ForEach不会更改/更改列表C#中的项目 [英] ForEach will not alter/change items in list C#

问题描述

我很确定这在我编写的某些代码中已经奏效，不确定我在这里缺少什么.

I'm pretty sure this has worked before in some code I have written, not sure what I am missing here.

以下代码不会更改列表中的 string 值.为什么?

The following code does not change the string values in my list. Why?

var items = listBox1.Items.Cast<string>().ToList();
items.ForEach(item => item = (!chkBox1.Checked) ? "move " + item : "move -check " + item);

编辑:为澄清起见，列表与创建时相同.

EDIT : Just to clarify, the list remains the same as when it was created.

编辑:抱歉，项目列表(listBox1.Items)包含字母数字字符串，如下所示.

EDIT : Apologies, the items list (listBox1.Items) contains alphanumeric strings as follows.

J00123456

J00123344

J00123458

另外，为了确认这一点，我已经使用T的自定义列表(如下所示)成功完成了此操作，这就是为什么我认为它可以在此处使用的原因.

Also, just to confirm, I have successfully done this with a custom list of T (shown below), which is why I thought it would work here.

var tapes = new List<TapeInfo>();
... //Populated list
tapes.ForEach(x => x.vItem = "tapelib 3592 -eject " + x.vItem);  //Works here

推荐答案

如果要获得不同的结果，则应使用 Select 函数.

You should use the Select function if you want to get a different result.

var items = listBox1.Items.Cast<string>().ToList();
listBox1.Items = items.Select(item => (!chkBox1.Checked) ? "move " + item : "move -check " + item).ToList();

ForEach 函数可以根据集合值执行某些操作，但不能对值本身执行操作.

The ForEach function can do something according to a collection value, but can't do it to the values themselves.

Select 函数将根据给定的集合创建一个新的集合.

The Select function will create a new collection according to the given collection.

修改

关于使用来成功更改值的编辑

About your edit of successfully changing values by using

tapes.ForEach(x => x.vItem = "tapelib 3592 -eject " + x.vItem);

您需要了解按值/引用传递参数的工作方式.

You need to understand how passing argument by value/reference works.

在c#中，当编写 var obj1 = new object()时， obj1 是指向存在于 new object()上的指针堆.

In c#, when writing var obj1 = new object(), obj1 is a pointer to the new object() that exists on the heap.

当您通过调用 Foo(obj1)将 obj1 传递给函数 public void Foo(object obj){//...} 时>，该函数会将参数作为 new 指针指向堆上的相同 对象.

When you pass obj1 to a function public void Foo(object obj) { //... } by calling Foo(obj1), the function will get the parameter as new pointer to the same object on the heap.

因此，当您在对象本身上使用ForeEach时，

Therefore, when you use ForeEach on the object itself,

object.ForEach(obj => obj = new object()) // No changed outside the Foreach

只有新的指针会被更改，它将指向一个新的对象，但原始的指针不会被更改.

only the new pointer will be changed and it will point on a new object but the original pointer isn't changed.

但是如果在内部对象上使用它，

But if you use it on an inner object,

object.ForEach(obj => obj.InnerObject = new object()) // Changed outside the Foreach

这将更改指向的内部对象，并且内部对象也将更改.

this will change the pointed inner object and the inner object will be changed.

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