Why is WCF client configuration different when running under the VS2015 debugger?

Question

In my WCF client code I add a message inspector (using an IEndpointBehavior).

class MyBehavior : IEndpointBehavior
{
    public void ApplyClientBehavior(ServiceEndpoint endpoint, ClientRuntime clientRuntime)
    {
        clientRuntime.ClientMessageInspectors.Add(new MyMessageInspector());
    }
}

public class MyMessageInspector : IClientMessageInspector
{
    public object BeforeSendRequest(ref Message request, IClientChannel channel)
    {
        Console.WriteLine(request.Properties.ContainsKey(HttpRequestMessageProperty.Name));
    }
}

Now here comes the part I do not understand:

• When I make a WCF call using a proxy configured to use this behavior, the following happens:

  • If the call is made from a process running inside the VS2015 debugger, True is printed to the console, meaning request.Properties["httpRequest"] is present.
  • If the call is made from a process running normally (not in the debugger), False is printed to the console.

Why is WCF configured differently when running inside/outside the VS2015 debugger? How does this happen? Is it documented anywhere?