Variable<string> message = new Variable<string> { Name = "message" };
Variable<string> echo = new Variable<string> { Name = "echo" };
Receive receiveString = new Receive
{
    OperationName = "Echo",
    ServiceContractName = "Echo",
    CanCreateInstance = true,
    //parameters for receive
    Content = new ReceiveParametersContent
    {
        Parameters = 
        {
            {"message", new OutArgument<string>(message)}
        }
    }
};

Sequence workflow = new Sequence()
{
    Variables = { message, echo },
    Activities =
        {
            receiveString,
            new WriteLine
            {
                Text = new InArgument<string>(env =>("Message received: " + message.Get(env)))
            },
            new Assign<string>
            {
                Value = new InArgument<string>(env =>("<echo> " + message.Get(env))),
                To = new OutArgument<string>(echo)
            },                        
            //parameters for reply
            new SendReply
            {                           
                Request = receiveString,                            
                Content = new SendParametersContent
                {
                    Parameters =
                    {
                        { "echo", new InArgument<string>(echo) }
                    },
                }
            },
            new WriteLine
            {
                Text = new InArgument<string>(env =>("Message sent: " + echo.Get(env)))
            },
        },
};

service = new WorkflowService
{
    Name = "Echo",
    Body = workflow
};