error while executing Update query in the asp.net

Question

I have used a class in which i have this update query for Payment Fees of a student whose Unique ID is UID and an Account ID is used for him i.e. AccId

public string updatePays(string UID,int AccId,float hostel,float book,float exam,float reval,float studentDevelop,float misc,float reregistration,float late,float dues,float arrear,float grandtotal)
    {
        string SQLQuery = " UPDATE Pays SET ExamFees = " + exam + ",ReValuationFees = " + reval + ",Hostelfees = " + hostel + ",BookFees = " + book + ",StudentDevelopFees = " + studentDevelop + ",ReregistrationFees = " + reregistration + ",MiscFees = " + misc + ",LateFees=" + late + ",DuesFees=" + dues + ",Backfees=" + arrear + ",GrandFees=" + grandtotal + " WHERE UID = '" + UID + "' AND AccId = " + AccId + "";
        SqlCommand command = new SqlCommand(SQLQuery, sqlConnection);
        string flag = "";

        sqlConnection.Open();
        try
        {
            flag = command.ExecuteScalar().ToString();
            if (flag.ToString() != null)
            {
                sqlConnection.Close();
                return flag;
            }
            else
            {
                sqlConnection.Close();
                return "Student Fees has not been Updated ";
            }
        }
        catch (Exception exr)
        {
            sqlConnection.Close();
            return exr.Message;
        }
    }

Now I call this function from a button click event

protected void Button1_Click(object sender, EventArgs e)
    {
        DatabaseLayer data = new DatabaseLayer();

        string UID = Session["UID"].ToString();
        int AccId = AccId = (int)(Session["Acc"]);
        string semfees = Session["semFees"].ToString();

        float hostel = 0;
        float book = 0;
        float exam = 0;
        float reval = 0;
        float studentDevelop = 0;
        float misc = 0;
        float reregistration = 0;
        float late = 0;
        float dues = 0;
        float arrear = 0;
        float grandtotal = 0;
        float sf = float.Parse(semfees.ToString());

        if (float.TryParse(TextBox2.Text, out hostel)) { }
        if (float.TryParse(TextBox3.Text, out book)) { }
        if (float.TryParse(TextBox4.Text, out exam)) { }
        if (float.TryParse(TextBox5.Text, out reval)) { }
        if (float.TryParse(TextBox12.Text, out studentDevelop)) { }
        if (float.TryParse(TextBox6.Text, out misc)) { }
        if (float.TryParse(TextBox7.Text, out reregistration)) { }
        if (float.TryParse(TextBox8.Text, out late)) { }
        if (float.TryParse(TextBox9.Text, out dues)) { }
        if (float.TryParse(TextBox10.Text, out arrear)) { }

        string flag = "";

        grandtotal = sf + hostel + book + exam + reval + studentDevelop + misc + reregistration + late + dues + arrear;

        flag = data.updatePays(UID, AccId, hostel, book, exam, reval, studentDevelop, misc, reregistration, late, dues, arrear, grandtotal);
        Literal1.Text = flag.ToString();
    }

I am getting eror saying

Object reference not set to an instance of an object

This error was shown in the literal1

Can any one tell where am i going wrong? I mean i saw to it that all the delcaration is done properly

Now after using (ctr+Alt+E) it gave error at

flag = command.ExecuteScalar().ToString();

saying NullReferenceException occured

Answer 1

This is a potential problem:

  flag = command.ExecuteScalar().ToString();

If ExecuteScalar() returns null then your ToString() will throw the null-ref exception.

Edit: since it seems to be this line, rewrite it as:

object tmp command.ExecuteScalar();
flag = tmp.ToString();

And use the debugger to see if tmp is null.

Edit2:

And it probably will be NULL. An UPDATE ... sql statement should be executed with ExecuteNonQuery, not with ExecuteScalar.