Discuss "Programmatically Add Item to Validation Summary" in the Code Samples forum.

Imports System.Web.UI
    Public Class ValidationError
        Implements IValidator
        Private _errorMessage As String = String.Empty
        Private _isValid As Boolean = False
        Public Shared Sub Display(ByVal message As String)
            Dim currentPage As Page = TryCast(HttpContext.Current.Handler, Page)
            currentPage.Validators.Add(New ValidationError(message))
        End Sub
        Public Sub New(ByVal message As String)
            ErrorMessage = message
            IsValid = False
        End Sub
        Public Property ErrorMessage() As String Implements System.Web.UI.IValidator.ErrorMessage
            Get
                Return _errorMessage
            End Get
            Set(ByVal value As String)
                _errorMessage = value
            End Set
        End Property
        Public Property IsValid() As Boolean Implements System.Web.UI.IValidator.IsValid
            Get
                Return _isValid
            End Get
            Set(ByVal value As Boolean)
                _isValid = value
            End Set
        End Property
        Public Sub Validate() Implements System.Web.UI.IValidator.Validate
        End Sub
    End Class

Page.Validate()
ValidationError.Display("Place error message here")