foreach循環（與包括LINQ）性能提高

Question

我希望簡化以下代碼包含foreach循環以最小化的迭代和/或增加，因爲LINQ和集合的性能在每次迭代被創建：

foreach (Contact contact in Contacts) // phone contacts, around 500-1000 
{ 
    IEnumerable<ContactEmailAddress> emails = contact.EmailAddresses; // each has multiple emails 

    foreach (Friend parseUser in parseUsers) // could be many thousands 
    { 
     if (emails.Where(e => e.EmailAddress == parseUser.Email).ToList().Count > 0) 
     {       
      parseUser.AddContact(contact); // function call 

      verifiedUsers.Add(parseUser); // add to my new aggregated list 
     } 
    } 
} 

感謝。

Answer 1

，而不是做對emails集合在parseUsers每一個項目，你可以使用一個集合的線性搜索，可以更加高效地搜索，如HashSet：

foreach (Contact contact in Contacts) // phone contacts, around 500-1000 
{ 
    HashSet<string> emails = new HashSet<string>(
     contact.EmailAddresses.Select(e => e.EmailAddress)); 

    foreach (Friend parseUser in parseUsers) // could be many thousands 
    { 
     if(emails.Contains(parseUser.Email)) 
     { 
      parseUser.AddContact(contact); // function call 

      verifiedUsers.Add(parseUser); // add to my new aggregated list 
     } 
    } 
} 

Answer 2

沒有多大改善性能，但提高readbility：通過替換

foreach (Friend parseUser in parseUsers) // could be many thousands 
{ 
    var filterContacts = Contacts.Where(contact => 
           contact.EmailAddresses.Contains(parseUser.Email)); 
    if (filterContact.Any()) 
    { 
     parseUser.AddContacts(filterContacts); 
     verifiedUsers.Add(parseUser); 
    } 
}