WCF single instance of service for VB (COM) client

I created a WCF service, and want to be used by multiple VB clients.

I need only a single instance of the service, even if multiple clients start, use it, or stop.

But the result seems to be against my expectation. My code is:

WCF service:

namespace xComm
{
    [ServiceBehavior(
        InstanceContextMode = InstanceContextMode.Single,
            ConcurrencyMode = ConcurrencyMode.Single)]
    public class SvcACS : ISvcACS
    {
        private int m_nCounter = 0;

        public int CounterUp()
        {
            m_nCounter++;
            return m_nCounter;
        }
        public int CounterGet()
        {
            return m_nCounter;
        }

    }
}

After building the WCF service, I can use it by COM reference from VB application, as the following one, a simple form application calling CountUp method and checking return value:

Option Explicit

Dim svcAcs As xComm.ISvcACS

Private Sub btnCmd_Click(Index As Integer)
    Dim nW1 As Integer
    
    nW1 = svcAcs.CounterUp()
    lblCounter.Caption = CStr(nW1)
End Sub

Private Sub Form_Load()
    Set svcAcs = New xComm.svcAcs                   
End Sub

I started for example 2 VB clients and press the btnCmd button respectively. What I want is:

1. client 1's btnCmd is clicked: client 1's lblCount shows 1
2. client 2's btnCmd is clicked: client 2's lblCount shows 2
3. client 1's btnCmd is clicked: client 1's lblCount shows 3 ...

But the result turned out that clients are serviced independently, instead of by using a single instance of service, which the counter would have been shared.

I have used InstanceContextMode.Single, is this not enough?

Thanks in advance.

Ting

some new findings(2022/7/26):

Instead of using COM reference directly, I tried the following VB code:

    Dim Client As New MSSOAPLib30.SoapClient30
    
    
    Set Client = CreateObject("MSSOAP.SoapClient30")    'xcomm.svcACS")
    Call Client.mssoapinit("http://localhost:8733/SvcACS?wsdl")
                    'localhost:8733/SvcACS" binding="basicHttpBinding"
    MsgBox (Client.CounterUp())

The result shows that several VB application instances consume a same service instance.

And I also tested C# clients by using SvcUtilexe generated client code, which shows the same result.

But in both the cases, changing InstanceContextMode to PerCall or PerSession makes not difference.

I guess, maybe it is the behavior of self hosting service, in which the active service works for all.

But still I dont understand how COM interface works. Seemingly by using COM interface, each time, a new service instance is created.