Question

struct SomeStruct
{
    public int Num { get; set; }
}

class Program
{
    static Action action;

    static void Foo()
    {
        SomeStruct someStruct = new SomeStruct { Num = 5 };
        action = () => Console.WriteLine(someStruct.Num);
    }

    static void Main()
    {
        Foo();
        action.Invoke();
    }
}

1. Is a copy of someStruct created when the lambda is created?
2. Is a copy of someStruct created when Foo returns?
3. Can I verify that copying doesn’t occur? In C++ I’d implement the copy constructor and print from inside it.

Citations from the standard will be appreciated. Any relevant online articles as well.

Answer 1

There will be no copies. Lambdas capture variables, not values.

You can use Reflector to look at the compile code: the compiler will move the “someStruct” variable into a helper class.

private static void Foo()
{
    DisplayClass locals = new DisplayClass();
    locals.someStruct = new SomeStruct { Num = 5 };
    action = new Action(locals.b__1);
}
private sealed class DisplayClass
{
    // Fields
    public SomeStruct someStruct;

    // Methods
    public void b__1()
    {
        Console.WriteLine(this.someStruct.Num);
    }
}

Copying structures will never cause user-defined code to run, so you cannot really check it that way.
Actually, the code will do a copy when assigning to the “someStruct” variable. It would do that even for local variables without any lambdas.