objDataReader 是 Null - ASP.NET C#

Question

I am quite new to ASP.NET and C#, so I still do not have much of an idea as to how things work. I basically get an error when I run my program and create a maintenance task. My code is shown right below:

private DataTable getMaintenance()
        {
            DataTable maintenance_dt = new DataTable();
            maintenance_dt.Columns.Add("maintenance_ID");
            maintenance_dt.Columns.Add("DAILY_MAINTENANCE");
            maintenance_dt.Columns.Add("ADMIN_COMMENT");
            string SQLstr = "SELECT MAINTENANCE_ID,DAILY_MAINTENANCE,ADMIN_COMMENT FROM " + maintenance_table + " where " + key + " like " + value + " order by MAINTENANCE_ID ";
            using (DataTableReader objDataReader = OS.OSFunctions.executeSQLQuery(SQLstr))
            {
                while (objDataReader.Read())
                {
                    DataRow mItem = maintenance_dt.NewRow();
                    mItem[0] = objDataReader["MAINTENANCE_ID"].ToString();
                    mItem[1] = objDataReader["DAILY_MAINTENANCE"].ToString();
                    if (objDataReader["ADMIN_COMMENT"] != DBNull.Value)
                    {
                        mItem[2] = objDataReader["ADMIN_COMMENT"].ToString();
                    }
                    else
                    {
                        mItem[2] = "";
                    }
                    maintenance_dt.Rows.Add(mItem);
                }
            }
            return maintenance_dt;
        }

The error I get from running this states

Object reference not set to an instance of an object. objDataReader was null

This occurs when I attempt to create a maintenance task. The code for that is also below right here:

protected void createMaintenance_Click(object sender, System.EventArgs e)
        {
            string SQLstr;

                if (txtMaintenanceName.Text.Length > 0)
                {
                    if (maintenance_table == "ACTIVE_DAILYMAINTENANCE")
                    {
                        SQLstr = "SELECT TOP(1) MAINTENANCE_ID FROM ACTIVE_DAILYMAINTENANCE WHERE SCHEDULE_DATE = " + value + " ORDER BY MAINTENANCE_ID desc";
                        using (DataTableReader objDataReader = OS.OSFunctions.executeSQLQuery(SQLstr))
                        {
                            if (objDataReader.Read())
                            {
                                int id = Convert.ToInt32(objDataReader["Maintenance_ID"]) + 1;
                                SQLstr = "insert into " + maintenance_table + " (maintenance_id, DAILY_MAINTENANCE, " + key + ", ADMIN_COMMENT) values ('" + id + "',"
                                    + " '" + txtMaintenanceName.Text + "'," + value + ",'" + txtAdminMaintenanceComment.Text + "')";
                                OS.OSFunctions.executeSQLNonQuery(SQLstr);
                            }
                        }
                    }
                    else
                    {
                        SQLstr = "insert into " + maintenance_table + "(DAILY_MAINTENANCE, " + key + ", ADMIN_COMMENT) values ('" + txtMaintenanceName.Text + "'," + value + ",'" + txtAdminMaintenanceComment.Text + "')";
                        OS.OSFunctions.executeSQLNonQuery(SQLstr);
                    }
        }

Again, it is the getMaintenance() method giving me the error. This also isn't all my code, I do call the getMaintenance() function sometime later in the code for CreateMaintenance. Any help would be greatly appreciated.

EDIT: CODE TRYING OUT DATA SET

private DataSet getMaintenance()
{
   DataSet maintenance_ds = new DataSet();
            string SQLstr= "SELECT MAINTENANCE_ID,DAILY_MAINTENANCE,ADMIN_COMMENT FROM " + maintenance_table + " where " + key + " like " + value + " order by MAINTENANCE_ID ";
            using(SqlConnection connection=new SqlConnection(ConfigurationManager.ConnectionStrings["SQLConnectionString"].ConnectionString))
            {
                SqlDataAdapter adapter = new SqlDataAdapter();
                adapter.SelectCommand = new SqlCommand(SQLstr, connection);
                adapter.Fill(maintenance_ds);
                return maintenance_ds;
            }
}

Answer 1

So, you execute

DataTableReader objDataReader = OS.OSFunctions.executeSQLQuery(SQLstr)

in your using . SQLstr is

"SELECT MAINTENANCE_ID,DAILY_MAINTENANCE,ADMIN_COMMENT FROM " + maintenance_table + " where " + key + " like " + value + " order by MAINTENANCE_ID ";

You will need to use a debugger and jump to this line just before the error is thrown. First of all, you will need to find out what maintenance_table , key and value is. Try finding out what the generated query is and run it in your RDBMS, I think it will most likely return a null for some reason.

