<asp:TextBox ID="txtTest" runat="server" />
<asp:RequiredFieldValidator ID="rfvTest" ControlToValidate="txtTest" ErrorMessage="Il campo Test deve essere valorizzato!" runat="server" />
<asp:ValidationSummary ID="vsTest" ShowMessageBox="true" ShowSummary="false" runat="server" />
<asp:Button ID="btnTest" Text="Test" runat="server" />