Question

我使用了一个课程，其中我有一个学生的付款费用的更新查询，其唯一ID是UID，并且他使用了帐户ID，即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;
        }
    }

现在我通过按钮点击事件调用此函数

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();
    }

我正在说

对象引用未设置为对象的实例

此错误显示在literal1

中

任何人都可以告诉我哪里出错了吗？我的意思是我看到所有的演化都已正确完成

现在使用（ctr + Alt + E）后，它在

处出错

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

说发生NullReferenceException

Answer 1

这是一个潜在的问题：

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

如果ExecuteScalar（）返回null，那么你的ToString（）将抛出null-ref异常。

编辑：因为它似乎是这一行，所以将其重写为：

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

使用调试器查看tmp是否为null。

EDIT2：

它可能是NULL。应使用 UPDATE ... 执行 ExecuteNonQuery sql语句，而不是 ExecuteScalar 。

在asp.net中执行更新查询时出错

1 个答案:

EDIT2：