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access function in aspx page to master page

I have a.master and b.aspx .
i have some functions in my aspx page.
how to access that functions in a.master page.
thank you

Let's say, you want to call Foo from b.aspx from a.master. So first thing is that you have make the method internal (or public) and then you can use code such as below in master page is call that method.
var page = (b)this.Page;
page.Foo();
Note that b will be the code behind class name in b.aspx. Note that above code will fail if you use another page c.aspx and use the same master a with it. Generally, I will say that invoking page specific functions from master does not make sense unless functions are present in some base page class and in such case you should be casting to that base page class.
Edit: More elaborate example as requested by Asif:
Consider your content page b.aspx such as
<%# Page Language="C#" MasterPageFile="a.Master" Title="Page B" AutoEventWireup="true"
CodeBehind="b.aspx.cs" Inherits="YourProject.b" %>
And in code behind file (b.aspx.cs), you have a method Foo such as
namespace YourProject
{
public partial class b : System.Web.UI.Page
{
void Foo(string someParameter)
{
Label1.Text = someParameter
}
...
}
}
Now in code behind (a.master.cs) of a.master page
namespace YourProject
{
public partial class a : System.Web.UI.MasterPage
{
protected void Page_Load(object sender, EventArgs e)
{
b contentPage = (b)this.Page;
contentPage.Foo("Hello");
}
....
}
}

Of course you can make the method in b.aspx be public to call it in a.master. However I suggest you consider your design carefully. Because it's really weird just like that you call a method of a child class from its parent class (even though it's theoretically possible). Before your modification, ask yourself:
Is it necessary to call this method in the master page? If yes, do I have a better place to put the method?

As others have said, it's possible to do this. However, it's an odd way of doing things. You are probably going to be better off doing whatever you want to do in a different way. The whole idea of a master page is that it "wraps" many kinds of content pages. What if you content page doesn't have the function you want to call?
You could make sure all your content pages have the function, but then why not just put it in the master page?
Perhaps if you descired what you wanted to do a little better, we could advies you on a better way to handle things.

To access, either:
Make that method a static method.
Move your code in App_Code folder.
Move your code out of your web project, into some generic assembly and use that as a reference.

How to make a code run on all aspx pages?

I need to create a code that reads the QueryString and set a value on the Session and on the end of the page I need to clear the Session.
How can I make a code like this to run on all .aspx pages?

Two possibilities are:
Create a class that inherits from System.Web.UI.Page. Insert your code there and all your pages inherit from that class instead of System.Web.UI.Page.
Create a HttpModule

well as i see it you got 2 solutions
Use a master page and do it there
Inherit Page and use that as base class on all your pages
One question why must it be stored in a session? this will give your problems if the same user executes 2 pages at the same time (the first to finish will clear the sesson for the other page)
if you only need the data while the page runs you can just save it in a normal variable, else use the viewState!

Or just use Global.asax: http://en.wikipedia.org/wiki/Global.asax

Create a class that inherit from Page that you will use instead of Page.
Alternatively, you can use a MasterPage if your application design allows that.

By putting the code in a basepage and letting all your .aspx pages inherit from the basepage.

An easy way to include code that is part of all pages is to subclass Windows.Web.UI.Page, and have all your pages inherit from this new class, instead of Windows.Web.UI.Page. The new class can register for page events, independently from each individual page.
Another option, if you don't want it to be part of each page, and you want to ensure that it runs even if a developer doesn't inherit your new page subclass, is to write an HTTPModule. This hooks into the ASP.NET processing pipeline, and you can trigger off pipeline events, such as authentication or displaying pages. You can write an HTTPHandler and run it out of the pipeline as well. (All pages implement IHTTPHandler, somewhere up the chain.)

