There are differences in code for .Net framework 4.0 and earlier releases (such as framework 3.5 and 2.0) that may impact applications were created by using earlier releases.
Differences Between .NET Framework 4.0 And Earlier Releases
- 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
Read more: ASP.NET 4 Breaking Changes
