Outlook "Check Names" feature in visual basic.net or C#

One code snippet is attached, the other is here ... This solved my issue.

 Public Shared Function AuthenticateUserPassword(ByVal szUserName As String, ByVal szPassword As String) As Boolean
        Dim result As Boolean = False
        Dim szUserDomain As String = ""
        Dim myLDAPPath As String

        ' Determine what the domain name should be.
        If szUserName.Contains("\") Then
            ' Pull the domain out of the user name.
            szUserDomain = szUserName.Substring(0, szUserName.IndexOf("\"))
            ' Set user name to just user name.
            szUserName = szUserName.Substring(szUserName.IndexOf("\") + 1)
        ElseIf szUserName.Contains("@") Then
            ' User Name is in form of "user@domain"
            ' Pull the domain out of the user name.
            szUserDomain = szUserName.Substring(szUserName.IndexOf("@") + 1)
            ' Set user name to just user name.
            szUserName = szUserName.Substring(0, szUserName.IndexOf("@"))
        Else
            ' A "." refers to the local system.
            szUserDomain = Environment.MachineName
            szUserName = szUserName.Substring(szUserName.IndexOf("\") + 1)
        End If

        ' Determine which entry it will need to be.
        If String.Compare(szUserDomain, Environment.MachineName, True) = 0 Then
            myLDAPPath = "WinNT://" & szUserDomain
        Else
            myLDAPPath = "LDAP://" & szUserDomain
        End If

        Try
            Dim entry As DirectoryEntry = New DirectoryEntry(myLDAPPath, szUserName, szPassword)
            Dim nativeObject As Object = entry.NativeObject
            result = True 'no exception thrown, user must exist
            nativeObject = Nothing  'be sure and clean up these object as this service could be used many times
            entry = Nothing
        Catch ex As Exception
            result = False  'exception thrown - no user with that name/pwd combination
        End Try
        Return result
    End Function

Public Shared Function AD(ByVal strQuery As String, ByVal strReturn As String, ByRef listbox1 As ListBox)
        If strQuery = "" Then
            Exit Function
        End If
        'strQuery = "khalid.mehmood@mobilink.net"

        Dim oroot As DirectoryServices.DirectoryEntry = New DirectoryServices.DirectoryEntry("LDAP://mobilink")
        Dim osearcher As DirectoryServices.DirectorySearcher = New DirectoryServices.DirectorySearcher(oroot)
        Dim result As DirectoryServices.SearchResult
        Dim resultAll As DirectoryServices.SearchResultCollection
        'osearcher.Filter = "(&(objectCategory=person)(objectClass=user)(mail=" & strQuery & "))"  ' search filter
        osearcher.Filter = "(&(anr=" & strQuery & ")(mail=*))"  ' search filter
        resultAll = osearcher.FindAll()

        If resultAll.Count > 0 Then
            For i As Integer = 0 To (resultAll.Count - 1)
                listbox1.Items.Add(resultAll(i).Properties("mail")(0).ToString())
            Next
        End If

        oroot = Nothing
        osearcher = Nothing
        result = Nothing
        resultAll = Nothing

        Exit Function
End Function

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Thanks