添加 3 个标签控件的值以在另一个标签控件中输出结果时出错

Question

Here is the full description of what I am trying to achieve.

The way I designed it is that I have

• 3 Checkboxes
• 3 DropDown controls: Quantity
• 7 Label controls: 3 Labels for Unit Price, 3 Labels for SubTotal and one Label for GrandTotal

On page load, DropDown will be disabled, Unit Price Labels will have values as "100", "200" and "100" respectively and the 3 SubTotal Labels will have initial values as "0", and GrandTotal Label will be "0".

When a user clicks on any of the Checkboxes, the DropDown attached to that clicked Checkbox will be enabled and when user selects quantity from DropDown, the value of Unit Price Label will be multiplied by the value in the quantity DropDown and output the result in SubTotal Labels.

Finally, all the values of SubTotal Labels will be added and the result will be displayed in the GrandTotal Label.

This is how the view will look like

Product        Unit Price       Quantity     SubTotal      GrandTotal

Product1          100              3            300

Product2          200              1            200

Product3          100              2            200          
                                                               700

Here is my CODE (forgive the way I display my code. I tried to upload my code but it did not enter the code window, since I'm using mobile device to ask this question)

protected void Page_Load(object sender, EventArgs e)
{
    unitprice1.Text = ("100").ToString();
    unitprice2.Text = ("200").ToString();
    unitprice3.Text = ("100").ToString();
}

protected void CheckBox1_CheckedChanged(object sender, EventArgs e) 
{
    Product1DropDown.Enabled = CheckBox1.Checked;
    Subtotal1.Text = ("0").ToString();
}

protected void CheckBox2_CheckedChanged(object sender, EventArgs e)
{
    Product2DropDown.Enabled = CheckBox2.Checked;
    Subtotal2.Text = ("0").ToString();
}

protected void CheckBox3_CheckedChanged(object sender, EventArgs e)
{
    Product3DropDown.Enabled = CheckBox3.Checked;
    Subtotal3.Text = ("0").ToString();
}

protected void Product1DropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    if (Product1DropDown.SelectedIndex > 0)
    {
        Qty1.Text = Product1DropDown.SelectedItem.Text;
        Subtotal1.Text = Convert.ToString(Convert.ToInt32(Qty1.Text) * Convert.ToInt32(unitprice1.Text)).ToString();
    }
}

protected void Product2DropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    if (Product2DropDown.SelectedIndex > 0)
    {
        Qty2.Text = Product2DropDown.SelectedItem.Text;
        Subtotal2.Text = Convert.ToString(Convert.ToInt32(Qty2.Text) * Convert.ToInt32(unitprice2.Text)).ToString();
    }
}

protected void Product3DropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    if(Product3DropDown.SelectedIndex > 0)
    {
        Qty3.Text = Product3DropDown.SelectedItem.Text;Subtotal3.Text = Convert.ToString(Convert.ToInt32(Qty3.Text) * Convert.ToInt32(unitprice3.Text)).ToString();
    }
}    

Answer 1

Ensure that you've included AutoPostBack="true" for each CheckBox :

<asp:CheckBox ID="checkBox1" runat="server" Checked="false" OnCheckedChanged="checkBox1_CheckedChanged" AutoPostBack="true" />

and also for each DropDownList :

<asp:DropDownList id="product1DropDown" runat="server" OnSelectedIndexChanged="product1DropDown_SelectedIndexChanged" AutoPostBack="true" Enabled="false" style="align-content:center;width: 75px" />

Create a method to calculate the subtotal. Since we have to calculate subtotals for 3 different products, we'll create a method. The parameters will be the quantity from the DropDownList, and the unit price from the label.

private decimal CalculateSubtotal(string strQty, string strUnitPrice)
{
    int qty = 0;
    decimal unitPrice = 0m;
    decimal subtotal = 0m;

