How local variables are handled when referenced in another scope?

Question

I've been writing things like this in my implementations:

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();
}

However, I don't know exactly what's happening here...

Maybe everything was "copied" to the delegate. Or maybe, like reference types, all value types will have a copy associated to the Task delegate until it exists?

I'm just trying to understand what exactly happens in latest C# versions for performance matters.

Answer 1

Excellent question; captured variables and closure contexts. Decompiling it shows that the current compiler creates 2 capture context objects here:

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;
}

If your objective is to minimise context objects, you could perform the closures manually:

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);
    }
}

You can also avoid a delegate creation per-task by caching a single delegate that passes in the state separately:

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);
    }
}

or (cleaner, 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);
    }
}