how to execute sql select statement in c#

Question

i have a little problem here: i have a combobox that gets its values from column in database and i use it to enter data back to another palce in database like

comboBox2.DataSource = ds1.Tables[0];
comboBox2.DisplayMember = "DoctorName";
comboBox2.ValueMember = "DoctorCode";
comboBox2.BindingContext = this.BindingContext;

this fill the combobox with the name of doctors and the value will be the code of doctor then

 SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=c:\users\administrator\documents\visual studio 2010\Projects\Clinic\Clinic\Clinc.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True");
        con.Open();
        SqlCommand cmd1 = new SqlCommand("SELECT   Doctors.DoctorCode, Doctors.DoctorName, SessionReservations.SessionCode, SessionReservations.PatientCode, SessionReservations.ExaminationCode, SessionReservations.DoctorCode AS Expr1, SessionReservations.SessionMonth, SessionReservations.SessionYear   FROM  Doctors INNER JOIN   SessionReservations ON Doctors.DoctorCode = SessionReservations.DoctorCode    WHERE  (Doctors.DoctorCode = @DoctorCode) AND (SessionReservations.SessionMonth = @month) AND (SessionReservations.SessionYear = @year)", con);
        SqlDataAdapter da2 = new SqlDataAdapter(cmd1);
        DataSet ds2 = new DataSet();

        try
        {

            da2.InsertCommand.Parameters.Add("@DoctorCode", SqlDbType.Int).Value = Convert.ToInt32(comboBox2.SelectedValue);
            da2.InsertCommand.Parameters.Add("@month", SqlDbType.Int).Value = comboBox1.SelectedValue;
            da2.InsertCommand.Parameters.Add("@year", SqlDbType.Int).Value = textBox2.Text;

            da2.Fill(ds2);
            cmd1.ExecuteReader();
            con.Close();
}

this code is for selecting specific rows and the select statment is working right in sql manager but while running it gives error that

"System.NullReferenceException: Object reference not set to an instance of an object. at Clinic.DoctorMoneyCall.button1_Click(Object sender, EventArgs e) in C:\Users\Administrator\documents\visual studio 2010\Projects\Clinic\Clinic\DoctorMoneyCall.cs:line 45"

i just don't understand what's going wrong

@fenonoga - This is just a null reference problem. This isn't unique to the running a SQL query. — Security Hound
– Security Hound, Commented Nov 26, 2012 at 14:34
try debuging and see if your checkboxes and text box have any value to pass as a parameter — Gustav Klimt
– Gustav Klimt, Commented Nov 26, 2012 at 14:36
@MarcGravell line 45 is where it adds the doctor code da2.InsertCommand.Parameters.Add("@DoctorCode", SqlDbType.Int).Value = Convert.ToInt32(comboBox2.SelectedValue); — feno noga
– feno noga, Commented Nov 26, 2012 at 15:39

gashach · Accepted Answer · 2012-11-26 15:17:34Z

2

It seems like you are trying to run a select command butt you are adding your parameters to the insert command.

        da2.SelectCommand.Parameters.Add("@DoctorCode", SqlDbType.Int).Value = Convert.ToInt32(comboBox2.SelectedValue);
        da2.SelectCommand.Parameters.Add("@month", SqlDbType.Int).Value = comboBox1.SelectedValue;
        da2.SelectCommand.Parameters.Add("@year", SqlDbType.Int).Value = textBox2.Text;

answered Nov 26, 2012 at 15:17

gashach

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4 Comments

feno noga Over a year ago

yeah i want select different thing each time, so i have to make variables in the query statment

gashach Over a year ago

but you added them to the Insert command, not the select command. SQL commands have 4 states: Select, Update, Insert, Delete. it comes from the SQL querys you can run. I changed your "insertcommand"'s to "selectCommand"'s

feno noga Over a year ago

oh my god thank you very much, i feel stupid actually trying for days to know where the mistake, very stupid thing, i have to be more attention again thank you

gashach Over a year ago

no prob! its often something simple. happened to me recently too.

Luo · Accepted Answer · 2012-11-26 14:30:54Z

0

The error message means that one of the objects you used was null. Is the con object correctly initialized?

answered Nov 26, 2012 at 14:30

Luo

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1 Comment

feno noga Over a year ago

yes the connection is working right and i have displayed the values of my textboxes and comboboxes in labels to be sure they working right and they do

hermann · Accepted Answer · 2012-11-26 14:31:26Z

0

It seems like you're passing a variable that is null in line 45 of that document. Make sure all values are non-null.

answered Nov 26, 2012 at 14:31

hermann

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1 Comment

Security Hound Over a year ago

This doesn't make a great deal of sense. What he is passing couldn't be null since its the contents of a control that exists on the form and both of those values have a default value.

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