public User Login(string userName, string password)
{
    using (DirectoryEntry entry = new DirectoryEntry(config.Path, config.UserDomainName + "\\" + userName, password))
    {
        using (DirectorySearcher searcher = new DirectorySearcher(entry))
        {
            // ruleid: ldap-injection
            searcher.Filter = String.Format("({0}={1})", SAMAccountNameAttribute, userName);
            searcher.PropertiesToLoad.Add(DisplayNameAttribute);
            searcher.PropertiesToLoad.Add(SAMAccountNameAttribute);
            var result = searcher.FindOne();
            if (result != null)
            {
                var displayName = result.Properties[DisplayNameAttribute];
                var samAccountName = result.Properties[SAMAccountNameAttribute];
                
                return new User
                {
                    DisplayName = displayName == null || displayName.Count <= 0 ? null : displayName[0].ToString(),
                    UserName = samAccountName == null || samAccountName.Count <= 0 ? null : samAccountName[0].ToString()
                };
            }
        }
    }
}

public User Login(string userName, string password)
{
    using (DirectoryEntry entry = new DirectoryEntry(config.Path, config.UserDomainName + "\\" + userName, password))
    {
        using (DirectorySearcher searcher = new DirectorySearcher(entry))
        {
            // ok: ldap-injection
            searcher.Filter = String.Format("({0}={1})", Encoder.LdapFilterEncode(SAMAccountNameAttribute), Encoder.LdapFilterEncode(userName));
            searcher.PropertiesToLoad.Add(DisplayNameAttribute);
            searcher.PropertiesToLoad.Add(SAMAccountNameAttribute);
            var result = searcher.FindOne();
            if (result != null)
            {
                var displayName = result.Properties[DisplayNameAttribute];
                var samAccountName = result.Properties[SAMAccountNameAttribute];
                
                return new User
                {
                    DisplayName = displayName == null || displayName.Count <= 0 ? null : displayName[0].ToString(),
                    UserName = samAccountName == null || samAccountName.Count <= 0 ? null : samAccountName[0].ToString()
                };
            }
        }
    }
}