Saving multiple data–part 31

I want to load the exchange rates from NBR and ECB and load at once .From here,the challenges of programming for a simple task like this:

1. How we can display the errors ?
2. What if the data exists already for this day and I cannot save into database,because it exists ?
3. How to acknowledge what exists and what not,in one operation ?
4. How to report success even if data exists ? Should we report number of records?
5.  How we can perform async all that stuff and,however,report errors ?

If you know the answer to all that,I have a challenge for you: see the file at https://github.com/ignatandrei/InfoValutar/blob/master/InfoValutar/InfovalutarLoadAndSave/LoadAndSaveLastData.cs– and improve it.

Until then,I come with this simple code

public class ResultsLoadBankData
    {
        public string Bank { get; internal set; }
        public int NrRecords { get; internal set; }
        public bool HasSuccess { get; internal set; }
        public string ErrorMessage { get; internal set; }
    }

//in some class below
public async Task<ResultsLoadBankData[]> LoadAndSave()
        {
            
            
            var items= providers.Banks().Select(it =>
                new KeyValuePair<string,ResultsLoadBankData>(it,
                new ResultsLoadBankData()
                {
                    Bank = it,
                    ErrorMessage=null,
                    HasSuccess=true,
                    NrRecords=0
                })
             );
            var lst = new Dictionary<string,ResultsLoadBankData>(items);


            var rates = 
                providers.LoadExchange()
                .Select(it => it.GetActualRates())
                .ToArray();
            //TODO: how to load async all async enumerables?
            //TODO: how to report error if one fails?
            foreach (var rateAsync in rates)
            {
                
                await foreach(var rate in rateAsync)
                {
                    var item = lst[rate.Bank];
                    try
                    {
                        
                        if (await ret.Exists(rate))
                            continue;
                        var nr = await save.Save(rate);
                        item.NrRecords++;
                    }
                    catch(Exception ex)
                    {
                        //TODO:log
                        item.ErrorMessage = ex.Message;
                        item.HasSuccess = false;
                    }
                }
                
                
            }
            return lst.Values.ToArray();
        }