列表作为“out”参数导致错误。为什么？

发布于 2024-11-09 06:46:02 字数 521 浏览 0 评论 0原文

在此代码中：

public bool SomeMethod(out List<Task> tasks)
{
    var task = Task.Factory.StartNew(() => Process.Start(info));
    tasks.Add(task);
}

我收到错误“使用未分配的输出参数‘任务’”。为什么？

在 MSDN 示例中，仅使用了 out 参数

class OutExample
{
    static void Method(out int i)
    {
        i = 44;
    }

    static void Main()
    {
        int value;
        Method(out value);
        // value is now 44
    }
}

，是因为 List 吗？

原文

In this code:

public bool SomeMethod(out List<Task> tasks)
{
    var task = Task.Factory.StartNew(() => Process.Start(info));
    tasks.Add(task);
}

I get an error, "Use of unassigned out parameter 'tasks'". Why?

In an MSDN example there's just use of out parameter

class OutExample
{
    static void Method(out int i)
    {
        i = 44;
    }

    static void Main()
    {
        int value;
        Method(out value);
        // value is now 44
    }
}

Is it because of List<T>?

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青衫负雪 2024-11-16 06:46:02

您必须在方法主体中初始化 out 参数（即创建一个新的 List 实例并将其分配给 out 参数)：

public bool SomeMethod(out List<Task> tasks) {
  var task = Task.Factory.StartNew(() => Process.Start(info);
  tasks = new List<Task>() { task };
  ...
}

我使用集合初始值设定项语法将任务添加到列表中，但如果您愿意，也可以调用 Add 方法。

您应该这样调用该方法：

List<Task> tasks;
SomeMethod(out tasks);
var newTask = tasks[0]; // Access the task just created.

C# 7.0 引入了新的更简单的语法，您可以使用 out 参数在函数调用中声明变量：

SomeMethod(out var tasks);
var newTask = tasks[0]; // Access the task just created.

作为 List 通过引用传递，您可以去掉 out 参数。然后，您必须在调用该方法之前创建列表：

public bool SomeMethod(List<Task> tasks) {
  var task = Task.Factory.StartNew(() => Process.Start(info);
  tasks.Add(task);
  ...
}

并像这样调用它：

var tasks = new List<Task>();
SomeMethod(tasks);
var newTask = tasks[0]; // Access the task just created.

一般来说，避免使用 out 参数是一种很好的做法，因为它们可能会造成混淆。

You have to initialize the out parameter in the method body (that is create a new List<Task> instance and assign it to the out parameter):

public bool SomeMethod(out List<Task> tasks) {
  var task = Task.Factory.StartNew(() => Process.Start(info);
  tasks = new List<Task>() { task };
  ...
}

I'm using the collection initializer syntax to add the task to the list, but you could call the Add method instead if you prefer.

You should call the method like this:

List<Task> tasks;
SomeMethod(out tasks);
var newTask = tasks[0]; // Access the task just created.

C# 7.0 has introduced new simpler syntax where you declare the variable in the call to the function with the out parameter:

SomeMethod(out var tasks);
var newTask = tasks[0]; // Access the task just created.

As a List<T> is passed by reference you can get rid of the out parameter. You then have to create the list before calling the method:

public bool SomeMethod(List<Task> tasks) {
  var task = Task.Factory.StartNew(() => Process.Start(info);
  tasks.Add(task);
  ...
}

And call it like this:

var tasks = new List<Task>();
SomeMethod(tasks);
var newTask = tasks[0]; // Access the task just created.

In general it is good practice to avoid out parameters because they can be confusing.

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夏见 2024-11-16 06:46:02

out 表示您的方法需要创建对象，然后将其分配给参数：

List<Task> tasks = new List<Task>();
tasks.Add(task);

out means that your method needs to create the object, then assign it to the parameter:

List<Task> tasks = new List<Task>();
tasks.Add(task);

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追星践月 2024-11-16 06:46:02

这是因为您没有为 tasks 变量分配值...在本例中，该值将是对 List 类型实例的引用。

将 tasks = new List(); 添加到 SomeMethod 的主体中，一切都会正常工作：

public bool SomeMethod(out List<Task> tasks) {
   tasks = new List<Task>();
   var task = Task.Factory.StartNew(() => Process.Start(info);
   tasks.Add(task);
}

It's because you didn't assign a value to the tasks-variable... in this case that would be a reference to a instance of type List<Task>.

Add tasks = new List<Task>(); to the body of SomeMethod and everything will work fine:

public bool SomeMethod(out List<Task> tasks) {
   tasks = new List<Task>();
   var task = Task.Factory.StartNew(() => Process.Start(info);
   tasks.Add(task);
}

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