使用ADO.NET刪除數據庫條目

Question

我有以下代碼連接到我的數據庫，並檢查電子郵件地址是否存在於表中，如果存在，它將刪除它，如果沒有，它將添加它。

但這兩個選項都不起作用。我得到了兩個動作相同的錯誤：

對象不能被刪除，因爲它沒有在 ObjectStateManager找到。

代碼：

protected OEntities database = new OEntities(); 

public string NewsletterSubscriptionValidation(string emailAddress) 
{ 
    string validationMessage = ""; 
    Newsletter newsletter = new Newsletter(); 

    var identifier = database.Newsletters.Select(
     id => id.EmailAddress); 

    if (identifier == null) 
    { 
     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = true; 

     database.Newsletters.AddObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You are now subscribed to our periodic Newsletter."; 
    } 
    else 
    { 
     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = false; 


     database.DeleteObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You have been unsubscribed from our periodic Newsletter."; 
    } 

    return validationMessage; 
} 

難道我甚至增加/刪除數據庫中的正確方法，在這裏？

解決方案

---

protected OEntities database = new OEntities(); 

public string NewsletterSubscriptionValidation(string emailAddress) 
{ 
    string validationMessage = ""; 
    Newsletter newsletter = database.Newsletters.FirstOrDefault(
     nl => nl.EmailAddress == emailAddress); 

    if (newsletter == null) 
    { 
     newsletter = new Newsletter(); 

     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = true; 

     database.AddToNewsletters(newsletter); 
     //database.Newsletters.AddObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You are now subscribed to our periodic Newsletter."; 
    } 
    else 
    { 
     newsletter.EmailAddress = emailAddress; 
     newsletter.Subscribed = true; 

     database.AddToNewsletters(newsletter); 
     //database.Newsletters.AddObject(newsletter); 
     database.SaveChanges(); 

     validationMessage = "You have been unsubscribed from our periodi Newsletter."; 
    } 

    return validationMessage; 
} 

Answer 1

您需要刪除

database.Attach(newsletter); 
    database.DeleteObject(newsletter); 
    database.SaveChanges(); 

之前附加，但你可能想選擇現有的資料更多類似這樣

Newsletter newsletter = database.Newsletters.FirstOrDefault(nl => nl.EmailAddress == emailAddress); 
if (newsletter == null) 
{ 
    newsletter = new Newsletter(); 
    ... 
} 
else 
{ 
    database.Attach(newsletter); 
    database.DeleteObject(newsletter); 
    database.SaveChanges(); 
}