How can I conditionally prevent a form from submit

Question

When my "submit" (Save) button is clicked, one thing it does is verifies that the two "totals" vals match, and returns if they don't, fingerwagging the user with a red 'bad boy!' message:

message = new LiteralControl();
AddVerticalSpace();
Controls.Add(message);
decimal? paymentTot = TryConvertToDecimal(boxPaymentAmount.Text);
if ((null == paymentTot) || (paymentTot < 0))
{
    paymentTot = 0M;
}
decimal? grandTot = TryConvertToDecimal(boxGrandTotal.Text);
if ((null == grandTot) || (grandTot < 0))
{
    grandTot = 0M;
}
if (grandTot != paymentTot)
{
    AddVerticalSpace();
    message.Text = "<span style='color:red'>Total and Payment Total do not match; Please enter the same amount for both values and try again.</span>";
    return;
}

What I intend with the "return" is that the submit is aborted; yet, with the code above, the form is still submitted - it reverts to its initial appearance/state. How can I conditionally prevent this automatic submit behavior?

UPDATE

This is for Tyree Jackson:

Here's the server-side save button. First, it is conditionally dynamically created and configured thus:

if (AnyCheckboxSelected())
{
    // Create Save button
    this.Controls.Add(new LiteralControl("<br />"));
    btnSave = new Button();
    btnSave.ID = "btnSave";
    btnSave.Text = "Save";
    btnSave.Click += new EventHandler(btnSave_Click);
    this.Controls.Add(btnSave);

    AddVerticalSpace();
}

...and here is the event handler:

protected void btnSave_Click(object sender, EventArgs e)
{
    LiteralControl message = null;
    try
    {
        fullAddressChosen = rbFullAddress.Checked;
        paymentToAnIndividualDropDownChosen = rbPaymentToIndividual.Checked;
        paymentToAVendorDropDownChosen = rbPaymentToVendor.Checked;

        message = new LiteralControl();
        AddVerticalSpace();
        Controls.Add(message);
        decimal? paymentTot = TryConvertToDecimal(boxPaymentAmount.Text);
        if ((null == paymentTot) || (paymentTot < 0))
        {
            paymentTot = 0M;
        }
        decimal? grandTot = TryConvertToDecimal(boxGrandTotal.Text);
        if ((null == grandTot) || (grandTot < 0))
        {
            grandTot = 0M;
        }
        if (grandTot != paymentTot)
        {
            AddVerticalSpace();
            message.Text = "<span style='color:red'>Total and Payment Total do not match; Please enter the same amount for both values and try again.</span>";
            return;
        }
        ConditionallyCreateList();
        SaveInputToList();
        listOfListItems = ReadFromList();
        message.Text = "Saving the data has been successful";

        // Re-visiblize any rows with vals
        if (RowContainsVals(3))
        {
            foapalrow3.Style["display"] = "table-row";
        }
        if (RowContainsVals(4))
        {
            foapalrow4.Style["display"] = "table-row";
        }
        if (RowContainsVals(5))
        {
            foapalrow5.Style["display"] = "table-row";
        }
        if (RowContainsVals(6))
        {
            foapalrow6.Style["display"] = "table-row";
        }

        AddVerticalSpace();

        //CreatePDFGenButton(); 
        GeneratePDFAndMsg();
        btnGeneratePDF.Visible = true;

        AddVerticalSpace();
        GenerateLegalNotice();
    }
    catch (Exception ex)
    {
        message.Text = String.Format("Exception occurred: {0}", ex.Message);
    }
}

Answer 1

First Default.aspx source:

    <script type="text/javascript" language="javascript">
        function validateData() {
            var payment = parseFloat(document.getElementById('boxPaymentAmount').value).toFixed(2);
            var total = parseFloat(document.getElementById('boxGrandTotal').value).toFixed(2);
            if (payment == null) payment = 0;
            if (total == null) total = 0;
            if (payment == total) {
                return true;
            } else {
                alert('Total and Payment Total do not match. Please enter the same amount for both values and try again!');
                return false;
            }
        }
    </script>

    <asp:TextBox ID="boxPaymentAmount" runat="server"></asp:TextBox><br />
    <asp:TextBox ID="boxGrandTotal" runat="server"></asp:TextBox><br />
    <asp:Button ID="btnSave" runat="server" Text="Button" OnClientClick="return validateData();" OnClick="btnSave_Click" />

And code for Default.aspx.cs:

public partial class Default : System.Web.UI.Page
{

    protected void btnSave_Click(object sender, EventArgs e)
    {
        //TODO: do your stuff here...
    }

}

Answer 2

If you are talking about preventing a form from submitting in a web browser, you should use client side code (i.e. JavaScript) for that. Even if there was a way to do it in code behind, it would just be translated into client side code. Otherwise, the form will submit even if it retains the edited values.