The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.

The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Runtime.InteropServices.COMException: The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace:

[COMException (0x8102006d): The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.]
Microsoft.SharePoint.Library.SPRequestInternalClass.UpdateRoleAssignment(String bstrUrl, Guid& pguidScopeId, Int32 lPrincipalID, Object& pvarArrIdRolesToAdd, Object& pvarArrIdRolesToRemove) +0
Microsoft.SharePoint.Library.SPRequest.UpdateRoleAssignment(String bstrUrl, Guid& pguidScopeId, Int32 lPrincipalID, Object& pvarArrIdRolesToAdd, Object& pvarArrIdRolesToRemove) +119
[SPException: The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.]
OfficialMail.WebParts.TestWebPart.btn_Click(Object sender, EventArgs e) +94
System.Web.UI.WebControls.Button.OnClick(EventArgs e) +105
System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +107
System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5102

Solutions :
Central Administration > Application Management > Web Application General Settings ->Select web application

In Web Page Security Validation , change security validation is off.