Imagine you want to use your ValidationSummary to display server errors as client error, there is a simple way to do that.

First, declare your ValidationSummary, and a CustomValidator with Display property set to “None”

<asp:CustomValidator ID="ServerErrorCustomValidator"
Display="None" runat="server" ValidationGroup="MyValidationGroup" />

<asp:ValidationSummary ID="AdministrationTaskValidationSummary" runat="server" ValidationGroup="MyValidationGroup"/>

Next, in your application code, when an error occurs : set the CustomValidator’s ErrorMessage and IsValid properties :

ServerErrorCustomValidator.ErrorMessage = "Server error :  error description";
ServerErrorCustomValidator.IsValid = false;