How to trigger a OnChange type of event in ASP.NET C#

Question

I have this onchange event in my JS that according to a select list, changes the maxlength for the textboxes under it. Now i want to be able to do this server side just incase if a user has JS turned off.

I have some code behind code but it only works after the selection is made and then the page is refreshed, i want to be able to work without the refresh factor or some sort of auto refresh as soon as the user selects an option from the dropdown.

This is my Code Behind:-

protected void ListPayment_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ListPayment.SelectedIndex == 4)
        {

            TextBoxCCN.MaxLength = 15;
            TextCSS.MaxLength = 4;
        }

        else if (ListPayment.SelectedIndex != 4)
        {

            TextBoxCCN.MaxLength = 16;
            TextCSS.MaxLength = 3;
        }
    }

This is my HTML

<asp:DropDownList ID="ListPayment" runat="server" 
  onchange="ValidateCCN();" 
  OnSelectedIndexChanged="ListPayment_SelectedIndexChanged">
    <asp:ListItem Value="0">Select...</asp:ListItem>
    <asp:ListItem Value="Visa">Visa</asp:ListItem>
    <asp:ListItem Value="MasterCard">MasterCard</asp:ListItem>
    <asp:ListItem Value="Discover">Discover</asp:ListItem>
    <asp:ListItem Value="American Express">American Express</asp:ListItem>
    </asp:DropDownList>

user1017882 · Accepted Answer · 2014-08-26 11:37:16Z

2

This is what the AutoPostBack property is for:

<asp:DropDownList AutoPostBack="true"...

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.listcontrol.autopostback(v=vs.110).aspx

EDIT:

Full revised code:

<asp:DropDownList ID="ListPayment" runat="server" AutoPostBack="true"
  onchange="ValidateCCN();" 
  OnSelectedIndexChanged="ListPayment_SelectedIndexChanged">
    <asp:ListItem Value="0">Select...</asp:ListItem>
                    <asp:ListItem Value="Visa">Visa</asp:ListItem>
                    <asp:ListItem Value="MasterCard">MasterCard</asp:ListItem>
                    <asp:ListItem Value="Discover">Discover</asp:ListItem>
                    <asp:ListItem Value="American Express">American Express</asp:ListItem>
    </asp:DropDownList>

answered Aug 26, 2014 at 11:37

user1017882

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8 Comments

rookie26 Over a year ago

where do i add that? above the select list?

user1017882 Over a year ago

I've added it to your DropDownList tag in my answer. See my edit for full code.

rookie26 Over a year ago

yep okay, so do i have to move all my code behind code into the page load function? because it isnt working as of now

Coding Flow Over a year ago

@rookie26 no, setting AutoPostBack="true" should do the trick. When you change the value in the select list is it causing the page to postback? If not I would look what ValidateCCN() is doing client side as this may be preventing the postback.

user1017882 Over a year ago

Why do you think you'd have to do that as well as the suggestion I've just made?

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