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Assigning objects within public method

I wish to create a public method on my masterpage, that I can call from within every subpage.
I am trying to wrap my head around how this should be done.
On my subpages I've made this method to fill a panel with an errormessage.
protected void errorMessage (string errorText) {
HtmlGenericControl divTag = new HtmlGenericControl("div");
Panel_Name.Controls.Add(divTag);
divTag.InnerHtml = errorText;
}
Now if I were to make this function public on my masterpage, It wont recognize my Panel as it havent been showed yet. I'm guessing the answer involves FindControl
(Sorry for my rubbish code english)
How should I do this ?

To be fair, for your scenario, I would use a UserControl (.ascx) on your Pages (.aspx).
Then, in the UserControl, have your error message markup, such as:
Code front (ErrorMessage.ascx)
<asp:Panel runat="server" ID="PanelErrorMessage" /> // creates a <div>
Code behind (ErrorMessage.ascx.cs)
public string ErrorMessage
{
get {}
set
{
PanelErrorMessage.Text = value; // sets the panel text (<div>text</div>) to value when property is set
}
}
Use your UserControl on your Page (you'll need to define this as a control tag on your page too with the prefix/suffix):
<myControls:ErrorMessage runat="server" ID="MyErrorControl" />
You can also do this in many places on your page, if you require different errors.
Then, when you have an error, you'll simply do:
MyErrorControl.ErrorMessage = "This is my error message";
Job done!

Passing data between master and content pages

This is my task . I have to create a master page and its two child pages. Details are below
Declare 3 session variables in master page.
Pages_viewed
ChildAppData1
ChildAppData2
Declare 1 session variable in each child application
ChildVar1
ChildVar2
Once the child application loads
Pages_viewed ++;
Set ChildVar1 = ChildAppData1 = datetime.now(); or ChildVar2 = ChildAppData2 = datetime.now();
Add a button to the master page that resets ALL variables.
Display all 5 variables in master page as well as active child application.
I have to show the no of times a child page is visited in page_viewed variable.

Simplest way is to Redirect to self with some information/command in the get parameter to self page.
like Default.master enclosing
Default.aspx
->Redirect to
`Default.aspx?cmd=clearAllVariables`
On Page_Load check
if(!PostBack && (Request["cmd"] != null) && (Request["cmd"].ToString() == "clearAllVariables"))
{
clearAllVariablesImplementation();
}

Master Page is adding additional text in TextBox ID

I have a master page which has a content section with the id cpMainContent.
I am using this master page on every webform I am creating for college project. One of such form is frmSearchPersonnel. The purpose of frmSearchPersonnel is to ask user last name of the person they want to search in a textbox and then click on search button. The ID of TextBox is
txtSearchName
Search button will do postbackUrl transfer to another form which I have named frmViewPersonnel.
In frmViewPersonnel I am trying to use following code.
NameValueCollection myRequest = Request.Form;
if(!string.IsEmptyOrNull(myRequest["txtSearchName"]))
string strSearch = myRequest["txtSearchName"];
The problem I ran into is that this didn't find any control with the name of txtSearchName. While debugging I found this in myRequest object,
[5] "ctl00$cpMainContent$txtSearchName" string
Even though when I added textbox I gave it ID of txtSearchName but when page is rendered it is adding extra string from master page.
How can I stop this? I have to use master page so don't say not to use master page :)
Why is it doing that?
Update
While Googling and Binging I found that I can use Control.ClientID in this case so looking into it.
Update 2
As suggested below to add ClientIDMode="static" in the html of control or add it in page directive. What it does is, it keeps the ID static to txtSearchName but problem is this,
<input name="ctl00$cpMainContent$txtSearchName" type="text" id="txtSearchName" />
Here name is still using ctl00 and the code I showed above,
string strSearch = myRequest["txtSearchName"]
it still won't work because nvc collection is either searchable by index or name not the id directly.
==============

You need to add a ClientIDMode="Static" to the html of the textbox:
<asp:TextBox ID="txtSearchName" runat="server" ClientIDMode="Static" />
It happens to prevent duplicate ID's. Usually it happens when you use master pages as it contains nested pages
If you want all controls with ClientIDMode="Static", you can put it in the page header of the master file.
<%# Page Language="C#" ClientIDMode="Static" %>

If you are posting to another page that uses the same master page (called SiteMaster in my case), the name of the textbox should be same the same.
string val = Request[((SiteMaster)Master).txtSearchName.UniqueID];
If you're NOT posting to a page with the same master, well, then are you using the viewstate for the textbox at all since you're posting to another page? If not, just make the control a non asp.net control:
<input type="text" name="txtSearchName"/>
If you are using viewstate and posting to another page with a different master page, well, you should use PreviousPage.

