When we migrated our project from .NET 3.5 to .NET 4.0, we can get this error:

Error: System.Web.HttpRequestValidationException
A potentially dangerous Request.Form value was detected from the client (ctl00$CC$txtAnswer=\”… World\r\n\r\nI am doing Testin…\”).”} System.Web.HttpRequestValidationException

at System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection)
   at System.Web.HttpRequest.ValidateNameValueCollection(NameValueCollection nvc, RequestValidationSource requestCollection)
   at System.Web.HttpRequest.get_Form()
   at System.Web.HttpRequest.get_HasForm()
   at System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull)
   at System.Web.UI.Page.DeterminePostBackMode()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.displaypost_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\a37c2f81\cfc4c927\App_Web_i2rujncl.9.cs:line 0
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

What is the Cause?

In ASP.NET 4, by default, request validation is enabled for all requests, because it is enabled before the BeginRequest phase of an HTTP request. As a result, request validation applies to requests for all ASP.NET resources, not just .aspx page requests. This includes requests such as Web service calls and custom HTTP handlers. Request validation is also active when custom HTTP modules are reading the contents of an HTTP request.

Solution:

To revert to the behavior of the ASP.NET 2.0 request validation feature, add the following setting in the Web.config file:

<system.web>
 <httpRuntime requestValidationMode=”2.0? />
</system.web>