It is possible that you are just missing a wildcard character of % being wrapped around value if you have the intention to have a "contains" rather than an "equals" check.

Anyway, in order to detect what the error is you will need to find out what is being generated and why your query results in a null . Once you know what the problem is, you will also know what you need to fix, which largely simplifies the problem.

Since you do not use a parameterized query, I have to mention that if any of the dynamic values you concatenate to the query may come from untrusted sources, such as user input, then your query is vulnerable to SQL injection and you will need to protect your project against this potential exploit.

Answer 2

You do realize that you can send the sql to a datatable, and the columns and the data table is created for you.

so, use this code to get/return a data table.

It not clear if you "else" is to update a existing row, or insert a new one, but the code can look somthing like this:

  protected void createMaintenance_Click(object sender, System.EventArgs e)
    {
        DateTime value = DateTime.Today;
        string maintenance_table = "";
        string SQLstr = "";
        string key = "";

        if (txtMaintenanceName.Text.Length > 0)
        {

            if (maintenance_table == "ACTIVE_DAILYMAINTENANCE")
            {
                // add new row
                int id = NextMaintID(value);
                string strSQL = @"SELECT * FROM " + maintenance_table + " WHERE Maintenance_ID = 0";
                DataTable rstSched = MyRst(strSQL);
                DataRow MyNewRow = rstSched.NewRow();

                MyNewRow["maintenance_id"] = id;
                MyNewRow["DAILY_MAINTENANCE"] = txtMaintenanceName.Text;
                MyNewRow["ADMIN_COMMENT"] = txtAdminMaintenanceComment.Text;
                rstSched.Rows.Add(MyNewRow);
                MyUpdate(rstSched, strSQL);
            }
        }
        else
        {
            // update (or add to daily?????
            string strSQL = @"SELECT * FROM " + maintenance_table + " WHERE Maintenance_ID = " + key;
            DataTable rstSched = MyRst(strSQL);
            DataRow MyRow = rstSched.Rows[0];
            MyRow["DAILY_MAINTENANCE"] = txtMaintenanceName.Text;
            MyRow["ADMIN_COMMENT"] = txtAdminMaintenanceComment.Text;
            MyUpdate(rstSched, strSQL);
        }
    }

So, I only need a few helper routines - (make them global in a static class - you can then use it everywhere - saves boatloads of code.

so these were used:

    public DataTable MyRst(string strSQL)
    {
        // return data table based on sql
        DataTable rstData = new DataTable();
        using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.TEST4))
        {
            using (SqlCommand cmdSQL = new SqlCommand(strSQL, conn))
            {
                cmdSQL.Connection.Open();
                rstData.Load(cmdSQL.ExecuteReader());
            }
        }
        return rstData;
    }


    public DataTable MyRstP(SqlCommand cmdSQL)
    {
        // return data table based on sql command (for parmaters)
        DataTable rstData = new DataTable();
        using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.TEST4))
        {
            using (cmdSQL)
            {
                cmdSQL.Connection = conn;
                conn.Open();
                rstData.Load(cmdSQL.ExecuteReader());
            }
        }
        return rstData;
    }

    void  MyUpdate(DataTable rstData, string strSQL)
    {
        using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.TEST4))
        {
            using (SqlCommand cmdSQL = new SqlCommand(strSQL, conn))
            {
                conn.Open();
                SqlDataAdapter da = new SqlDataAdapter(cmdSQL);
                SqlCommandBuilder daU = new SqlCommandBuilder(da);
                da.Update(rstData);
            }
        }
    }

and of course this:

    int NextMaintID (DateTime value)
    {
        int result = 0;
        string SQLstr = @"SELECT TOP(1) MAINTENANCE_ID FROM ACTIVE_DAILYMAINTENANCE 
                        WHERE SCHEDULE_DATE = @scDate ORDER BY MAINTENANCE_ID desc";
        SqlCommand cmdSQL = new SqlCommand(SQLstr);
        cmdSQL.Parameters.Add("@scDate", SqlDbType.Date).Value = value;

        DataTable rstNextID = MyRstP(cmdSQL);
        result = ((int)rstNextID.Rows[0]["Maintenance_ID"]) + 1;

        return result;

    }

So, how do you eat a elephant? Answer: One bite at a time!!!

So, break out just a "few" helper routines that allows data operations against a data table object. That update command will work with edits, adds to rows, and even delete row method of a single row. All such updates can be thus be done with ONE simple update command.