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ASP.NET Textbox Mask

So I want to include a mask to my textbox. I have researched for different options and I have tried all of them but none seem to work. Im using VS2013 and C# and I want to be able to use textbox.text on my code too.
I have something like this:
<script type="text/javascript" src="/js/jquery-2.1.4.min.js"></script>
<script type="text/javascript" src="/js/jquery.maskedinput.js"></script>
<script type="text/javascript">
$(function () {
$("#TxtDOB").mask("999-999-9999");
});
</script>
asp:TextBox ID="TxtDOB" runat="server" CssClass="textbox1" Width="130px" />
TextBox won't recognize the mask.

I'm a bit late to the party, but adding ClientIDMode="static" should fix the issue here. By default, .Net will convert the ID ("TxtDOB" in this case) to something other than what you set it to. Adding ClientIDMode="static" tells .Net not to change the ID and make it available as-is on the client-side.
<asp:TextBox ID="TxtDOB" runat="server" ClientIDMode="static" CssClass="textbox1" Width="130px" />

asp.net textbox mask using jquery not working

I'm trying to use the jquery mask to apply to an ASP.NET web form textbox. Nothing I do works... what am I missing?
<script type="text/javascript" src="/js/jquery.maskedinput-1.2.2.js"></script>
<script type="text/javascript" src="/js/jquery-2.1.4.min.js"></script>
<script type="text/javascript">
$(function() {
$("#phone").mask("999-999-9999");
});
</script>
This is the text box code:
<asp:TextBox runat="server" ID="phone" CssClass="rightsection" ToolTip="Please enter a phone number."></asp:TextBox>
<asp:RequiredFieldValidator runat="server" ControlToValidate="phone" Display="Dynamic" ForeColor="Red" Text="Required field" ValidationGroup="formfields" />

the textbox's id changes when it gets rendered to include the parent containers id, you can get the rendered id with ClientID like this
$("#<%= phone.ClientID%>").mask("999-999-9999");
The ClientID value is generated by concatenating the ID values of each parent naming container with the ID value of the control. In data-binding scenarios where multiple instances of a control are rendered, an incrementing value is inserted in front of the control's ID value. Each segment is separated by an underscore character (_).

This is the solution:
<script type="text/javascript" src="js/jquery-2.1.4.min.js"></script>
<script type="text/javascript" src="js/jquery.maskedinput.js"></script>

asp.net passing values from JS/jquery to code behind c#

I have tried "every" possible way of sending the screen.width vlaue from the JS script on the aspx page to the c# in the code behind and while I can see the screen.width being assigned correctly it never gets assigned to my hidden field value.
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<asp:HiddenField ID="hiddenfield" runat="server" />
<script type="text/javascript" language="javascript">
$(function(){
$('#hiddenfield').val(screen.width);
});
</script>
other content
</asp:Content>
and the code behind:
protected void btnChartGo_Click(object sender, EventArgs e)
{
string s = hiddenfield.Value;
}
No matter what I try s is always ""
Something wrong with the above, everyone seems to be doing it like that and it works?

The ID of the rendered hidden field isn't "hiddenfield" - it'll be something like ctl00_bodycontent_hiddenfield.
Try using
$('[id$="hiddenfield"]')
as the selector instead.

<asp:HiddenField ID="hiddenfield" runat="server" ClientIDMode="Static">
</asp:HiddenField >
Make sure client ID mode of your hidden field is static if you are using ASP.NET 4 or use
$('#<%= hiddenfield.ClientID %>').val(screen.width);

This should get the right selector:
$('#<%= hiddenfield.ClientID %>').val(screen.width);

Check the view source of the page and find out proper id of the element and then use jquery selector over it and then at the page load check for request.form collection to check if hidden variable is coming in post request or not

is there a way to process .NET/C# variables on external .js?

I have my variable in an aspx.cs , such as :
protected string myVar="Hello";
Now, if I go to my scripts.js file added as :
<head>
<script src="/scripts/scripts.js" type="text/javascript"></script>
</head>
and I try this :
var myVarJs="<%=myVar&>";
it doesnt get the .NET myVar value.
Is there a way to catch it or am I dreaming?

Insert the variable before the script:
<head>
<script type="text/javascript"> var myVarJs="<%=myVar%>"; </script>
<script src="/scripts/scripts.js" type="text/javascript"></script>
</head>

You can also register/render client script. So you can declare variables in the backend and then render the javascript variables.

I don't think it is possible to directly access C# variable in javascript code.
As C# is client side and Javscript is server side.
Unless on the Asp.net page you save the variable in a hidden field or label as text which is not visible.
Asp:
<asp:HiddenField ID="hidden" runat="server" value="<%=strvariable %>" />
Javascript:
function Button_Click()
{
alert(document.getElementById('hidden').value);
}
So this would get the hidden field with the ID of "hidden".
This could work I think.

Find the ID after compiling

When you assign something an ID in asp.net such as:
<asp:Label runat="server" ID="mylabel" Text="some text"></asp:Label>
The code behind ID you use to reference that object is mylabel.
However after the site has compiled and is running the ID in the HTML changes to something a bit more abscure
<asp:Label runat="server" ID="ct100_blahblah_what_mylabel" Text="some text"></asp:Label>
And I cannot (don't know how) to reliably predict what it will be to select it with javascript.
Is there a way to ask the server what the ID for that label is at any given moment?
What are some keywords could use to find out more about this phenomenon?

You need the ClientID property. This contains the generated id of the control at runtime. You could emit some JavaScript in the code behind that sets the client id of your control to a JavaScript variable so you can reference it.
EDIT: added example
The following will find the label using the ClientID and then update its text.
<html>
<body>
<asp:Label runat="server" ID="mylabel" Text="some text"></asp:Label>
<script language="javascript">
var mylabelID = "<%= mylabel.ClientID %>";
var label = document.getElementById(mylabelID);
label.innerHTML += " changed";
</script>
</body>
</html>

I believe you are looking for the ClientID property.
The ClientID value is often used to
programmatically access the HTML
element rendered for a control in
client-side script.
Your other option is to wrap your control in an HTML element like this:
<span id="foo">
<asp:Label runat="server" ID="mylabel" Text="some text"></asp:Label>
</span>
Since this outer element does not have the runat="server" attribute, its id will not be changed on render. This would mean that your JavaScript function would have to to be changed to pull an inner element from the span foo or if you are using a JavaScript framework like jQuery you could pull the label back like this:
$("span#foo span");