public Element GetElement(int id)
{
  if (id < 0)
    return null;

  var index = this.Elements.IndexOf(id);
  if (index < 0)
    return null;

  return this.Elements[id];
}

public Element GetElementOrDefault(int id)
{
  if (id < 0)
    return new Element(); // Ok;

  var index = this.Elements.IndexOf(id);
  if (index < 0)
    return null; // ай, шайтан! проглядел.

  return this.Elements[id];
}

public Element GetElementOrDefault(int id)
{
  var result = default(Element);
  if (id >= 0)
  {
    var index = this.Elements.IndexOf(id);
    if (index >= 0)
      result = this.Elements[id];
    else
      result = new Element(); // Ok;
  }
  else
    result = null; // опять забыл!

  return result;
}

public Element GetElementOrDefault(int id)
{
  var result = new Element();
  if (id >= 0)
  {
    var index = this.Elements.IndexOf(id);
    if (index >= 0)
      result = this.Elements[id];
  }
  return result;
}

N_P>foreach
N_P>{
N_P>   Element ElementB;
N_P>   if (InputStore.TryGet(ElementA.ID, out ElementB))
N_P>   {
N_P>      ElementA.Value = ElementB.Value;
N_P>   }
N_P>   else
N_P>   {
N_P>      ElementA.Value = ElementFormat.DefaultValue;
N_P>   }
N_P>   ElementA.ModificationTime = DateTime.UTCNow
N_P>}
N_P>

N_P>Разбить на 3 функции с простыми названиями и foreach в каждой ?

void ApplyValuesFromInputStore(IEnumerable elements, Store inputStore)
{
  Element source;
  foreach(target in elements)
  {
    target.Value 
      = inputStore.TryGet(target.ID, out source) 
        ? source.Value 
        : ElementFormat.DefaultValue;
    target.ModificationTime = DateTime.UTCNow; // кстати, вот это должно быть частью сеттера Value;
  }
}

Action<Element> ApplyValuesFromInputStore(Store inputStore)
{
  return (Element target) => {
    Element source;
    target.Value 
      = inputStore.TryGet(target.ID, out source) 
        ? source.Value 
        : ElementFormat.DefaultValue;
    target.ModificationTime = DateTime.UTCNow; 
  }
}

Document.Elements.ForEach(ApplyValuesFromInputStore(inputStore));

protected string GetClass(object url)
{
  string result = string.Empty;

  if (SiteMap.CurrentNode != null && SiteMap.CurrentNode.Url == url.ToString())
    result = "class=\"active\"";

  return result;
}

protected string GetClass(object url)
{
  if (SiteMap.CurrentNode != null && SiteMap.CurrentNode.Url == url.ToString())
    return "class=\"active\"";

  return string.Empty;
}

{
  if (CaсhedWebsites != null)
      return CaсhedWebsites;
}
...
private List<Website> CaсhedWebsites
{
  get
  {
    return RequestState.GetValue<List<Website>>("Backoffice", "AllWebsites");
  }
}

public string GetUserAddress(string username)
{
  if (string.IsNullOrEmpty(username) || username.Length > maxUserNameLength)
    return string.Empty;

  IShopAccount account =
      ShopContext.CustomerManager.GetShopAccountByEmail(username, true);

  return account.Address;
}

public string GetUserAddressOrDefault(string username)
{
  if (string.IsNullOrEmpty(username) || username.Length > maxUserNameLength)
    return string.Empty;

  IShopAccount account =
      ShopContext.CustomerManager.GetShopAccountByEmail(username, true);

  return account.Address;
}

public string GetUserAddress(string username)
{
  if (string.IsNullOrEmpty(username) || 
                username.Length > maxUserNameLength)
    throw new ArgumentException("Invalid username length", "username");

  IShopAccount account =
     ShopContext.CustomerManager.GetShopAccountByEmail(username, true);

  return account.Address;
}

public bool TryGetUserAddressOrDefault(string username, out string userAddress)
{
....
}

public IList<Website> AllWebsites
{
  get
  {
    if (CaсhedWebsites != null)
      return CaсhedWebsites;

    List<Website> websites = IsRoleDealer ? DealerWebsites 
      : new List<Website>(Framework.Content.GetWebsites());

    SetCache(websites);

    return websites;
  }
}

WebSites sites = new WebSites();
sites.AllWebSites.RemoveAt(0);

public int Quantity
{
  get
  {
    int quantity;
    if(int.TryParse(TxtQuantity.Text, out quantity))
      return quantity;
    
    return 0;
  }
}

public int Quantity
 {
   get
   {
     if (!int.Valid(TxtQuantity.Text))
       return 0;

     return int.Parse(TxtQuantity.Text);
   }
 }

private bool OrderPaid(IPaymentContext context)
{
  return context.Order.PaymentStatus == PaymentStatus.Paid;
}

private bool OrderMailSent(IPaymentContext context)
{
  return context.Order.Fields.ContainsKey("OrderMailSend");
}

private void MarkOrderAsSent(IPaymentContext context)
{
  context.Order.Fields.Add(new EntityField { Name = "OrderMailSend", Value = 
    true });
}

        public void UpdateBasket(IEnumerable<IBasketLineData> linesData)
        {
            IBasket customerBasket = GetCurrentBasket();
            if (customerBasket == null)
                return;

            foreach (IBasketLineData lineData in linesData)
            {
                BasketLine line = GetLine(customerBasket.BasketLines, lineData);

                if (line != null) //<--Bad
                {
                    ProcessLine(customerBasket, lineData.Stock, line);
                }
            }