I used the method of writing a class to inherit from System.Web.UI.Page. However, I did not find the implementation of this method to be obvious. Here is the code that I eventually wrote:
public class BasePage : System.Web.UI.Page
{
protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
// Code here will be run when any page loads that inherits from BasePage
}
}
}

Can't access master page from user control

My master page code looks something like this:
namespace Recipes
{
public partial class MasterPage : System.Web.UI.MasterPage
{
...
public void UpdateUserLogin()
{
NicknameLbl.Text = ((Recipes.BasePage)Page).CurrentUser.Nickname;
}
...
}
}
I want to call the UpdateUserLogin() method from a user control, something like this:
((Recipes.MasterPage)this.Page.Master).UpdateUserLogin();
But for some reason the compiler doesn't know Recipes.MasterPage (are you missing an assembly blablabla).
Sorry I can't show the exact error message, it's in French.
Maybe the problem is that I added the Recipes namespace around MasterPage manually, it wasn't added by VS.
By the way I'm using VS Web Developer Express 2008.
Do you have any idea how I can make this call work?

Both the MasterPage and the UserControl are child controls of the page they are used by. Your UserControl could potentially be used in a page that doesn't use your MasterPage, and so calling UpdateUserLogin() would not be valid.
You can check it like this, however, and make your call conditionally:
if (Page.Master is MasterPage)
{
((MasterPage)Page.Master).UpdateUserLogin();
}
UPDATE
It seems you were already aware of that, sorry. Your question is about the reference not working. What is the namespace of your UserControl?

I would recommend data binding the NicknameLbl to the CurrentUser.Nickname property. Then the NicknameLbl text will get updated automatically if the property changes.

include a MasterType directive at the top of the Content page ASPX file:
'<%# MasterType virtualpath="~/DetailsMaster.master" %>'
include a public method in the Master page
public void UpdateUserLogin(string value)
{
NicknameLbl.Text = value;
}
access the method from the Content page using the Master syntax:
Master.UpdateUserLogin(Some Text");
http://www.codeproject.com/KB/aspnet/Master_and_Contents.aspx

If your project is a Web Site Project (instead of a Web Application Project), then you do not have a project namespace. All code that is referenced from aspx.cs or master.cs files needs to be stored inside the App_Code directory, as the ASP.Net compiler will create several assemblies instead of just 1, and its not predictable which assembly will contain which aspx code.
Update after 1st comment:
The .ascx.cs and .aspx.cs stay where VS puts them. But it you want to reference classes etc, this needs to be placed inside App_Code, e.g. your Recipes.MasterPage or Recipes.BasePage objects.

Partial User Control

In Asp.net Web forms why must a user control be partial? I tried to remove the partial keyword and it says:
Missing partial modifier on
declaration of type 'test'; another
partial declaration of this type
exists
Where is the other declaration?
I am trying to pass a generic type with the user control how can I do that? I can't unless I change the other declaration too. I couldn't find it so I removed the partial keyword.
Note:
you do have 3 files if your making WebApplication but if your making website you only get 2 files !
UserControl.ascx
UserControl.ascx.cs
so in website where is the other declaration ?
the reason i want generic is because im making a Grid User Control so i want to pass the type the datasource is going to have.

ASP.NET uses partial classes for the codehind because it has to generate all the server side controls you have declared in your ASPX file to a class and merge all the other data files that come along with your codebind. It allow the classes ASP.NET uses to be distributed across multiple files.
From MSDN:
The code file contains a partial
class—that is, a class declaration
with the keyword partial (Partial in
Visual Basic) indicating that it
contains only some of the total code
that makes up the full class for the
page. In the partial class, you add
the code that your application
requires for the page.
This is the key part:
When the page is compiled, ASP.NET
generates a partial class based on the
.aspx file; this class is a partial
class of the code-behind class file.
The generated partial class file
contains declarations for the page's
controls. This partial class enables
your code-behind file to be used as
part of a complete class without
requiring you to declare the controls
explicitly.
What are you trying to do by adding a generic type to a user control? Can you accomplish this by adding a Type property to the UserControl and then using that?

