public event ExportBeginEventHandler ExportBegin

Public Event ExportBegin As ExportBeginEventHandler

void ReportViewer1_ExportBegin(object sender, Telerik.ReportViewer.Silverlight.ExportBeginEventArgs args)
{
    if (true) // some logic here
    {
        // Cancel is false by default
        args.Cancel = true;
    }

    switch (args.Format)
    {
        case "Excel":
            // do something
            break;
        case "PDF":
            // do something
            break;
    }
}

Private Sub ReportViewer1_ExportBegin(sender As Object, args As Telerik.ReportViewer.Silverlight.ExportBeginEventArgs)
    If True Then
        ' some logic here
        ' Cancel is false by default
        args.Cancel = True
    End If

    Select Case args.Format
        Case "Excel"
            ' do something
            Exit Select
        Case "PDF"
            ' do something
            Exit Select
    End Select
End Sub