Programmatically set InstanceContextMode

I’m answering my own question as I’ve found a solution in another answer on stackoverflow, and I think stackoverflow is a great place to search without even having to ask a question, so hopefully I will add to that richness by answering my own question with a link to the other answer and not just closing my own question.

My code now looks like this …

public HttpWebService(string baseUri, string acceptType, TApi serviceInstance = null)
{
    _baseUri = baseUri;
    _acceptType = acceptType.ToLower();

    if (serviceInstance != null)
    {
        _host = new HttpServiceHost(serviceInstance, baseUri);
        var behaviour = _host.Description.Behaviors.Find<ServiceBehaviorAttribute>();
        behaviour.InstanceContextMode = InstanceContextMode.Single;
    }
    _host = _host ?? new HttpServiceHost(typeof (TApi), baseUri);

    _host.Open();
    _client = new HttpClient();
    _client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue(_acceptType));
}

I changed this …

_host = serviceInstance == null
            ? new HttpServiceHost(typeof (TApi), baseUri)
            : new HttpServiceHost(serviceInstance, baseUri);

… to this …

if (serviceInstance != null)
{
    _host = new HttpServiceHost(serviceInstance, baseUri);
    var behaviour = _host.Description.Behaviors.Find<ServiceBehaviorAttribute>();
    behaviour.InstanceContextMode = InstanceContextMode.Single;
}
_host = _host ?? new HttpServiceHost(typeof (TApi), baseUri);