为什么 dynamic .ToString() (string.Format(), Convert.ToString()) returns 是动态的而不是字符串？

Question

此问题仅用于教育目的。我有这段代码在我想将行写入文件的行的编译时失败。 (File.WriteAllLines(@"C:\temp\processed.txt",contents);)

错误信息：

Argument 2: cannot convert from 'System.Collections.Generic.List<dynamic>' to 'string[]' C:\hg\PricingEngine\Source\App\Support\PricingEngine.Migrator\Program.cs 49 57 PricingEngine.Migrator

Error 6 The best overloaded method match for 'System.IO.File.WriteAllLines(string, string[])' has some invalid arguments C:\hg\PricingEngine\Source\App\Support\PricingEngine.Migrator\Program.cs 49 13 PricingEngine.Migrator

如果我注释掉最后一行并用断点检查所有行都是字符串，一切正常。

代码：

public static void Main()
{
    var t = new List<dynamic>
    {
        new {name = "Mexico", count = 19659},
        new {name = "Canada", count = 13855},
    };

   var stringed =  t.Select(o => string.Format("{0} {1}", o.name, o.count)).Select(o => Convert.ToString(o)).ToList();
   File.WriteAllLines(@"C:\temp\processed.txt", stringed);
 }

这是怎么回事为什么动态 ToString()、string.Format() 和 Convert.ToString() 是动态的？我在这里遗漏了什么明显的东西吗？

Answer 1

由于动态的工作原理，编译器不知道 t.name 的类型，因此无法为 File.WriteAllLines 方法找到正确的重载。一种解决方案可能是将 t.name 显式转换为 string，但在您的情况下，您可以对数组使用隐式类型并完全停止使用动态：

var t = new[]
{
    new {name = "Mexico", count = 19659},
    new {name = "Canada", count = 13855},
    new {name = "U.K.", count = 3286},
    new {name = "France", count = 2231},
    new {name = "Italy", count = 2201},
    new {name = "Germany", count = 1688},
    new {name = "Jamaica ", count = 1688},
    new {name = "Bahamas ", count = 1538},
    new {name = "Japan", count = 1538},
    new {name = "People's Republic of China", count = 1327},
    new {name = "Spain", count = 995},
    new {name = "Netherlands", count = 904},
    new {name = "Hong Kong", count = 904},
    new {name = "India", count = 904},
    new {name = "Ireland", count = 844},
    new {name = "Republic of China (Taiwan)", count = 693},
    new {name = "Switzerland ", count = 633},
    new {name = "Republic of Korea", count = 633},
    new {name = "Australia", count = 603},
    new {name = "Greece", count = 482},
};

Answer 2

在我看来你的问题真的可以归结为：

dynamic d = "x";
var v = Convert.ToString(d);

... v 的编译时类型是 dynamic，如在 Visual Studio 中将鼠标悬停在它上面所示，您会期望它是 string.不需要列表或文件。

那么，这是为什么呢？基本上，C# 有一个简单的规则：几乎所有使用动态值的操作的结果都是 dynamic。这意味着在执行时是否可以使用编译时不知道的额外重载并不重要，例如。

我知道涉及动态值的操作结果不是动态的唯一操作是：

is 运算符，例如var b = d is Foo; // Type of b is bool
演员表，例如var x = (string) d; // Type of x is string
as 运算符，例如var y = d as string; // Type of y is string
构造函数调用，例如var z = new Bar(d); // Type of z is Bar

对于方法调用的简单情况，C# 5 规范的第 7.6.5 节明确指出 Convert.ToString(d) 的类型为 dynamic:

An invocation-expression is dynamically bound (§7.2.2) if at least one of the following holds:

The primary-expression has compile-time type dynamic.

At least one argument of the optional argument-list has compile-time type dynamic and the primary-expression does not have a delegate type.

In this case the compiler classifies the invocation-expression as a value of type dynamic.

（作为旁注，“并且 主要表达式 没有委托类型”部分似乎没有在任何地方得到解决，或者被编译器。如果你有一个 Func<string, string> func = ...; var result = func(d);，result 的类型似乎仍然是 dynamic 而不是 string。我会调查一下...）

为什么 dynamic .ToString() (string.Format(), Convert.ToString()) returns 是动态的而不是字符串？

Why dynamic .ToString() (string.Format(), Convert.ToString()) returns dynamic and not string?

c#

dynamic

tostring