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

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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

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