指定的转换无效-System.InvalidCastException

Question

I've read many of similar questions and tried several different things, but I do not understand why this is not working. The column that is being summed is of the int type. I have tried casting to uint and int32. I appreciate direction. Thank you.

        int i = 1;
        int total=0;
        while (i < 7)
        {

            string conString = @"Data Source=UATDB2\sqleuat;Initial Catalog=CVNee;User ID=appuser;Integrated Security=true";

            SqlConnection connection = new SqlConnection(conString);
            connection.Open();
            SqlCommand cmd = connection.CreateCommand();
            cmd.CommandText = "SELECT SUM(AMOUNT) FROM TRANSACTIONS_DETAIL2 WHERE TRANSACTIONS_DETAIL2.TRANS_TYPE =" + i + "AND TRANSACTIONS_DETAIL2.HOH_UPI = '185292000'";

           int amt = ((int)cmd.ExecuteScalar());
            //amt = cmd.ExecuteScalar();

            if (i == 1)
            {
                total = total + amt;
            }

            if (i == 2)
            {
                total = total - amt;
            }

            if (i == 3)
            {
                total = total - amt;
            }

            if (i == 4)
            {
                total = total + amt;
            }

            if (i == 5)
            {
                total = total + amt;
            }

            if (i == 6)
            {
                total = total + amt;
            }

            connection.Close();
            ViewBag.TotalBalance = total;

            i++;
        }

Answer 1

The illegal cast exception occurs, when ExecuteScalar returns null . You could either check if the result is null before converting to int ,

 object o = cmd.ExecuteScalar();
 int result = o == null ? 0 : (int)o;

or you could use a nullable int int? which can also be checked if it has a value

 int? result = (int?)cmd.ExecuteScalar();
 if (!result.HasValue) result = 0;

or you could adjust your query to return 0 even if no matching rows are found. The TSQL function COALESCE checks if the first parameter is NULL , if no it returns the value, if yes, it returns the value of the second parameter.

 string query = "SELECT COALESCE(SUM(AMOUNT),0) FROM TRANSACTIONS_DETAIL2 WHERE TRANSACTIONS_DETAIL2.TRANS_TYPE =" + i + "AND TRANSACTIONS_DETAIL2.HOH_UPI = '185292000'";

Answer 2

according to your comment

If you call ExecuteScalar , you might get null or DBNull in return, and you should check both. You can first fill it into a variable of type object , then check for DBNull.Value and null and react on this:

int amt=0;
object amtUnchecked = cmd.ExecuteScalar();
if(amtUnchecked != DBNull.Value && amtUnchecked!=null)
    amt=(int)amtUnchecked;

Or you change the SELECT to return a sure zero: SELECT ISNULL(SUM(AMOUNT),0) ...