Error when checking if SqlDataReader column has null value

Question

One of the guys working on a project with me keeps getting an error when the code trys to validate if SqlDataReader returns a null value in a column. He runs this code:

if (!DB_Conn.isConnected()) DB_Conn.Connect();
using (SqlDataReader dr = DB_Conn.QueryDB(query))
{
   if (dr.HasRows && !dr.IsDBNull(0))
   {
       maxID = dr.GetInt32(0);
   }
}

But gets an error that Its an invalid attemtp to read when no data is present at the !dr.IsDBNull(0) command.

If i run this same code but i query a different table, it works.

Also, i run both queries and they return the expected null value. The querys are:

SELECT MAX(ID) FROM Loan;
SELECT MAX(ID) FROM InternationalSwap;

I dont think the querys have any affect on the reason why we are getting this error at one machine and not the other.

Answer 1

You need to call the Read method before trying to access any columns:

while (dr.Read())
{
    if (!dr.IsDBNull(0))
        maxID = dr.GetInt32(0);
}

But... if you only need a single value then you should probably use something like ExecuteScalar rather than a datareader.