Protected Overrides Function CheckAccessCore(ByVal operationContext As OperationContext) As Boolean 
    ' Extract the action URI from the OperationContext. Match this against the claims.
    ' in the AuthorizationContext.
    Dim action As String = operationContext.RequestContext.RequestMessage.Headers.Action
    
    ' Iterate through the various claimsets in the AuthorizationContext.
    Dim cs As ClaimSet
    For Each cs In  operationContext.ServiceSecurityContext.AuthorizationContext.ClaimSets
        ' Examine only those claim sets issued by System.
        If cs.Issuer Is ClaimSet.System Then
            ' Iterate through claims of type "http://www.contoso.com/claims/allowedoperation".
            Dim c As Claim
            For Each c In  cs.FindClaims("http://www.contoso.com/claims/allowedoperation", _
                 Rights.PossessProperty)
                ' If the Claim resource matches the action URI then return true to allow access.
                If action = c.Resource.ToString() Then
                    Return True
                End If
            Next c
        End If
    Next cs 
    ' If this point is reached, return false to deny access.
    Return False

End Function