C# 使用 Action<T> 创建任务
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Create a Task with an Action<T>
提问by alwayslearning
I somehow feel I am missing something basic. Here's my problem.
我不知何故觉得我缺少一些基本的东西。这是我的问题。
I am trying to create a System.Threading.Tasks.Task instance to execute an action that accepts a parameter of a certain type. I thought I could do something like
我正在尝试创建一个 System.Threading.Tasks.Task 实例来执行接受某种类型参数的操作。我以为我可以做类似的事情
void DoWork(MyClass obj) {} //My action that accepts a parameter of type 'MyClass'
MyClass obj = new MyClass();
Action<MyClass> action = DoWork; //action that points to the method
Task task = new Task(action,obj); //task that would execute 'DoWork' with 'obj' as the parameter when I call Start.
Obviously this does not compile. It seems I can only use an Action<object>and not an Action<T>for a task and then cast the 'object' to T inside my method.
显然这不能编译。看来我只能对任务使用 anAction<object>而不是 an Action<T>，然后在我的方法中将“对象”转换为 T 。
How can I achieve what I want most effectively and efficiently?
我怎样才能最有效和最高效地实现我想要的？
回答by MartinStettner
You could use
你可以用
Action<Object> action = o => DoWork((MyClass)o);
Task task = new Task(action, obj);
If you're using .NET 4.0 or above, you can use Contravarianceto achieve your goal without introducing a new delegate
如果您使用 .NET 4.0 或更高版本，则可以使用逆变来实现您的目标，而无需引入新的委托
//INCORRECT Code, casts InvalidCastException at runtime Action action = DoWork; Task task = new Task((Action)action, obj);
//INCORRECT Code, casts InvalidCastException at runtime Action action = DoWork; Task task = new Task((Action)action, obj);
EDIT:
编辑：
Thanks for @svick for pointing out, that the second option is not correct: I was too busy sorting out, whether Action is co- or contravariant (it is in fact contravariant, I was right about this at least) that I oversaw, that I would need Covariancein this case.
感谢@svick 指出，第二个选项是不正确的：我太忙于整理，我监督的 Action 是协变还是逆变（实际上是逆变的，至少我是对的），那在这种情况下，我需要协方差。
Contravariance means that you can do
逆变意味着你可以做到
Action<object> action1 = someAction;
Action<SubClass> action2 = action1;
without explicit casting.
没有显式转换。
回答by cuongle
You also can use directly:
也可以直接使用：
MyClass obj = new MyClass();
Task task = Task.Run(() => DoWork(obj));
