列表中已存在检查或昵称<User>

Question

I am creating an ASP website with a possibility to register.

The nickname that visitors choose to register has to be unique.

Everytime when an user registers, I select all users from the database, and then I am using a foreach loop to check or username already exists:

private List<User> listExistingUsers;
listExistingUsers = Database.GetUsers();
foreach (User u in listExistingUsers)
{
    if (u.Nickname == txtNickname.text)
    {
        Error = "Username already in use.";
    }
}

But the code above doesn't work properly. It doesn't check all the items in the list which are read from the database. So it is possible to have users with the same usernames, which I don't want.

What can I do to solve this problem? I read about LINQ, but I think that this is the wrong way of checking usernames with List<> in my opinion. I think this username-check must be done in another way.

Can you experts help me? I could also do this check with a SQL-query, but I would like to do it in c#.

Answer 1

而不是从DB返回所有用户，将用户名传递给Query /存储过程，让后端进行检查，然后返回状态标志1/0 - 存在/不存在。

Answer 2

if (Database.GetUsers().Select(x => x.Nickname).Contains(txtNickname.Text)) should do what you want.

I've condensed everything into a single line so I'll give a quick explanation; First I use your Database.GetUsers() method to retrieve the users, then I use select to project the Nickname since that's what we're comparing. If that were to execute on it's own it would result in an IEnumerable<string> with all of the Nicknames. From there I use contains to see if that list contains the nickname that (I'm assuming) has been entered in the UI.

Answer 3

You can use Contains operator in order tocheck

listExistingUsers.Select(x => x.Nickname).Contains(txtNickname.text);

link : http://msdn.microsoft.com/fr-fr/library/bhkz42b3%28v=vs.80%29.aspx

Remark : You can use Any or count (very expensive last solution)

Answer 4

Use Any operator. It checks whether any element of a sequence satisfies some condition. In your case condition is user nickname equals to text in textBox :

if (Database.GetUsers().Any(u => u.Nickname == txtNickname.Text))
    Error = "Username already in use.";

BTW if you change GetUsers to return IQueryable<User> then check will occur on server side.

Answer 5

Do get a list of NickNames once

var nickNames = new List<string>();

for(int i=0;i<listExistingUsers.Count;i++)
{
     nickNames.Add(listExistingUsers.NickName);
}

Then u can simply use

if(nickNames.Contains(txtNickname.text))
{
     Error = "Username already in use.";
}

Answer 6

1) Have you verified that Database.GetUsers() is actually returning the full list, with no SQL issues?

2) Do you need it to be case-insensitive?

3) You can use the LINQ to do the query like this:

if (listExistingUsers.Any(u => string.Equals(u, txtNickname.Text, StringComparison.CurrentCultureIgnoreCase)))
{
   // Process error
}

Answer 7

If Database.GetUsers() return all the users from database, so do not use it! Imagine if you have already 1000 of users, for each new user it will load all the users, and you will have performance issues.

Instead, create a new method that search your database and return only one result, case it exists. Something like :

private bool Database.UserExists(txtNickname.text) {
      //Your query to database with a where statment looking for the nickname. It could be a LINQ query, or any other way you use in your system.
      //If it brings 1 result, it has to return true. 
 }

Answer 8

I think the most tricky part of your task is to fill the database Correctly.

Particularly:

• Cut off trailing and ending spaces
• Decide if the user names should becase sensitive
• Make sure that when creating a new user name you do not have the nick already

About Loading users and checking:

As mentioned above LINQ is the most effective a C# like checking for duplicates ( if (Database.GetUsers().Select(x => x.Nickname).Contains(txtNickname.Text)))

I am more used to writing SQL statements than using LINQ. If you've got lots of users SQL will read only the selected ones but I don't know if the LINQ statement above pulls all users into the memory pool or just the one(s) with the same nickname.