When i select from DropDownList then press button print i get this error.

Server Error in '/' Application.

An invalid event-dependent reposting or call argument. Event validation is enabled using <pages enableEventValidation="true"/> in the configuration or <%@ Page EnableEventValidation="true" %> on a page. For security purposes, this feature verifies that arguments for the event-dependent reposting or invocation events were created from the server control that originally provided them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method to register the reposting or event-dependent call data for the validation process.

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.ArgumentException: An invalid event-dependent reposting or call argument. Event validation is enabled using <pages enableEventValidation="true"/> in the configuration or <%@ Page EnableEventValidation="true" %> on a page. For security purposes, this feature verifies that arguments for the event-dependent reposting or invocation events were created from the server control that originally provided them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method to register the reposting or event-dependent call data for the validation process.

C#

protected void BtnUser_Click(object sender, EventArgs e)
{

}