    //clear msg
    lblMsg.Text = string.Empty;

    if (!String.IsNullOrEmpty(strQty))
    {
        //try to convert
        Int32.TryParse(strQty, out qty);
    }
    else
    {
        throw new Exception("Error: Qty is null or empty");
    }

    if (!String.IsNullOrEmpty(strUnitPrice))
    {
        //try to convert
        Decimal.TryParse(strUnitPrice, out unitPrice);

        //calculate
        subtotal = qty * unitPrice;
    }
    else
    {
        throw new Exception("Error: UnitPrice is null or empty");
    }
    
    return subtotal;
}

 

Next, create a method to calculate the grand total. This method will call CalculateSubtotal for each of the products.

CalculateGrandTotal

private void CalculateGrandTotal()
{
    decimal subtotal1 = 0m; //use 'm' for decimal
    decimal subtotal2 = 0m; //use 'm' for decimal
    decimal subtotal3 = 0m; //use 'm' for decimal

    //calculate subtotal1 - only calculate if CheckBox is enabled
    if (checkBox1.Enabled && !String.IsNullOrEmpty(product1DropDown.Text))
    {
        //calculate subtotal
        subtotal1 = CalculateSubtotal(product1DropDown.Text, lblProduct1UnitPrice.Text);
    }

    //set value
    lblProduct1Subtotal.Text = subtotal1.ToString("$###,##0.00");

    //calculate subtotal 2 - only calculate if CheckBox is enabled
    if (checkBox2.Enabled && !String.IsNullOrEmpty(product2DropDown.Text))
    {
        //calculate subtotal
        subtotal2 = CalculateSubtotal(product2DropDown.Text, lblProduct2UnitPrice.Text);
    }

    //set value
    lblProduct2Subtotal.Text = subtotal2.ToString("$###,##0.00");

    //calculate subtotal3 - only calculate if CheckBox is enabled
    if (checkBox3.Enabled && !String.IsNullOrEmpty(product3DropDown.Text))
    {
        //calculate subtotal
        subtotal3 = CalculateSubtotal(product3DropDown.Text, lblProduct3UnitPrice.Text);
    }

    //set value
    lblProduct3Subtotal.Text = subtotal3.ToString("$###,##0.00");

    //calculate
    decimal grandTotal = subtotal1 + subtotal2 + subtotal3;

    //set value
    lblGrandTotal.Text = grandTotal.ToString("$###,##0.00");
}

In each of the CheckBox CheckChanged event handlers, not only do we need to determine whether or not to enable the drop down, but also re-calculate the grand total. I've also decided to reset the quantity and subtotal to 0 if the checkbox is disabled.

ex :

protected void checkBox1_CheckedChanged(object sender, EventArgs e)
{
    ////uncomment for debugging
    //lblMsg.Text = "Info: checkBox1.Checked: " + checkBox1.Checked.ToString();
    //System.Diagnostics.Debug.WriteLine("Info: checkBox1.Checked: " + checkBox1.Checked.ToString());

    //set value
    product1DropDown.Enabled = checkBox1.Checked;

    if (!product1DropDown.Enabled)
    {
        //not enabled - reset values
        product1DropDown.Text = string.Empty;
        lblProduct1Subtotal.Text = "$0.00";
    }

    //calculate
    CalculateGrandTotal();
}

Since, we need to re-calculate the grand total each time a quantity is changed, as well as, the subtotal for that particular product, we just call CalculateGrandTotal(); .

ex :

protected void product1DropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    CalculateGrandTotal();
}

Here's the complete solution-it's been tested:

VS 2019 :

Create a new ASP.NET Web Application (.NET Framework)

• Project Name: BMICalc
• Click Create
• Click Empty
• Click Create

Add Web Form

• In VS menu, click Project
• Select Web Form (name: default.aspx)

default.aspx :

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="TestAspNetDU._default" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">

