How does List<T>.IndexOf() perform comparisons on custom objects?

Question

I wrote a class of account objects and hold a static List<T> of those account objects. My program loops through each account in the list, performing some work with the account, and then resetting at the top when it reaches the end of the list.

My issue is that I need to be able to reinsert the account into the list after my program finishes working with it, with some updated info added. Can I do this as written below, using the IndexOf() function to check for the object in the static list or will it fail because I added data to it? I don't understand which fields it compares to see if the two objects are the same.

Note: no duplicates are allowed in the list so there is no risk of updating the wrong item

public class Account
{
   public string name;
   public string password;
   public string newInfo;
}

public static class Resources
{
   private static List<Account> AccountList = new List<Account>();
   private static int currentAccountIndex = 0;

   public static Account GetNextAccount()
   {
      if (currentAccountIndex > AccountList.Count)
         currentAccountIndex = 0;
      return AccountList[currentAccountIndex++];
   }

   public static void UpdateAccount(Account account)
   {
      int index;
      if ((index = AccountList.IndexOf(account)) >= 0)
         AccountList[index] = account;
   }
}

public class Program
{
   public void PerformWork()
   {
      Account account = Resources.GetNextAccount();
      // Do some work
      account.newInfo = "foo";
      Resources.UpdateAccount(account);
   }
}

Answer 1

Another option is to use List.FindIndex, and pass a predicate. That is:

if ((index = AccountList.FindIndex(a => a.name == account.name)) >= 0)
    AccountList[index] = account;

That way you can search on any arbitrary field or number of fields. This is especially useful if you don't have access to the source code for Account to add an overloaded Equals method.

Answer 2

One thing the accepted answer did not cover is you are supposed to override Equals(object) and GetHashCode() for IEquatable<T> to work correctly. Here is the full implementation (based off of keyboardP's answer)

public class Account : IEquatable<Account>
{
    public string name;
    public string password;
    public string newInfo;

    private readonly StringComparer comparer = StringComparer.OrdinalIgnoreCase;

    public override bool Equals(object other)
    {
        //This casts the object to null if it is not a Account and calls the other Equals implementation.
        return this.Equals(other as Account);
    }

    public override int GetHashCode()
    {
        return comparer.GetHashCode(this.newInfo)
    }

    public bool Equals(Account other)
    {
       //Choose what you want to consider as "equal" between Account objects  
       //for example, assuming newInfo is what you want to consider a match
       //(regardless of case)
       if (other == null) 
             return false;

       return comparer.Equals(this.newInfo, other.newInfo);
    }
}

Answer 3

Your object should implement the IEquatable interface and override the Equals method.

public class Account : IEquatable<Account>
{
    public string name;
    public string password;
    public string newInfo;

    public bool Equals(Account other)
    {
       //Choose what you want to consider as "equal" between Account objects  
       //for example, assuming newInfo is what you want to consider a match
       //(regardless of case)
       if (other == null) 
             return false;

       return String.Equals(this.newInfo, other.newInfo, 
                           StringComparison.OrdinalIgnoreCase);
    }
}