请问C＃解析动态对象时，选择了错误的类型变种？

问题描述

我使用下面的代码部分的Json转换成动态对象。当我使用DateTime.Parse我的动态类型的属性我期望VAR猜测，它的类型是一个DateTime ......相反，它保持为动态。这不可能是正确的，可以吗？

下面完整的示例。

  VAR设置=新的JavaScriptSerializer（）反序列化<动态>（JSON）; 
 
变种的startDate = DateTime.Parse（settings.startDate）; 
 VAR结束日期= DateTime.Parse（settings.endDate）; 
 VAR用户id = int.Parse（settings.userId）; 
  

的startDate，结束日期和用户id都仍然是动态的，这意味着我再不能在以后的LAMBDA使用它们表达式。很显然，我可以修复的代码：

 日期时间的startDate = DateTime.Parse（settings.startDate）; 
 DateTime的结束日期= DateTime.Parse（settings.endDate）; 
 INT用户id = int.Parse（settings.userId）; 
  

..但似乎编译器做一个坏的猜测。谁能解释这样对我？

感谢

解决方案

这基本上意味着大多数操作（类型在规范的7.2节所列），它有一个声明为动态将被评估为动态任何元素，结果将是一个动态。

在你的情况，这种说法：

  VAR设置=新。的JavaScriptSerializer（）反序列化<动态>（JSON）; 
  

使用动态，因此，它getst reated作为一个动态的表情。由于方法调用是C＃操作主题之一结合（7.2），编译器将这种动态绑定，这将导致该评价为：

 动态设置=新的JavaScriptSerializer（）反序列化<动态>（JSON）; 
  

这反过来又导致 DateTime.Parse 表达式是动态绑定的，这反过来又使他们返回动态。

您的作品修复的时候您日期时间的startDate = DateTime.Parse（settings.startDate）; ，因为这种力量的结果的隐式动态转换（在规范第6.1.8所述） DateTime.Parse方法为DateTime：

When you use dynamic, the entire expression is treated at compile time as a dynamic expression, which causes the compiler to treat everything as dynamic and get run-time binding.

This is explained in 7.2 of the C# Language specification:

This basically means that most operations (the types are listed in section 7.2 of the spec) which have any element that is declared as dynamic will be evaluated as dynamic, and the result will be a dynamic.

In your case, this statement:

var settings = new JavaScriptSerializer().Deserialize<dynamic>(json);

Uses dynamic, so, it getst reated as a dynamic expression. Since "Method invocation" is one of the C# operations subject to binding (7.2), the compiler treats this as dynamic bound, which causes this to evaluate to:

dynamic settings = new JavaScriptSerializer().Deserialize<dynamic>(json);

This, in turn, causes the DateTime.Parse expressions to be dynamic bound, which in turn makes them return dynamic.

Your "fix" works when you do DateTime startDate = DateTime.Parse(settings.startDate); because this forces an implicit dynamic conversion (described in section 6.1.8 of the spec) of the result of the DateTime.Parse method to a DateTime:

In this case, the conversion is valid, so you effectively switch everything back to static binding from then on.

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