On postback, how can I add a error message to validation summary?

Two questions:

On postback when a user clicks submit, how can I add a error message to validation summary?

Is it also possible to highlight a particular textbox using the built in .net validation controls?

Dynamically create a CustomValidator control and add it directly to the Page.Validators collection.

Dim err As New CustomValidator
err.ValidationGroup = "MyGroup"
err.IsValid = False
err.ErrorMessage = "The password is invalid"
Page.Validators.Add(err)

Unlike adding the CustomValidator to the markup, this method allows you to add any number of arbitrary error messages based on server-side business logic.

Note that you can also add it to the page directly, but there are a couple of rules to follow:

1. You must add the control to the same naming container as the controls of the validation group.
2. If you don't want the validation message to appear in a random position in the page, you will either have to add the validator to a specific container or you will need to supress it using a CSS class or style.

You can also create a custom class and implement IValidator, which enables you to add the message with one line of code, but this method doesn't support Validation Groups.

Per Anders Fjeldstad's suggestion, here are a set of handy extension methods.

Imports Microsoft.VisualBasic
Imports System.Runtime.CompilerServices

Public Module PageExtensions

	<Extension()> _
	Public Sub AddValidationError(ByVal p As System.Web.UI.Page, ByVal errorMessage As String)
		p.AddValidationError(errorMessage, String.Empty)
	End Sub

	<Extension()> _
	Public Sub AddValidationError(ByVal p As System.Web.UI.Page, ByVal errorMessage As String, ByVal validationGroup As String)
		Dim err As New CustomValidator
		err.ValidationGroup = validationGroup
		err.ErrorMessage = errorMessage
		err.IsValid = False
		p.Validators.Add(err)
	End Sub

End Module

Add a custom validator and manually set it's IsValid and ErrorMessage properties. Sort of like this:

<asp:panel ID="ErrorsPanel" runat="server" CssClass="ErrorSummary">
    <asp:CustomValidator id="CustomValidator1" runat="server" 
        Display="None" EnableClientScript="False"></asp:CustomValidator>
    <asp:ValidationSummary id="ErrorSummary" runat="server" 
        HeaderText="Errors occurred:"></asp:ValidationSummary>
</asp:panel>

In the code behind:

//
// Update the database with the changes
//
string ErrorDetails;
if (!Db.Update(out ErrorDetails))
{
    CustomValidator1.IsValid = false;
    CustomValidator1.ErrorMessage = ErrorDetails;
}

Here's a little extension to the good idea from NightOwl888:

public class ValidationError : CustomValidator
{
    public ValidationError(string group, string msg)
        : base()
    {
        base.ValidationGroup = group;
        base.ErrorMessage = msg;
        base.IsValid = false;
    }
}

public static class PageExtensions
{
    public static void ErrorMessage(this Page page, string group, string msg)
    {
        page.Validators.Add(new ValidationError(group, msg));
    }
}

Whenever you want to issue an error message, simply call Page.ErrorMessage; the mechanism is the same as he suggested.

Well all you need to do is create a Custom Validator and add it to the Validator collection of your page, whenever the condition to do so arises.

CustomValidator cv = new CustomValidator();

if(condition)
{
cv.IsValid = false;
cv.ErrorMessage = "This Catalog Data already exists.";
cv.ValidationGroup = "InputList";
this.Page.Validators.Add(cv);
}

NOTE: Dont forget to specify the ValidationGroup, or the error message is not going to be displayed inspite of the custom validator being added to your page. And ya, if you do get an answer to your 2nd question(highlighting textbox) do post it!

To add error message on validation summary you can use EnableClientScript property of ValidationSummary and the other validation controls. Set EnableClientScript to false all of them :

<asp:ValidationSummary
HeaderText="You must enter a value in the following fields :"
DisplayMode="BulletList"
EnableClientScript="false"
runat="server"/>

For highlighting a control, no it's not possible with current controls.

But I put my validation controls near the related controls, and I set their Text property as "*". Then if the validation fails, it appears near failed control.

Maybe you can use custom validator to highlight the failed control. But you should write your own implementation.

Here is a version of the above answers that is an extension method for ValidationSummary, which takes care of the validation group ID.

public static void AddCustomMessage(this ValidationSummary summaryControl, string message)
{
	summaryControl.Page.Validators.Add(new CustomValidator {
		ValidationGroup = summaryControl.ValidationGroup,
		ErrorMessage = message,
		IsValid = false
	});
}