Validate user against LDAP...

    ...
    using System.DirectoryServices;

        ...
        public bool IsValidLDAPUser(string userName, string password, string ldapPath)
        {
            DirectoryEntry entry = new DirectoryEntry(ldapPath, userName, password);
            DirectorySearcher searcher = new DirectorySearcher(entry);
            searcher.SearchScope = SearchScope.OneLevel;

            try
            {
                SearchResult result = searcher.FindOne();
                return result != null ? true : false;
            }
            catch
            {
                return false;
            }
            finally
            {
                if (searcher != null) searcher.Dispose();
                if (entry != null) entry.Dispose();
            }
        }
        ...

Retrieving data from LDAP...

    //person 
    strFilter = String.Format("(&(objectCategory=person)(objectClass=user)({0}={1}))", strFieldNm, strValue);
    //distribution group 
    strFilter = String.Format("(&(objectCategory=group)(objectClass=group)({0}={1}))", strFieldNm, strValue);

    private SearchUsersList GetUsersOrDistributionGroups(string strFilter, string strLDAPPath, bool blnIsGroup) 
    { 
        SearchUsersList objResult = new SearchUsersList(); 
        DirectorySearcher search = null; 

        try 
        { 
            if (strFilter != String.Empty) 
            { 
                search = new DirectorySearcher(new DirectoryEntry(strLDAPPath), strFilter); 
            } 
            else 
            { 
                search = new DirectorySearcher(strFilter); 
            } 

            if (search != null) 
            { 
                foreach (SearchResult result in search.FindAll()) 
                { 
                    DirectoryEntry entry = result.GetDirectoryEntry(); 

                    if (entry.Properties["mail"].Value != null && !String.IsNullOrEmpty(entry.Properties["mail"].Value.ToString())) 
                    { 
                        SearchUsers objUser = new SearchUsers(); 
                        objUser.ID = entry.Properties["samaccountname"].Value.ToString(); 
                        objUser.MailAddress = entry.Properties["mail"].Value.ToString(); 
                        objUser.UserName = entry.Properties["name"].Value.ToString(); 
                        objUser.UserType = blnIsGroup ? "LDAP Distribution Group" : "LDAP User"; 

                        objResult.Add(objUser); 
                    } 
                } 
            } 
        } 
        catch 
        { 
            throw; 
        } 
        return objResult; 
    }