幫助EntityQuery工作的Silverlight C＃

Question

我有以下代碼：

var data = (sender as Button).DataContext as Web.Booking; 

EntityQuery<Web.Ticket> ticketQuery = 
    from t in _ticketContext.GetTicketsQuery() 
    where t.ticketId == data.ticketId 
    select t; 

LoadOperation<Web.Ticket> loadTicket = this._ticketContext.Load(ticketQuery); 
loadTicket.Completed += (s, args) => { MessageBox.Show("Loaded Tickets!"); }; 

ticketDomainDataSource.DataContext = loadTicket.AllEntities; 

var ticketData = ticketDomainDataSource.DataContext as Web.Ticket; 
string ticketName = ticketData.ticketName; 

是我遇到的麻煩的主要代碼是：

var ticketData = ticketDomainDataSource.DataContext as Web.Ticket; 
    string ticketName = ticketData.ticketName; 

它返回一個錯誤：

Object reference not set to an instance of an object.

任何人都可以幫助我瞭解我在做什麼錯在這裏，我無法弄清楚什麼是空以及如何返回適當的數據。

感謝

Answer 1

你的問題很可能是DataContext的不能轉換爲Web.Ticket。如果您查看C＃的文檔 - as在轉換失敗時返回null。見 - http://msdn.microsoft.com/en-us/library/cscsdfbt%28v=vs.71%29.aspx

如果你改變你的代碼行

var ticketData = (Web.Ticket) ticketDomainDataSource.DataContext; 

你應該得到更好的鑄件錯誤。