ASP / ASP.NET

Last Updated: 2001

Topics

General
Validation Controls
Auto-PostBack
<SCRIPT runat=server>...</SCRIPT> blocks vs. <%...%> blocks
Tips & Tricks

General

ASPX page requires a header (automatically added by VS): <% Page language="language" Codebehind="codeBehindFile" AutoEventWireup="boolWireUp" Inherits "baseClassFile" %>
Where:
    langauge is C#, VB or JScript
    codeBehindFile is the name of the support file (generated & maintained automatically by VS) that contains things like the implementation of RequiredFieldValidator (you can add you own functions to this file if you wish) and the Using statements.
    boolWireUp     Whether or not handlers the plumbing for registering events such as Page_Load is automatically generated or not. Counterintuitively, Forms that are created in VS.NET default this to false.
    baseClassFile   Name of base class the web form inherits from, typically contained in the codeBehindFile
/> can be used as a tag terminator if included in the original tag. For example: <asp:Textbox id=txtName runat=server />
A single expression can be outputted using the <%= syntax. For example: The time is <% = DataTime.Now.ToString() %>. To output within an existing <% ... %> block, use the Response.Write function.
Lines / blocks of code can be marked a comments by using the <%-- and --%> markers
To include another source file in the current file
Specify the start web page by right-clicking on a page in Solution Explorer and select 'Set as Start Page'
Page_Load occurs after Page_Init. During Page_Init the controls will still have their default values (i.e. the hidden field hasn't been read yet)
The server controls live under System.Web.UI.Controls and comprise of:

Control Description

AdRotator Ad Rotator

Button General purpose button. Post button by default (?? although this might just be to insure the button is processed ??)

Calendar Calendar control (with date range selection capabilities)

CheckBox CheckBox (single)

CheckBoxList CheckBox (multiple)

DataGrid Can be connected to an ADO.NET data source (basically anything that implements ICollection interface, so we talking arrays as well as SQL result sets). See example below

DataList ??. Can be connected to an ADO.NET data source

DropDownList Combo box

HyperLink HyperLink

Image Image

ImageButton Button with image instead of text

Label Label

LinkButton HyperLink button

Panel ??

RadioButton Radio Button (single)

RadioButtonList Radio Button (multiple)

Repeater Include template code multiple times

Table Table

TextBox Textbox

These controls are referred to in code with the <asp: reference and require the runat=server setting. For example: <asp:Button id=myButton text=Submit runat=server /> (Page compiles find without runat=server, but the code behind the controls will not have been "expanded" by the server so are unlikely to do anything useful)

Server controls can be data bound by setting their text properties to <% expr %>. Data binding is one way as (in MS words) "A majority of Web application data scenarios do not involve data update... The binding performed with data binding expressions is therefore one-way... If you want to write data back to a data store, you must do so yourself".!
Clicking most buttons causes the validation on the page to be run (even a button labelled "Cancel"). To prevent this, set the button's CausesValidation attribute to false, for example:
<input type="button" id="cmdHelp" value="Help" runat=server CausesValidation=false onserverclick="myHelpFunction">

Examples

DataGrid
Add a method to the "CodeBehind" file that returns a DataTable object (or in fact any object that supports the ICollection interface - alledgidly!). For example:

public DataTable GetData() {
    DataSet ds = new DataSet("myDataSet");
    DataTable dt;
    SqlDataAdapter sqlDA = new sqlDataAdapter(
        "select * from Employees",
        "server=myServer;user id = sa;password=;database=northwind");
    sqlDA.Fill(ds, "Employees");
    return ds.Tables["Employees"];
}

Then add the following code to the ASP page:
<%
    dg1.DataSource = GetData();
    dg1.DataBind();
%>
<asp:DataGrid AutoGenerateColumns=True runat=server id=dg1/>

Note: It took a long time to work this out as (a) All the beta 1 examples don't work with Beta 2 and (b) Beta 2 implementation of datasets/grids through ASP is very bugged. For example, if you move the dg1.DataSource/Bind code to after creation of dg1 is just doesn't do anything!

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Validation Controls

Control	Description
CompareValidator	Compares 2 controls (or one control and a constant value). Comparisions can be = =, >= etc.
CustomValidator	UDF. There has to be a server side implementation (sure??), but a client side implementation can additionally be included (advanced clients will then use the client side validation and older clients the round trip to the validation on the server).
RangeValidator	Check value is between a specified upper and lower boundary.Works with numbers, characters and dates.
RegularExpressionValidator	Applies regular expression matching to validation
RequiredFieldValidator	Checks field has been changed from initial value
ValidationSummary	Calls all other validations on the page to ensure all pass. Those that fail have their associated ErrorMessage text values harvested and displayed in one grouping by the ValidationSummary control. Different text (e.g. a red ) can be displayed for the validated control by specifying the error message in ErrorMessage and the in Text.

