指定的强制转换无效-C＃

Question

I'm trying to do an AppendLine with this

reportLayout.AppendLine(
  "Sold Items".PadRight(25) + 
   Convert.ToString(BatchCalculation.NoOfItems(dBCommand, transDate, reportType)));

And I'm getting an exception thrown

Specified Cast is not valid

my NoOfItems is a static method and it returns an int

EDIT: Last lines of code in NoOfItems method

            ...
            using (DbDataReader reader = dBCommand.ExecuteReader())
                {
                    if (reader.HasRows)
                    {
                        while (reader.Read())
                        {
                            //hsBatch.currentTC = (int)reader["CURRENTTC"];
                            //hsBatch.currentAmt = (decimal)reader["CURRENTPYMTAMT"];
                            //Cancellation fee
                            hsBatch.TotalNoOfitems = (int)reader["TOTALNOOFITEMS"];
                        }
                    }
                }
            }
        }
        finally
        {
            if (connection.State == ConnectionState.Open)
                connection.Close();
        }

        return hsBatch.TotalNoOfitems;
    }

Answer 1

数据库可以返回null，所以我建议检查DBNull

hsBatch.TotalNoOfitems = reader["TOTALNOOFITEMS"] != DBNull.Value ? int.Parse(reader["TOTALNOOFITEMS"].ToString()):0;

Answer 2

It seems you have a problem with type mapping ( RDBMS Number is not necessary .Net int ):

   // (int) - cast like this may well be incorrect:
   // reader["TOTALNOOFITEMS"] is not nesseary int or can be cast to int
   hsBatch.TotalNoOfitems = (int)reader["TOTALNOOFITEMS"]; 

Instead of

  if (reader.HasRows)
  {
      while (reader.Read())
      {
          //hsBatch.currentTC = (int)reader["CURRENTTC"];
          //hsBatch.currentAmt = (decimal)reader["CURRENTPYMTAMT"];
          //Cancellation fee
          hsBatch.TotalNoOfitems = (int)reader["TOTALNOOFITEMS"];
      }
  }

Put

  // if: we don't want "while" since we read one record only
  // reader.HasRows is redundant - if we manage to read a record we have it  
  if (reader.Read()) 
  {
      // hsBatch.currentTC = Convert.ToInt32(reader["CURRENTTC"]);
      // hsBatch.currentAmt = Convert.ToDecimal(reader["CURRENTPYMTAMT"]);

      // What ever RDBMS type mapping is (byte, short, int, long, decimal 
      // or even string - depending on query, RDBMS and settings)
      // try convert it into int - Int32 
      hsBatch.TotalNoOfitems = Convert.ToInt32(reader["TOTALNOOFITEMS"]); 
  } 

There's one more (possible) issue: if TOTALNOOFITEMS field contains NULL value. If it's your case, you can put

      hsBatch.TotalNoOfitems = reader["TOTALNOOFITEMS"] == DBNull.Value
        ? 0 // or whatever value in case of null
        : Convert.ToInt32(reader["TOTALNOOFITEMS"]);