WebServices作为单例是否会导致不同用户出现问题?

我正在开发一个使用UPS Shipping Web服务的电子商务应用程序。我已经读到创建一个单例很好,所以在任何时候只有一个WebService实例。我的代码如下。

Public Class Ship
    Private Shared sync As New Object()
    Private Shared _Service As New ShipService

    Public Shared ReadOnly Property Service As ShipService
        Get
            If _Service Is Nothing Then
                SyncLock sync
                    If _Service Is Nothing Then
                        _Service = New ShipService
                    End If
                End SyncLock
            End If
            Return _Service
        End Get
    End Property

    Public Shared Function GetInstance() As ShipService
        Return Service()
    End Function
End Class

这是一个片段,它将在哪里使用。

Public Sub New(ByVal ToAddress As Address, ByVal WeightInLbs As String)
    //Not relevant code
    Ship.Service.UPSSecurityValue = Security
    //More not relevant code 
End Sub

Public Function ProcessShipment() As ShipmentResponse
    Return Ship.Service.ProcessShipment(ShipmentRequest)
End Function

在上面的构造函数行中,我必须设置服务的upSecurityValue。稍后我将调用processShipment函数。我的问题是:既然WebService被看作是一个单实例,那么不同的应用程序实例可以共享相同的upSecurityValue,并且在我设置它和调用processShipment之间会发生变化吗?