Little late here. Appreciate #aquinas and #rudeovski ze bear. Interesting and good answers.
I'd same issue and I solved it differently.
In fact, I used a public Interface.
public interface ISearch
{
string SearchText { get; }
}
Then implement ISearch interface in two aspx page say One.aspx and Two.aspx classes.
--One.aspx-- (Where I'v added TextBox1, and Button1 and set Button1.PostBackUrl="~/Two.aspx")
public partial class One : System.Web.UI.Page , ISearch
{
public string SearchText
{
get
{
return TextBox1.Text;
}
}
}
--Two.aspx--
public partial class Two : System.Web.UI.Page, ISearch
{
protected void Page_Load(object sender, EventArgs e)
{
ISearch search = (ISearch) PreviousPage;
Label1.Text = search.SearchText;
}
public string SearchText
{
get
{
throw new NotImplementedException();
}
}
}

If your try to access the input element value in code behind on post back instead of for example:
var emailAddress = Request.Form["ctl00$ctl00$ctl00$ContentContainer$MainContent$MainContent$ContentBottom$ProfileFormView$emailaddress1"];
Use
var emailAddressKeyName = Request.Form.AllKeys.FirstOrDefault(a => a.Contains("emailaddress1"));
var emailAddress = Request.Form[emailAddressKeyName];

Problem setting page title in content page when title is set in master page

I have a master page and in its page load event I am appending to the page title:
Title = Title + " MyWebsite";
Then in a page that uses the previous master page:
Title = "Home";
What ends up rendering is the value from the master page. A closer look at the debugger shows that it evaluates the Title attribute in my content page, but still says it's an empty string (even though I explicitly have a value set to it.)
Note: Content pages get evaluated before master pages ( I didn't realize this).

You need to set page title in Page_PreInit event
protected void Page_PreInit(object sender, EventArgs e)
{
//set here....
}

I have used the following... and it works as expected.
this.Page.Title = this.Page.Title + " Master Title";

Acess controls on ContentPages via Javascript by MasterPage

I need my MasterPage to be able to get ControlIDs of Controls on ContentPages, but I cannot
use <%= xxx.CLIENTID%> as it would return an error as the control(s) might not be loaded by the contentplaceholder.
Some controls have a so called BehaviourID, which is exactly what I would need as they can be directly accessed with the ID:
[Asp.net does always create unique IDs, thus modifies the ID I entered]
Unfortunately I need to access
e.g. ASP.NET Control with BehaviouraID="test"
....
document.getElementById("test")
if I were to use e.g. Label control with ID="asd"
....
document.getElementById('<%= asd.ClientID%>')
But if the Labelcontrol isn't present on the contentpage, I of course get an error on my masterpage.
I need a solution based on javascript. (server-side)
Thx :-)

You could use jQuery and access the controls via another attribute other than the ID of the control. e.g.
<asp:Label id="Label1" runat="server" bid="test" />
$('span[bid=test]')
The jQuery selector, will select the span tag with bid="test". (Label renders as span).

Best solution so far:
var HiddenButtonID = '<%= MainContent.FindControl("btnLoadGridview")!=null?
MainContent.FindControl("btnLoadGridview").ClientID:"" %>';
if (HiddenButtonID != "") {
var HiddenButton = document.getElementById(HiddenButtonID);
HiddenButton.click();
}
Where MainContent is the contentplace holder.
By http://forums.asp.net/members/sansan.aspx

You could write an json-object with all the control-ids which are present on the content-page and "register" that object in the global-scope of your page.
Some pseudo pseudo-code, because I can't test it at the moment...
void Page_Load(object sender,EventArgs e) {
System.Text.StringBuilder clientIDs = new System.Text.StringBuilder();
IEnumerator myEnumerator = Controls.GetEnumerator();
while(myEnumerator.MoveNext()) {
Control myControl = (Control) myEnumerator.Current;
clientIDs.AppendFormat("\t\"{0}\" : \"{1}\",\n", myControl.ID, myControl.ClientID);
}
page.ClientScript.RegisterStartupScript(page.GetType(),
"ClientId",
"window.ClientIDs = {" + clientIDs.ToString().Substring(0, clientIDs.ToString().Length - 2) + "};",
true);
}

It sounds like your issue is that you are using the master page for something it wasn't intended. The master page is a control just like any other control, and therefore cannot access any of the controls of its parent (the page). More info:
ASP.Net 2.0 - Master Pages: Tips, Tricks, and Traps
My suggestion is to inject the JavaScript from your page where the controls can actually be resolved. Here is a sample of how this can be done:
#Region " LoadJavaScript "
Private Sub LoadJavaScript()
Dim sb As New StringBuilder
'Build the JavaScript here...
sb.AppendFormat(" ctl = getObjectById('{0});", Me.asd.ClientID)
sb.AppendLine(" ctl.className = 'MyClass';")
'This line adds the javascript to the page including the script tags.
Page.ClientScript.RegisterClientScriptBlock(Me.GetType, "MyName", sb.ToString, True)
'Alternatively, you can add the code directly to the header, but
'you will need to add your own script tags to the StringBuilder before
'running this line. This works even if the header is in a Master Page.
'Page.Header.Controls.Add(New LiteralControl(sb.ToString))
End Sub
#End Region