public class SelectItem<T> where T : class
{
    public bool IsChecked { get; set; }
    public T Item { get; set; }
}

SelectItem<String> obj = new SelectItem<String> { Item = "Value" };

obj.IsChecked = true;

String objValue = obj;

String objValue = obj.Item;

public static implicit operator T(SelectItem<T> obj) {
    return obj.Item;
}