public event ToolTipOpeningEventHandler ViewerToolTipOpening

Public Event ViewerToolTipOpening As ToolTipOpeningEventHandler

private void reportViewer1_ViewerToolTipOpening(object sender, Telerik.ReportViewer.Common.ToolTipOpeningEventArgs args)
{
    if (args.ToolTip.Title.Contains("DoNotShow"))
    {
        args.Cancel = true;
    }
    else
    {
        args.ToolTip.Title = "The tooltip title is changed!";
        args.ToolTip.Text += " (changed)";
    }
}

' Handles the ViewerToolTipOpening event
' Do not forget to add the WithEvents clause on ReportViewer1 instantiation if needed.
Private Sub reportViewer1_ViewerToolTipOpening(sender As Object, args As Telerik.ReportViewer.Common.ToolTipOpeningEventArgs) Handles ReportViewer1.ViewerToolTipOpening
    If args.ToolTip.Title.Contains("DoNotShow") Then
        args.Cancel = True
    Else
        args.ToolTip.Title = "The tooltip title is changed!"
        args.ToolTip.Text += " (changed)"
    End If
End Sub