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Calling c# function on string value BEFORE postback?

I am new to ASP, and have jumped right in and started a new MVC 4 project.
I am using the standard template and am trying to edit the login page. The problem I am trying to solve is this:
If you open Fiddler and login you can see the user name and password in plain text. What I would like to do would be to use a C# function I have created in a helpers file BEFORE the post is submitted, for example on a button click event, is this possible?
If so can someone point me in the direction of a tutorial/ example please as this has baffled me for a few days now!
Thanks again for your help

Don't reinvent the wheel. Use https instead so that data does not travel as plain text.

You can't run a C# function before the postback, how would you accomplish that? C# code runs server-side, but you post the form from the client-side. You can't apply a C# method on something you haven't shown it yet.
You have basically two options:
1.) use javascript to somehow alter the data before sending it to the server
2.) use SSL to protect the channel
The problem with the first option is, that ANYONE who sees the form can see your javascript code as well. In other words, no matter how strong protection you come up with, the attacker sees the algorithm, so he can decode the data very easily... Probably the most reliable option is the second one - SSL. It isn't 100%, but at least it's much harder to penetrate...

If you want to encrypt the data before the form is submitted you can only rely on client side code - javascript. This is in no way the optimal solution, as already pointed out by others, you should use https.

ASP dynamically created image, unable to display in web browser control

I'm currently under the belief it may be to do with the header information but I'm really not too sure. The image on this page is the best example of it that I can give. It will display sometimes in a web browser control, and other times it just plain refuses to. Does anyone have any idea as to why?
The code I'm using to try and grab the image is simply:
WebBrowser.Navigate("http://www.lse.co.uk/tools/user/imgChatUsagePie.asp?nick=mulledwine", null, null, "image/gif");
It's really hard to ascertain as to what is causing the image to display sometimes and others not as it works completely fine within Chrome. Is this a problem related specifically to the web browser control?
Thank you in advance.

Apparently, the server side script checks the ASP Session ID cookie and displays the image depending on some session variable stored on the server.
Try navigating the to the HTML page first, then request that image.

Sometimes the data is a valid image, and sometimes it is not.
But seriously You need to give more information I assume you mean
http://www.lse.co.uk/tools/user/imgChatUsagePie.asp?nick=mulledwine
Maybe post the code you use to generate the image. Is this browser specific. Under what conditions does the pie chart fail to generate. Does the url give an error message when it refuses to display the image etc. I would assume some variable you use to generate the image is null and throws an unhandled exception causing the page to return 500 and some kind of error rather than image data.
If it is really just sometimes it works and sometimes it doesn't id recommend using some kind of test suit to keep requesting the image at random images until it fails and then isolate the error message. Selenium or Fiddler may be of some use to isolate the problem.

Capturing JavaScript Error in C#

I am developing an automation tool where one of my requirement is to detect whether the input page contains any js error. I have tried using response but this is not wokring.
Can you please help me out as I am stuck very badly.

You can't check if there is a JavaScript error on the page.
What you can do is on "trappable" errors, you can insert text into a hidden field and read that in your c# code.
But if there is an error in code then I don't think you'll be able to check that in c#.
Actually, you could have a hidden field on the page that already has text in it. Then use JavaScript to clear out the field.
In c# code, if you can see an entry in the field, then there was an error in the JavaScript and if it's empty then the JavaScript ran ok and there was no error.

Do you mean something like described here? Log client side js errors

IE shows a previously cached version of my page

my scenario is this; the user selects the list of reports they wish to print, once they select and click on the a button, i open up another page with the selected reports ready for printing. I am using a session variable to pass reports from one page to another.
first time you try it, it works fine, second time you try it, it opens the report window with the previous selected reports. I have to refresh the page to make sure it loads the latest selections.
is there a way to get the latest value from the session every time you use it? or is there a better way to solve this problem. open for suggestions...
Thanks
C# Asp.net, IE&7 /IE 8

After doing some more checking maybe if you check out COMET it might help.
The idea is that you can have code in your second page which will keep checking the server for updated values every few seconds and if it finds updated values it will refresh itself.
There are 2 very good links explaining the imlementation.
Scalable COMET Combined with ASP.NET
Scalable COMET Combined with ASP.NET - Part 2
The first link explains what COMET is and how it ties in with ASP.NET, the second link has an example using a chat room. However, I'm sure the code querying for updates will be pretty generic and can be applied to your scenario.
I have never implemented COMET yet so I'm not sure how complex it is or if it is easy to implement into your solution.
Maybe someone developing the SO application is able to resolve this issue for you. SO uses some real-time feature for the notifications on a page, i.e: You are in the middle of writing an answer and a message pops up in your client letting you know someone else has added an answer and to click "here" to refresh.

