'ms' because it is 'foreach iteration variable'

EDIT:

Extra question:

I just tried changing a string from MyFunction, and it doesn't actually update ms. BUT: If I go to quickwatch and assign the same value there, it does update ms. Why does this happen if it shouldn't even be compiling in the first place, shouldn't quickwatch throw an exception?

EDIT2:

Ok, quick watch also works on a copy of ms, so that's why I can edit it's value, it doesn't actually alter the contents of MyPrivateVariable.

解决方案

You're using them as mutable structs. Avoid doing that:

Why are mutable structs "evil"?

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