static Activity CreateWorkflow()
{
    Variable<int> numberBells = new Variable<int>()
    {
        Name = "numberBells",
        Default = DateTime.Now.Hour
    };
    Variable<int> counter = new Variable<int>()
    {
        Name = "counter",
        Default = 1
    };
 
   return new Sequence()
   {
        DisplayName = "Main Sequence",
        Variables = { numberBells, counter },
        ...
   }
}
Variables 为Sequence类的属性成员，声明如下：
public Collection<Variable> Variables { get; }
这不是只读属性吗？上面例子中的Sequence的构造函数中给成员Variables 赋值不懂？求解