How to test a client of a WCF service

I have a WCF service that exposes 1 method GetNextObjectInList(int id) which hits a DB.

The WCF service works, more or less, like this:

public class MyService : IDisposable
{
    public MyService() 
    { 
        this.IntializeDBConnection();
    }

    public int GetNextObjectInList(int id) 
    { 
        /* uses DB connection */ 
    }

    /* Dispose releases DB connection */
}

This makes the client code relatively simple:

public void UseNextElementInList()
{
    IMyService svc = new MyServiceClient();
    int nextID = svc.GetNextObjectInList(this.ID);

    /* use object */
}

I've written unit tests to test the WCF services objects, but I'd like to test the consumer code for various things like timing/performance/error handling but I don't know how to construct my tests such that the Service doesn't hit the DB.

Most of my tests (the tests that run against the service's objects for instance) DO create an in-memory DB but I don't know how to get the service to connect to that without test-specific code in the service.

I would create a test service for your unit tests. Typically what I do in these circumstances is create a config for the test project that is identical to the real one except the address would be local host, and the type would be my test service class:

        <service name="MyNamespace.TestService" behaviorConfiguration="BehaviorConfig">
            <endpoint address="net.tcp://localhost/MySolution/TestService"
                                binding="netTcpBinding"
                                bindingConfiguration="BindingConfig"
                                contract="MyNamespace.IMyService"/>

If you are using VS Test Project you can use the ClassInitialize / ClassCleanup attributes to set up / tear down the service:

    [ClassInitialize()]
    public static void MyClassInitialize(TestContext testContext) {
        mHost = new ServiceHost(typeof(TestService));
        mHost.Open();
        return;
    }
    [ClassCleanup()]
    public static void MyClassCleanup() {
        if(mHost != null) {
            mHost.Close();
        }
        return;
    }

Now inside of the TestService class (which would implement IMyService) you could provide whatever behavior necessary to test the client without worrying about your unit tests corrupting your production code