无法从SQL Server数据库读取浮点值

Question

I use SQL Server to build my database and SqlDataReader to read data from it.

command.Connection = cn;
command.CommandText = "SELECT * FROM test";
SqlDataReader rd = command.ExecuteReader();
while(rd.Read())
{
    double d = (double) rd.GetValue(0);
}

The column (0) I am trying to get value from is a 'float' type and has value '3.5' . As mapping data type from this MSDN link , the type of the object returned by rd.GetValue(0) must be 'double'. But the code above returns to variable 'd' value '0.0'. I tried this line:

double d = Convert.ToDouble(rd.GetValue(0));

But it still returns '0.0' to variable 'd'.

I tried searching on Google and StackOverflow but there is no result.

What am I missing? Help me!

Answer 1

As it is now, your code iterates over all the records (if there are many) an takes the last entry, which since you have no order by clause, may differ in every query execution. If indeed you want to only take 1 value, use ExecuteScalar together with an order by clause:

command.Connection = cn;
command.CommandText = "SELECT  TOP 1 * FROM test order by myfield desc"; //or asc
double result = (double)command.ExecuteScalar();

Otherwise have all the result saved in a list:

...

List<double> result = new List<doulbe>();
while(rd.Read())
{
    result.Add(double.Parse(rd[0].ToString());
}

Finally, if you need only the 1st field, for performance reasons, is far better not to use * but explicit set the field you want:

"SELECT  TOP 1 myfield  FROM test order by myfield desc"; //or asc

Answer 2

you can try it;

double d = (double) rd.GetValue(0);

to

double d = 0;
double.TryParse(rd["ColumnName"].ToString().Replace('.',','),out d);    

OR:

double d = double.Parse(rd["ColumnName"].ToString(), CultureInfo.InvariantCulture); 

Answer 3

This here works fine for me, im getting 3,5 in my list

List<double> columnData = new List<double>();
            using (SqlConnection connection = new SqlConnection("Server=EGC25199;Initial Catalog=LegOgSpass;Integrated Security=SSPI;Application Name=SQLNCLI11.1"))
            {
                connection.Open();
                    string query = "SELECT * FROM [dbo].[floattable]";
                using (SqlCommand command = new SqlCommand(query, connection))
                {
                    using (SqlDataReader reader = command.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            columnData.Add(reader.GetDouble(0));

                        }
                    }
                }
            }

Answer 4

Oh I have found the answer. Nothing wrong with the code I wrote. The problem is that I place the breakpoint on the 'double d = (double) rd.GetValue(0)' line. That is, 'd' value is not assigned yet so that on the debug screen it returns '0.0'. Sorry for this mistake and thank you all Stack-Over-flowers for spending your time helping me!!!!