C# Modify List item while in a foreach loop-CodePudding

as the title say's I am attempting to get to edit a list item inside of a foreach loop. I know it is not possible, so I am asking for solutions to work around this.

As this is taking data directly from a MySQL server, it need's to be done inside of this loop and with the correct indexes. I attempted to use a second list and change the data outside of the foreach loop, but that did not account for the index positions of the items.

foreach (string friend in friendsOnline)
{
    query = "SELECT `username` FROM `accounts` WHERE email = '"   friend   "'";
    
    cmd = new MySqlCommand(query, conn);
    rdr = cmd.ExecuteReader();
                
    while (rdr.Read())
    {
        for (int i = 0; i<friendsOnline.Count; i  )
        {
            if (friendsOnline[i].Contains(friend))
            {
                friendsOnline[i] = rdr[0].ToString();
            }
        }
    }
}

CodePudding user response：

As I can see, you have emails in the friendsOnline:

string[] friendsOnline = new[] {
  "[email protected]",
  "[email protected]",
};

and you want to change these emails into names:

string[] friendsOnline = new[] {
  "Frodo Baggins",
  "Sauron the Black",
};

First of all, I suggest executing query just once: SQL is too slow to be run in a loop; instead you can build a dictionary which eMail corresponds to which indexes:

using System.Linq;

...

// eMail - indexes correspondence
Dictionary<string, int[]> dict = friendsOnline
  .Select((name, index) => (name : name, index : index))
  .GroupBy(pair => pair.name, pair => pair.index, StringComparer.OrdinalIgnoreCase)
  .ToDictionary(group => group.Key, group => group.ToArray());

// We take all users (no WHERE), but once
// We read both username and email
string sql = 
  @"SELECT `username`,
           `email` 
      FROM `accounts`";
                
//DONE: using - we should Dispose IDisposable Command and Reader
using cmd = new MySqlCommand(sql, conn);
using rdr = cmd.ExecuteReader();

while (rdr.Read()) {
  string name = Convert.ToString(rdr[0]);
  string mail = Convert.ToString(rdr[1]); 

  // If we have correspondent indexes, we change friendsOnline
  if (dict.TryGetValue(mail, out var indexes))
    foreach (var index in indexes)
      friendsOnline[index] = name;
}