            SaveBasket(customerBasket);
        }

        private void ProcessLine(IBasket customerBasket, int quantity, BasketLine line)
        {            
        //...
        }

        private BasketLine GetLine(BasketLines basketLines, IBasketLineData lineData)
        {
            if (!IgnoreLine(lineData.BasketLineId))
                return null; //<--Bad

            return basketLines.GetById(lineData.BasketLineId);
        }

AK>protected string GetClass(object url)
AK>{
AK>  string result = string.Empty;

AK>  if (SiteMap.CurrentNode != null && SiteMap.CurrentNode.Url == url.ToString())
AK>    result = "class=\"active\"";

AK>  return result;
AK>}
AK>

AK>protected string GetClass(object url)
AK>{
AK>  if (SiteMap.CurrentNode != null && SiteMap.CurrentNode.Url == url.ToString())
AK>    return "class=\"active\"";

AK>  return string.Empty;
AK>}
AK>

protected string GetClass(object url)
{
  string result = string.Empty;

  if (SiteMap.CurrentNode != null && SiteMap.CurrentNode.Url == url.ToString())
    result = "class=\"active\"";
  
  if (...)
    result = "class=\"disabled\"";

  return result;
}

foreach
{
   Element ElementB;
   if (InputStore.TryGet(ElementA.ID, out ElementB))
   {
      ElementA.Value = ElementB.Value;
   }
   else
   {
      ElementA.Value = ElementFormat.DefaultValue;
   }
   ElementA.ModificationTime = DateTime.UTCNow
}

// Не отправлено ли уведомление ранее и наступило ли время отправки уведомления.
if (!context.Order.Fields.ContainsKey("OrderMailSend") 
  && (context.Order.PaymentStatus == PaymentStatus.Paid 
      || (context.Order.PaymentStatus == PaymentStatus.InProgress 
        && GetPaymentMethod(context).StartsWith("banktransfer")
     )
   )
)
{
  IEnumerable<string> email = GetEmailAddress(context);
  MailNotification.SendOrderConfirmation(email, context.Order);
  // Отмечаем что уведомление о заказе отправлено
  context.Order.Fields.Add(
    new EntityField { Name = "OrderMailSend", Value = true });
}

if (!OrderMailSent(context) && MailNotificationRequired(context))
{
  IEnumerable<string> email = GetEmailAddress(context);
  MailNotification.SendOrderConfirmation(email, context.Order);
  MarkOrderAsSent(context);
}

public int Quantity
{
  get
  {
    int quantity;
    if(int.TryParse(TxtQuantity.Text, out quantity))
      return quantity;
    
    return 0;
  }
}

public int Quantity
 {
   get
   {
     if (!int.Valid(TxtQuantity.Text))
       return 0;

     return int.Parse(TxtQuantity.Text);
   }
 }

U>// Не отправлено ли уведомление ранее и наступило ли время отправки уведомления.
U>if (!context.Order.Fields.ContainsKey("OrderMailSend") 
U>  && (context.Order.PaymentStatus == PaymentStatus.Paid 
U>      || (context.Order.PaymentStatus == PaymentStatus.InProgress 
U>        && GetPaymentMethod(context).StartsWith("banktransfer")
U>     )
U>   )
U>)
U>{
U>  IEnumerable<string> email = GetEmailAddress(context);
U>  MailNotification.SendOrderConfirmation(email, context.Order);
U>  // Отмечаем что уведомление о заказе отправлено
U>  context.Order.Fields.Add(
U>    new EntityField { Name = "OrderMailSend", Value = true });
U>}
U>

U>if (!OrderMailSent(context) && MailNotificationRequired(context))
U>{
U>  IEnumerable<string> email = GetEmailAddress(context);
U>  MailNotification.SendOrderConfirmation(email, context.Order);
U>  MarkOrderAsSent(context);
U>}
U>

if (OrderMailNotSentAndMailNotificationRequired(context))
{
  SendOrderConfirmationAndMarkOrderAsSent(context);
}

U>public int Quantity
U>{
U>  get
U>  {
U>    int quantity;
U>    if(int.TryParse(TxtQuantity.Text, out quantity))
U>      return quantity;
    
U>    return 0;
U>  }
U>}
U>

U>public int Quantity
U> {
U>   get
U>   {
U>     if (!int.Valid(TxtQuantity.Text))
U>       return 0;

U>     return int.Parse(TxtQuantity.Text);
U>   }
U> }
U>

//отделяем логику обновления текущей корзины от собственно обновления корзины
public UpdateCurrentBasket(linesData : IEnumerable[IBasketLineData]) : void
{
    when (GetCurrentBasket() is Some(customerBasket))
    {
        UpdateBasket(customerBasket, linesData)
        SaveCurrentBasket(customerBasket);
    }
}
//Это возможно нужно сделать методом расширением
private UpdateBasket(customerBasket : IBasket, linesData : IEnumerable[IBasketLineData]) : void
{
    foreach (lineData in linesData)
        when (!IgnoreLine(lineData.BasketLineId))
        when (customerBasket.BasketLines.GetById(lineData.BasketLineId) is Some(line))
            if (lineData.Stock != 0)
                line.Quantity = lineData.Stock;
            else
                customerBasket.BasketLines.Remove(line);
}

  public variant option[T]
  {
    | None
    | Some { val : T; }
  }

when (GetCurrentBasket() is Some(customerBasket))