        <div style="top: 10px; left: 150px; position: absolute;">

            <asp:Table ID="tbl1" BorderStyle="none" Border="0px" CellPadding="2" CellSpacing="0" runat="server" Width="410">
                <asp:TableHeaderRow ID="tbl1HeaderRow">
                    <asp:TableHeaderCell ID="tbl1CheckBoxHeading" Text="" style="text-align:center" />
                    <asp:TableHeaderCell ID="tbl1ProductHeading" Text="Product" style="text-align:center" />
                    <asp:TableHeaderCell ID="tbl1UnitPriceHeading" Text="Unit Price" style="text-align:center" />
                    <asp:TableHeaderCell ID="tbl1QuantityHeading" Text="Quantity" style="text-align:center" />
                    <asp:TableHeaderCell ID="tbl1SubtotalHeading" Text="Subtotal" style="text-align:center" />
                </asp:TableHeaderRow>
                
                <asp:TableRow ID="tbl1Row1">
                    <asp:TableCell Width="35px" style="align-content:center;text-align:center;vertical-align:middle">
                        <asp:CheckBox ID="checkBox1" runat="server" Checked="false" OnCheckedChanged="checkBox1_CheckedChanged" AutoPostBack="true" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="text-align:left">
                        <asp:Label id="lblProduct1Name" runat="server" Text="Product1" style="text-align:left" />
                    </asp:TableCell>

                    <asp:TableCell Width="75px" style="align-content:center;text-align:center">
                        <asp:Label id="lblProduct1UnitPrice" runat="server" Text="100" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="align-content:center;text-align:center">
                        <asp:DropDownList id="product1DropDown" runat="server" OnSelectedIndexChanged="product1DropDown_SelectedIndexChanged" AutoPostBack="true" Enabled="false" style="align-content:center;width: 75px" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="text-align:right">
                        <asp:Label id="lblProduct1Subtotal" runat="server" Text="0.00" />
                    </asp:TableCell>
                </asp:TableRow>

               
                <asp:TableRow ID="tbl1Row2">
                    <asp:TableCell Width="35px" style="align-content:center;text-align:center;vertical-align:middle">
                        <asp:CheckBox ID="checkBox2" runat="server" Checked="false" OnCheckedChanged="checkBox2_CheckedChanged" AutoPostBack="true" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="text-align:left">
                        <asp:Label id="lblProduct2Name" runat="server" Text="Product2" style="text-align:left" />
                    </asp:TableCell>

                    <asp:TableCell Width="75px" style="align-content:center;text-align:center">
                        <asp:Label id="lblProduct2UnitPrice" runat="server" Text="200" style="text-align:center" />
                    </asp:TableCell>

                    <asp:TableCell Width="100" style="align-content:center;text-align:center">
                        <asp:DropDownList id="product2DropDown" runat="server" OnSelectedIndexChanged="product2DropDown_SelectedIndexChanged" AutoPostBack="true" Enabled="false" style="align-content:center;width: 75px" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="text-align:right">
                        <asp:Label id="lblProduct2Subtotal" runat="server" Text="0.00" />
                    </asp:TableCell>
                </asp:TableRow>

               
                <asp:TableRow ID="tbl1Row3">
                    <asp:TableCell Width="35px" style="align-content:center;text-align:center;vertical-align:middle">
                        <asp:CheckBox ID="checkBox3" runat="server" Checked="false" OnCheckedChanged="checkBox3_CheckedChanged" AutoPostBack="true" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="text-align:left">
                        <asp:Label id="lblProduct3Name" runat="server" Text="Product3" style="text-align:left" />
                    </asp:TableCell>

                    <asp:TableCell Width="75px" style="align-content:center;text-align:center">
                        <asp:Label id="lblProduct3UnitPrice" runat="server" Text="100" style="text-align:center" />
                    </asp:TableCell>

