Null out data dictionary after every run

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C#

WinForms

I have a very difficult question to ask, hopefully it will make sense. I am reading two different types of text files and checking the records within a dictionary. The textfiles can have different dates so I need to dump the dictionary after it checks one file and read again through database to fetch the new date of the textfile. How do I dumb the dictionary after every run?

What I have tried:

private Dictionary<DateTime, Dictionary<string, long>> DataByNum;
       private Dictionary<DateTime, Dictionary<string, long>> DataByID;

       private Dictionary<string, long> GetNumData(DateTime DailyFileDate)
       {
           DailyFileDate = DailyFileDate.Date;
           if (DataByNum == null || !DataByNum.ContainsKey(DailyFileDate))
           {
               CacheData(DailyFileDate);
           }
           return DataByNum[DailyFileDate];
       }

       private Dictionary<string, long> GetIDData(DateTime DailyFileDate)
       {
           DailyFileDate = DailyFileDate.Date;
           if (DataByID == null || !DataByID.ContainsKey(DailyFileDate))
           {
               CacheData(DailyFileDate);
           }
           return DataByID[DailyFileDate];
       }

       private void CacheData(DateTime DailyFileDate)
       {
           DailyFileDate = DailyFileDate.Date;

           if (DataByNum == null)
           {
               DataByNum = new Dictionary<DateTime, Dictionary<string, long>>();
           }
           if (DataByID == null)
           {
               DataByID = new Dictionary<DateTime, Dictionary<string, long>>();
           }


           var result = ExtractData(DailyFileDate);


           foreach (DataRow row in result)
           {
               var date = DateTime.Parse(row.GetString("FileDate")).Date;
               if (!DataByNum.ContainsKey(date))
               {
                   DataByNum[date] = new Dictionary<string, long>();
               DataByNum[date].Add(row.GetString("Num"), row.GetLong("RID").Value);
               }
               if (!DataByID.ContainsKey(date))
               {
                   DataByID[date] = new Dictionary<string, long>();
               DataByID[date].Add(row.GetString("CustomerID"), row.GetLong("RID").Value);
               }
           }