Question

I just wrote some code that I don’t quite comprehend how it works.

My question is about the local variables in the for loop which are then referenced when the radio button event occurs.

How does it track the different “versions” of these local variables and operate correctly? (i.e. the resulting radio buttons each fire an event with their corresponding value which is derived from an outer local variable)

public class RadioButtonPanel<T> : FlowLayoutPanel
{
    public RadioButtonPanel()
    {
        foreach (object value in Enum.GetValues(typeof(T)))
        {
            string name = Enum.GetName(typeof(T), value);
            var radioButton = new RadioButton { Text = name };
            radioButton.CheckedChanged += (s, e) =>
            {
                if (radioButton.Checked && this.Selected != null)
                    Selected((T)Enum.Parse(typeof(T), name));
            };
            this.Controls.Add(radioButton);
        }
    }

    public event SelectedEvent Selected;
    public delegate void SelectedEvent(T t);
}

Answer 1

This is done via a Closure.

Basically you can imagine that a small class has been created for you on your behalf, that has two properties for your local variables, and a single function. When your lambda gets called, it basically news one of those up and calls the function, thus preserving the values given to it.

The C# specification actually has some really good examples of how this is done by the compiler. Specifically section 6.5.3