                    <asp:TableCell Width="100" style="align-content:center;text-align:center">
                        <asp:DropDownList id="product3DropDown" runat="server" OnSelectedIndexChanged="product3DropDown_SelectedIndexChanged" AutoPostBack="true" Enabled="false" style="align-content:center;width: 75px" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="text-align:right">
                        <asp:Label id="lblProduct3Subtotal" runat="server" Text="0.00" />
                    </asp:TableCell>
                </asp:TableRow>
            </asp:Table>
        </div>

        <div style="top: 120px; left: 150px; position: absolute">
            <asp:Table ID="tbl1GrandTotal" BorderStyle="none" Border="0px" CellPadding="0" CellSpacing="0" runat="server" Width="410">
                <asp:TableRow ID="tbl1GrandTotalRow">
                    <asp:TableCell Width="160px" style="align-content:center;text-align:center;vertical-align:middle"></asp:TableCell>

                    <asp:TableCell Width="125px" style="font-weight:bold; text-align:center">
                        <asp:Label id="lblGrandTotalHeading" runat="server" Text="Grand Total:" />
                    </asp:TableCell>

                    <asp:TableCell Width="100px" style="font-weight:bold; text-align:right">
                        <asp:Label id="lblGrandTotal" runat="server" Text="0.00" />
                    </asp:TableCell>
                </asp:TableRow>
            </asp:Table>
        </div>

        <div style="top: 625px; left: 150px; position: absolute">
            <span style="color:red">
                <asp:Label ID="lblMsg" runat="server" Text="" />
            </span>
        </div>
    </form>
</body>
</html>

default.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace TestAspNetDU
{
    public partial class _default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            //set values
          
            lblProduct1UnitPrice.Text = "100";
            lblProduct2UnitPrice.Text = "200";
            lblProduct3UnitPrice.Text = "100";

            //populate DropDownList
            for(int i = 0; i < 100; i++)
            {
                if (i > 0)
                {
                    //add
             
                    product1DropDown.Items.Add(i.ToString());
                    product2DropDown.Items.Add(i.ToString());
                    product3DropDown.Items.Add(i.ToString());
                }
                else
                {
                    //add - for 0, show empty (blank)
                    product1DropDown.Items.Add("");
                    product2DropDown.Items.Add("");
                    product3DropDown.Items.Add("");
                }
            }
        }

        private void CalculateGrandTotal()
        {
            decimal subtotal1 = 0m; //use 'm' for decimal
            decimal subtotal2 = 0m; //use 'm' for decimal
            decimal subtotal3 = 0m; //use 'm' for decimal

            try
            {
                //calculate subtotal1 - only calculate if CheckBox is enabled
                if (checkBox1.Enabled && !String.IsNullOrEmpty(product1DropDown.Text))
                {
                    //calculate subtotal
                    subtotal1 = CalculateSubtotal(product1DropDown.Text, lblProduct1UnitPrice.Text);
                }

                //set value
                lblProduct1Subtotal.Text = subtotal1.ToString("$###,##0.00");
            }
            catch(Exception ex)
            {
                //ToDo: log msg

                //set value
                lblMsg.Text = "Error (CalculateGrandTotal 1) - " + ex.Message;

                //uncommenting the following line may be useful for debugging
                //throw ex;
            }

            try
            {
                //calculate subtotal 2 - only calculate if CheckBox is enabled
                if (checkBox2.Enabled && !String.IsNullOrEmpty(product2DropDown.Text))
                {
                    //calculate subtotal
                    subtotal2 = CalculateSubtotal(product2DropDown.Text, lblProduct2UnitPrice.Text);
                }

                //set value
                lblProduct2Subtotal.Text = subtotal2.ToString("$###,##0.00");
            }
            catch(Exception ex)
            {
                //ToDo: log msg

                //set value
                lblMsg.Text = "Error (CalculateGrandTotal 2) - " + ex.Message;