Examples:

<asp:RequiredFieldValidator ControlToValidate=txtName runat=server>You must supply a name!</asp:RequiredFieldValidator>

<asp:RangeValidator id="rangeTest" type="Integer" ControlToValidate = "txtMyControl" MaximumValue="30" MinimumValue="0" runat="server" />

<asp:RegularExpressionValidator id="exprTest" ControlToValidate="txtMyControl" ValidationExpression="^\d{3}$" Display="Static" runat="server">I was expecting 3 digits< asp:RegularExpressionValidator/>

Notes

Referencing Page.IsValid will cause all validation to be run. For example, writing "if (Page.IsValid) {o.text="Valid!"}; in code is fine (i.e. we don't have to call some sort of ValidatePage() function or each control's validation block first).

Referencing validator.IsValid is the same except obviously only that control's validation is re-evaluated. A useful trick is not to include any error message in the validator and in code check validator.IsValid and change a label manually.

Auto-PostBack

Events on the client that trigger code on the server tend to be postponed until the next submit. For example, place a listbox, a textbox and a button on a form. In the Page_Init add some enteries to the listbox. In the SelectedIndexChange of the listbox set the textbox's to display the current selection from the listbox. There is no need to do anything with the button.

Run the form. Change the listbox selection. Note that the textbox has NOT updated. Click the button. Even though the button has no code it has caused a postback and the textbox updated. Return to the editor and change the Listbox's AutoPostBack property to true. Run the form again. This time when the listbox is changed the textbox is automatically updated.

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<SCRIPT runat=server>...</SCRIPT> blocks vs. <%...%> blocks

To prevent confusion about when code embedded in an ASP.NET page is run <SCRIPT> blocks can only contain declarations, functions and methods while <%...%> blocks can only contain immediately executed code.

For example:

Allowed	Not Allowed
<SCRIPT language=c# runat=server> int fred=3; public int myFunc(int z) { return z*fred; } </SCRIPT>	<SCRIPT language=c# runat=server> Response.Write("Never work"); </SCRIPT>
<% Response.Write("<b>Got Here<\b>"); %>	<% public void() donothing{} %>

This is different to ASP, where anything goes. Anecdotal evidence suggests this is a major cause of problems when converting ASP code to ASP.NET.

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Tricks & Tips

<%Response.Write(strCode);%> to write code when the page is fetched from the server, or just place code outside of <% %> marker (for example:
    <% for(int i=0; i<7; i++) { %>
        <font size ="<% =i.ToString() %>"> Example </font><br>
    <%}%>
Session["strVariable"] to read/save values specific to that browser session
<input onclick="object.method()" type = "button" value = "Button Text"> to create a client-side button (e.g. to replay a media clip)
To obtain a reference to the selected item in a radio button group, oGroup.SelectedItem
To change page programmatically: Response.Redirect(strURL)
Request object can be used to refer to previous page values, e.g. Request.QueryString("control")
Properties in controls in a contained class can be set in <tags> by using "-" rather than ".". For example if "SomeTag" contains a format object which has a size property it can be accessed as <SomeTag Format-Size="10">
Use <%-- text --%> to create a comment block in a web page that (unlike ) won't be downloaded to the client. Doesn't appear to work in .js files.
[IE only] To ensure a particular control is in view when the page is (re)loaded on the client, add the following JScript to the page: ClientID.scrollIntoView();

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Control	Description
AdRotator	Ad Rotator
Button	General purpose button. Post button by default (?? although this might just be to insure the button is processed ??)
Calendar	Calendar control (with date range selection capabilities)
CheckBox	CheckBox (single)
CheckBoxList	CheckBox (multiple)
DataGrid	Can be connected to an ADO.NET data source (basically anything that implements ICollection interface, so we talking arrays as well as SQL result sets). See example below
DataList	??. Can be connected to an ADO.NET data source
DropDownList	Combo box
HyperLink	HyperLink
Image	Image
ImageButton	Button with image instead of text
Label	Label
LinkButton	HyperLink button
Panel	??
RadioButton	Radio Button (single)
RadioButtonList	Radio Button (multiple)
Repeater	Include template code multiple times
Table	Table
TextBox	Textbox