在另一个范围内引用时如何处理局部变量？

Question

在我的实现中,我一直在写这样的东西:

public void SomeMethod(int someValue, List<int> someValues)
{
  Task generatedTask = null;

  {
    int anotherValue = 2;
    object valuesRef = someValues;
    generatedTask = new Task(delegate{
      anotherValue += someValue + GetSum(valuesRef);
      Console.WriteLine(anotherValue);
    });
  }

  generatedTask.Start();
}

但是,我不知道到底发生了什么......

也许一切都被"复制"给代表.或者,与引用类型一样,所有值类型都将具有与Task委托关联的副本,直到它存在为止？

我只是想了解最新的C#版本在性能问题上究竟发生了什么.

Answer 1

优秀的问题; 捕获变量和闭包上下文.反编译显示当前编译器在此处创建了2个捕获上下文对象:

public void SomeMethod(int someValue, List<int> someValues)
{
    Task task;
    <>c__DisplayClass3 class2; // <== compiler generated type; unpronounceable
    <>c__DisplayClass1 class3; // <== compiler generated type; unpronounceable
    class3 = new <>c__DisplayClass1(); // outer-scope context
    class3.someValue = someValue;
    task = null;
    class2 = new <>c__DisplayClass3(); // <== inner-scope context
    class2.CS$<>8__locals2 = class3; // <== bind the contexts
    class2.anotherValue = 2;
    class2.valuesRef = someValues;
    task = new Task(new Action(class2.<SomeMethod>b__0));
    task.Start();
    return;
}

如果您的目标是最小化上下文对象,则可以手动执行闭包:

public void SomeMethod2(int someValue, List<int> someValues)
{
    Task generatedTask = null;
    {
        var ctx = new MyCaptureContext();
        ctx.anotherValue = 2;
        ctx.valuesRef = someValues;
        ctx.someValue = someValue;
        generatedTask = new Task(ctx.SomeMethod);
    }

    generatedTask.Start();
}

class MyCaptureContext
{
    // kept as fields to mimic the compiler
    public int anotherValue;
    public int someValue;
    public object valuesRef;
    public void SomeMethod()
    {
        anotherValue += someValue + GetSum(valuesRef);
        Console.WriteLine(anotherValue);
    }
}

您还可以通过缓存单独传递状态的单个委托来避免每个任务创建委托:

public void SomeMethod(int someValue, List<int> someValues)
{
    Task generatedTask = null;
    {
        var ctx = new MyCaptureContext();
        ctx.anotherValue = 2;
        ctx.valuesRef = someValues;
        ctx.someValue = someValue;
        generatedTask = new Task(MyCaptureContext.SomeMethod, ctx);
    }

    generatedTask.Start();
}
class MyCaptureContext
{
    // kept as fields to mimic the compiler
    public int anotherValue;
    public int someValue;
    public object valuesRef;
    public static readonly Action<object> SomeMethod = SomeMethodImpl;
    private static void SomeMethodImpl(object state)
    {
        var ctx = (MyCaptureContext)state;
        ctx.anotherValue += ctx.someValue + GetSum(ctx.valuesRef);
        Console.WriteLine(ctx.anotherValue);
    }
}

或(清洁,IMO):

public void SomeMethod(int someValue, List<int> someValues)
{
    Task generatedTask = null;
    {
        var ctx = new MyCaptureContext();
        ctx.anotherValue = 2;
        ctx.valuesRef = someValues;
        ctx.someValue = someValue;
        generatedTask = ctx.CreateTask();
    }

    generatedTask.Start();
}
class MyCaptureContext
{
    // kept as fields to mimic the compiler
    public int anotherValue;
    public int someValue;
    public object valuesRef;
    public Task CreateTask()
    {
        return new Task(someMethod, this);
    }
    private static readonly Action<object> someMethod = SomeMethod;
    private static void SomeMethod(object state)
    {
        var ctx = (MyCaptureContext)state;
        ctx.anotherValue += ctx.someValue + GetSum(ctx.valuesRef);
        Console.WriteLine(ctx.anotherValue);
    }
}