I have my own exception based on some condition and want to raise an alert when control comes in this catch block
catch (ApplicationException ex)
{
//want t
MessageBox like others said, or RegisterClientScriptBlock if you want something more arbitrary, but your use case is extremely dubious. Merely displaying exceptions is not something you want to do in production code - you don't want to expose that detail publicly and you do want to record it with proper logging privately.