Show MessageBox of ValidationSummary from Codebehind

Hi,
 I want to Show MessageBox of the ValidationSummary (ShowMessageBox="true")  from Code behind. Below is the code i used.Any tips.
Private Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
 Dim blnFlag = callmethod()
 If Not blnFlag Then
    Page.Validate()
    ValidationError.Display("Place error message here")
 End If
End Sub
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 Validati ...