                //uncommenting the following line may be useful for debugging
                //throw ex;
            }

            try
            {
                //calculate subtotal3 - only calculate if CheckBox is enabled
                if (checkBox3.Enabled && !String.IsNullOrEmpty(product3DropDown.Text))
                {
                    //calculate subtotal
                    subtotal3 = CalculateSubtotal(product3DropDown.Text, lblProduct3UnitPrice.Text);
                }

                //set value
                lblProduct3Subtotal.Text = subtotal3.ToString("$###,##0.00");
            }
            catch(Exception ex)
            {
                //ToDo: log msg

                //set value
                lblMsg.Text = "Error (product3DropDown_SelectedIndexChanged) - " + ex.Message;

                //uncommenting the following line may be useful for debugging
                //throw ex;
            }

            //calculate
            decimal grandTotal = subtotal1 + subtotal2 + subtotal3;

            //set value
            lblGrandTotal.Text = grandTotal.ToString("$###,##0.00");
        }

        private decimal CalculateSubtotal(string strQty, string strUnitPrice)
        {
            int qty = 0;
            decimal unitPrice = 0m;
            decimal subtotal = 0m;

            //clear msg
            lblMsg.Text = string.Empty;

            if (!String.IsNullOrEmpty(strQty))
            {
                //try to convert
                Int32.TryParse(strQty, out qty);
            }
            else
            {
                throw new Exception("Error: Qty is null or empty");
            }

            if (!String.IsNullOrEmpty(strUnitPrice))
            {
                //try to convert
                Decimal.TryParse(strUnitPrice, out unitPrice);

                //calculate
                subtotal = qty * unitPrice;
            }
            else
            {
                throw new Exception("Error: UnitPrice is null or empty");
            }
            
            return subtotal;
        }

        protected void checkBox1_CheckedChanged(object sender, EventArgs e)
        {
            ////uncomment for debugging
            //lblMsg.Text = "Info: checkBox1.Checked: " + checkBox1.Checked.ToString();
            //System.Diagnostics.Debug.WriteLine("Info: checkBox1.Checked: " + checkBox1.Checked.ToString());

            //set value
            product1DropDown.Enabled = checkBox1.Checked;

            if (!product1DropDown.Enabled)
            {
                //not enabled - reset values
                product1DropDown.Text = string.Empty;
                lblProduct1Subtotal.Text = "$0.00";
            }

            //calculate
            CalculateGrandTotal();
        }

        protected void checkBox2_CheckedChanged(object sender, EventArgs e)
        {
            //uncomment for debugging
            //lblMsg.Text = "Info: checkBox2.Checked: " + checkBox2.Checked.ToString();

            //set value
            product2DropDown.Enabled = checkBox2.Checked;

            if (!product2DropDown.Enabled)
            {
                //not enabled - reset values
                product2DropDown.Text = string.Empty;
                lblProduct2Subtotal.Text = "$0.00";
            }

            //calculate
            CalculateGrandTotal();
        }

        protected void checkBox3_CheckedChanged(object sender, EventArgs e)
        {
            //uncomment for debugging
            //lblMsg.Text = "Info: checkBox3.Checked: " + checkBox3.Checked.ToString();

            //set value
            product3DropDown.Enabled = checkBox3.Checked;

            if (!product3DropDown.Enabled)
            {
                //not enabled - reset values
                product3DropDown.Text = string.Empty;
                lblProduct3Subtotal.Text = "$0.00";
            }

            //calculate
            CalculateGrandTotal();
        }

        protected void product1DropDown_SelectedIndexChanged(object sender, EventArgs e)
        {
            CalculateGrandTotal();
        }

        protected void product2DropDown_SelectedIndexChanged(object sender, EventArgs e)
        {
            CalculateGrandTotal();
        }

        protected void product3DropDown_SelectedIndexChanged(object sender, EventArgs e)
        {
            CalculateGrandTotal();
        }
    }
}

Resources :

• OnCheckedChanged event not firing