what is the difference between 3.5 and 4.0 in net

You may change your question like this
What's New in the .NET Framework 4[^]
Anyway
Difference Between Microsoft .NET Framework 3.5 and .NET Framework 4.0[^]

Main Differences between ASP.NET 3.5 and ASP.NET 4.0.

ASP.NET 3.5 is having the following main features which are not availablle in the prior releases

1) AJAX integration
2) LINQ
3) Automatic Properties
4) Lambda expressions

I hope it would be useful for everyone to know about the differences about asp.net 3.5 and its next version asp.net 4.0

Because of space consumption I'll list only some of them here.

1) Client Data access:


ASP.NET 3.5: There is no direct method to access data from client side. We can go for any of these methods

1) Pagemethods of script manager
2) ICallbackEventHandler interface
3) XMLHttphanlder component

ASP.NET 4.0: In this framework there is an inbuilt feature for this. Following are the methods to implement them.

1) Client data controls
2) Client templates
3) Client data context

i.e we can access the data through client data view & data context objects from client side.

2) Setting Meta keyword and Meta description:


Meta keywords and description are really useful for getting listed in search engine.

ASP.NET 3.5: It has a feature to add meta as following tag


<meta name="keywords" content="These, are, my, keywords" />
<meta name="description" content="This is the description of my page" />



ASP.NET 4.0: Here we can add the keywords and description in Page directives itself as shown below.


< %@ Page Language="C#" CodeFile="Default.aspx.cs"
Inherits="_Default"
Keywords="Keyword1,Key2,Key3,etc"
Description="description" %>



3) Enableviewstage property for each control

ASP.NET 3.5: this property has two values "True" or "false"

ASP.NET 4.0: ViewStateMode property takes an enumeration that has three values: Enabled, Disabled, and Inherit.
Here inherit is the default value for child controls of a control.

4) Setting Client IDs
Some times ClientID property creates head ach for the programmers.

ASP.NET 3.5: We have to use ClientID property to find out the id which is dynamically generated

ASP.NET 4.0: The new ClientIDMode property is introduced to minimize the issues of earlier versions of ASP.NET.

It has following values.

AutoID - Same as ASP.NET 3.5
Static - There won't be any separate clientid generated at run time
Predictable-These are used particularly in datacontrols. Format is like clientIDrowsuffix with the clientid vlaue
Inherit- This value specifies that a control's ID generation is the same as its parent.




Following are list of Major Changes in .Net 4.0

ControlRenderingCompatabilityVersion Setting in the Web.config File
ClientIDMode Changes
HtmlEncode and UrlEncode Now Encode Single Quotation Marks
ASP.NET Page (.aspx) Parser is Stricter
Browser Definition Files Updated
System.Web.Mobile.dll Removed from Root Web Configuration File
ASP.NET Request Validation
Default Hashing Algorithm Is Now HMACSHA256
Configuration Errors Related to New ASP.NET 4 Root Configuration
ASP.NET 4 Child Applications Fail to Start When Under ASP.NET 2.0 or ASP.NET 3.5 Applications
ASP.NET 4 Web Sites Fail to Start on Computers Where SharePoint Is Installed
The HttpRequest.FilePath Property No Longer Includes PathInfo Values
ASP.NET 2.0 Applications Might Generate HttpException Errors that Reference eurl.axd
Event Handlers Might Not Be Not Raised in a Default Document in IIS 7 or IIS 7.5 Integrated Mode Changes to the ASP.NET Code Access Security (CAS) Implementation
MembershipUser and Other Types in the System.Web.Security Namespace Have Been Moved
Output Caching Changes to Vary * HTTP Header
System.Web.Security Types for Passport are Obsolete
The MenuItem.PopOutImageUrl Property Fails to Render an Image in ASP.NET 4
Menu.StaticPopOutImageUrl and Menu.DynamicPopOutImageUrl Fail to Render Images When Paths Contain Backslashes

New Feature
Asp.net 3.5
1.
New Features
Asp.net 3.5 includes the following new features
1) Listview Control
2) DataPager Control
3) Nested Master Pages
4) Linq DataSource
2.
Multi Targeting
Asp.net 3.5 supports multi –targeting What is multi – targeting? Check below description
3.
Ajax Support
In Asp.net 3.5, Ajax is integrated in .NET framework, thereby making the process of building intuitive cool user interfaces easier
4.
Silverlight Support
It support for silverlight
5.
JavaScript Debugging
It support for JavaScript debugging
6.
LINQ Support
It supports LINQ