用于 VB (COM) 客户端的 WCF 单一服务实例

Question

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

Answer 1

This is just a speculation on my part, but the Instancing article notes, that

Single: A single instance of the service class handles all client requests for the lifetime of the application .

(Emphasis mine)

Which to me sounds that because you are using two applications, each creating its own instance of the service, you experience the expected behavior. So I assume you need to persist & restore the return value between calls and share it between the service instances.