如何从SqlDataReader同时读取两行值

Question

How can I read values from two consecutive rows from SqlDataReader at the same time ?!

SqlCommand query2 = new SqlCommand("SELECT lon , lat FROM Student Where B_ID = @B_ID ORDER BY Distance ASC");
// some value for @B_ID
            query2.Connection = cn;
SqlDataReader readStd = query2.ExecuteReader();
while (readStd.Read())
            {
                // here I want to calculate the distance between each consecutive rows 
// by taking the latitude and longitude from each row
                    double d = DistanceBetweenPlaces(Convert.ToDouble(readStd.GetValue(0)), Convert.ToDouble(readStd.GetValue(1)), ???, ???);
                    totalDistance = totalDistance + d;
                //}
            }

Answer 1

You can't do that directly. Each call to SqlDataReader.Read advances the reader, allowing you to retrieve the values of the row it is pointing too. Therefore you can't "read two rows at the same time". Refer to the MSDN Documentation .

What you can do, however, is using some temporary values like that :

SqlCommand query2 = new SqlCommand("SELECT lon , lat FROM Student Where B_ID = @B_ID ORDER BY Distance ASC");
    // some value for @B_ID
    query2.Connection = cn;
    SqlDataReader readStd = query2.ExecuteReader();
    double tempLong = 0.0;
    double tempLat = 0.0;
    bool isFirstIteration = true;
    while (readStd.Read())
        {
            if(isFirstIteration)
            {
                isFirstIteration = false;
            }
            else
            {
                double d = DistanceBetweenPlaces(tempLong, tempLat, Convert.ToDouble(readStd.GetValue(0)), Convert.ToDouble(readStd.GetValue(1)));
                totalDistance = totalDistance + d;
            }
            tempLong = Convert.ToDouble(readStd.GetValue(0));
            tempLat = Convert.ToDouble(readStd.GetValue(1));            
        }

Not tested (and maybe wrong because I don't know your method definition), but I think you will get the general idea.