They have to be partial because the designer autogenerates the stuff you do in the Design window. Usually that code is in the foo.designer.cs (or whatever the extension for an asp.net form is). If you want to change the class, you have to generate the UI manually without the designer or use the designer and copy the code over to your proper class.

There are actually three files for each user control.
UserControl.ascx
UserControl.cs
UserControl.designer.cs
The designer file has a partial implementation because of that your code behind also has to be partial.

They don't have to be partial, indeed I've never used a partial class for one. Some of the code-generation tools will use partial so that they can more readily alter bits based on GUI-work you do without you and it getting in each others way, but that's for the tools rather than for the code.

New to C# and trying to use a global variable

Is it possible to use global variables in C#? I'm coming from mainly a PHP background so variables are either accessible everywhere or just a global definition away.
My main issue is I have a User class that I built myself to wrap around the current users table on my company's database. I am defining it in the MasterPage but can't seem to access it from the actual pages (I don't know if there's a better word to describe them but they are the pages that inherit the styles and format from the MasterPage)
Any general tips or implementation practices for me?
EDIT: here's some code snippets of what I'm trying to do:
Site.master.cs
public partial class SiteMaster : System.Web.UI.MasterPage
{
public User user = new User();
}
logout.aspx
<%# Page Title="" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeFile="logout.aspx.cs" Inherits="logout" %>
<%# MasterType virtualPath="~/Site.master"%>
logout.aspx.cs
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
User user = Master.user;
}
}

No, it is impossible. It is possible to create singletons or public static classes, but this is bad practice.
C# was designed for object oriented programming. If you haven't written programs using object oriented paradigm before it can be a bit hard to switch to it in the beginning. OOP (http://en.wikipedia.org/wiki/Object-oriented_programming) is built on three main concepts: inheritance, polymorphism and encapsulation.
You can defined classed apart of the pages/masterpages, it is good practice to define them in the App_Code folder.

Have a public static class and declare public static member variables.
That's what I do when I need some globals, though I try to avoid using them when I can.

If Page is inheriting from MasterPage, then make User property protected in MasterPage and it will be visible to Page.

The Master page class can be accessible to the pages that use it by setting the MasterPageClass in your .aspx page like so:
<%# MasterType TypeName="MyTypeName" VirtualPath="~/MasterPageName.master" %>
It sounds to me like you may just need to put your code in a slightly different place. A typical User class would be accessible to your project through a stand-alone class, and not bundled into a master page or a master type.
I might suggest that you add your User class into a new classfile in the /AppCode directory of your project instead, (User.cs). That would let you have access to it from your pages without having to muck with the MasterType.

See my answer to this question. Non-static class-level variables do no persist once the response is sent to the browser. This is because each Page object is going to be a new instance of the class, not the same one from your last request.
Use the "Session" property instead as I show in the link.

Is your problem, from your page, get to data stored in the masterpage (assuming we're talking about the ASP.Net MasterPage mechanism here)?
If so, you should look at strongly-typed access to masterpages. Basically, what you do is create a public property in your MasterPage class. Then, in your Page, declare the MasterPageFile and MasterType, like this:
public partial class MasterPage
{
public User CurrentUser{...}
}
In your page aspx, declare to use the masterpage and which master type to use.
<%# Page masterPageFile="~/MasterPage.master"%>
<%# MasterType virtualPath="~/MasterPage.master"%>
You will then be able to access the property from within your page class like this:
var user = Master.CurrentUser;
Then, for the question on where to initialize the CurrentUser object, look at the list of page lifecycle events. As you can see, MasterPage.Init fires before Page.Init and MasterPage.Load fires before Page.Load. You can use either MP.Init or MP.Load to make sure the data is ready for when the page events fire, though Init is preferred.

There are at least a couple different ways to achieve what you want:
Use the Application object - It can be used to store things globally and is part of ASP.Net.
Use static classes - This is another option for creating a singleton.