The proper fix is to set the caching directives on the HTTP response correctly, so that the cached response is not reused without validation from the server.
When you fail to specify the cache lifetime, the client has to "guess" how long the response is good for, and the browser's guess probably isn't what you want. See http://blogs.msdn.com/b/ie/archive/2010/07/14/caching-improvements-in-internet-explorer-9.aspx

It's better to use URL paramaters. So you have a view of value of the paramaters.

Displaying expected errors to users in ASP.NET

I have found huge amounts of information (ie, this) on how to handle unexpected errors in ASP.NET, using the Page_Error and Application_Error methods as well as the customErrors directive in Web.config.
However, my question is what is the best way to handle EXPECTED errors. For example, I have a page to display a record. Each record has a specific list of users who are allowed to see it. Since many users may have the "View Records" role that are not on said list, I have to write some code on the page to filter them.
protected void Page_Load(object sender, EventArgs e)
{
var user = Membership.GetUser();
if (!CanUserViewThisRecord(Request["id"], user.Username)
{
// Display an error to the user that says,
// "You are not allowed to view this message", and quit.
}
else
{
// Display the page.
}
}
What are the best practices for handling this kind of error? I can think of a few possibilities:
Redirect to an error page.
Put a label on every page called "lblErrorText". Leave it blank unless there is an error.
Raise an exception and let the standard error handling deal with it.
This feels like a basic question and for that I apologize, but just about everything I've found has been in reference to unexpected exceptions. It's not that any of the above possibilities are hard to implement, but I'd like to use a standard, recommended method if possible.
NOTE: Thanks everyone for the answers. I want to clarify that users would NOT have the ability to click links to records they're allowed allowed to view. This question is more in the interest of being defensive. For example, since the record ID is in the URL someone could potentially enter the ID of a forbidden record in the address bar. Or User A who is allowed might e-mail a link to User B who is not. It seems I may not be using the words "exception" and "error" in the correct way, but hopefully the scenario makes sense.

In the interest of failing gracefully, I'd go with the option to display a message on the page.
Even better is error prevention; if you know ahead of time that the user won't be able to do anything on the page, don't provide a link to it. Generally, users should only see the things that they are allowed to do.

As others have mentioned, I would prefer to prevent this before it gets sent, either by disabling the functionality for these users, or catching it with javascript before the page is sent.
you would still need to check on the server that the user is allowed to make use of a control, and in such cases the suggested label would be preferable as a solution to the other 3 given.
A further solution however would be to provide a hidden value to the page which is checked by javascript within the page, generating either an alert or a more easily spotted error dialogue than a label somewhere which might be missed leading to confusion as to why nothing happened.
Edit based on questioner's comments: if modifying a number in a URL is all that is required to point to records the user is unauthorized to use, would POST perhaps be a better method to use than GET? that way the way this error is handled is less important, as no standard user would encounter it.

Of your three options, the third would be my least favored. It's not really an exception for a user to try to view a record you told him was there. Redirecting to an error page is more reasonable, as is the error label. However, neither is particularly user-friendly.
I don't know how your UI is structured, but it seems to me that you shouldn't let the user try to view a record if you know that the user isn't allowed to view it. That is, if you know that the user can't view that record, then don't give him the opportunity to click on it. Never get to the point where you have to say, "You can't view this record."
If you can't prevent the user from trying to view the record, I think a pop-up message box would be preferable to either of your first two options.

in my opinion this question is more methodology then tech..
i think is more right to show the error message near the object/action that cause the error.
if you send him to another page he will lost is orientation and it not be clear what cause this error.
so in my opinion is more right to put the error message in the same page.
and maybe give him the opportunity to correct..