泛型类型推理与活力的工作？ [英] Generic type inference not working with dynamic?

问题描述

我最近在玩海量，一个Micro-ORM返回的IEnumerable<的集合;动态方式>

I have recently been playing with Massive, a Micro-ORM which returns collections of IEnumerable<dynamic>.

我发现了一个意想不到的问题，当我试图查询的一种使用LINQ的集合。

I discovered an unexpected issue when I tried to query one of those collections using LINQ.

虽然编译器似乎已经没有任何问题的工作指出，即使的String.Format返回一个字符串的时候传递给它的论据之一是声明为动态...

While the compiler seems to have no issues whatsoever to work out that string.Format returns a string even when one of the arguments passed to it is declared as dynamic...

dynamic dynamicString = "d"; // just using a string here for simplicity, same problem occurs with any other type
string explicitString = string.Format("string is {0}", dynamicString); // works without issues

...这似乎并不能够推断事实以下情形：

...it doesn't appear to be able to infer that fact in the following scenario:

IEnumerable<string> strings = new[] { "a", "b", "c" };
IEnumerable<dynamic> dynamics = strings;

IEnumerable<string> output = dynamics.Select(d => string.Format("string is {0}", d)); // compiler error on this line

编译器会抱怨：

The compiler complains:

"Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<dynamic>' to 'System.Collections.Generic.IEnumerable<string>'. An explicit conversion exists (are you missing a cast?)"

由于编译器应该能够推断出我的lambda表达式返回一个字符串，我本来期望它也推断选择的TResult应该是字符串类型（而不是动态）。

Since the compiler should be able to infer that my lambda expression returns a string, I would have expected it to also infer that the TResult of the Select should be of type string (and not dynamic).

这是很容易通过指定修复在TSource和TResult明确如下：

It was easy to fix by specifying the TSource and TResult explicitly like this:

IEnumerable<string> output2 = dynamics.Select<dynamic, string>(d => string.Format("string is {0}", d)); // works !!!

或者，我可以把结果赋值给一个IEnumerable<动态> ...

Or I could assign the result to an IEnumerable<dynamic>...

IEnumerable<dynamic> output3 = dynamics.Select(d => string.Format("string is {0}", d)); // also works

我也证实，当我代替我的IEnumerable<不会出现此问题;动态> ;有一个IEnumerable<对象>

I have also confirmed that this problem does not occur when I replace my IEnumerable<dynamic> with an IEnumerable<object>:

IEnumerable<object> objects = strings;
IEnumerable<string> output4 = objects.Select(o => string.Format("string is {0}", o)); // works

有趣的是，即使以下工作：

And interestingly even the following works:

IEnumerable<string> output5 = dynamics.Select(d => string.Format("string is {0}", (object)d)); // works
IEnumerable<string> output6 = dynamics.Select(d => string.Format("string is {0}", (string)d)); // works

任何人能解释这是怎么回事呢？ ？这是C＃编译器的一个已知限制或有我发现了另一个错误

Can anybody explain what's going on here? Is this a know limitation of the C# compiler or have I found yet another bug?

推荐答案

您需要：

IEnumerable<string> output = dynamics.Select(d => (string)string.Format(
       "string is {0}", d));

它无法推断返回类型为字符串，因为动态意味着它必须承担回报动态，如果有更合适的超载的的String.Format 为提供（用不同的返回类型）的特定类型。即使我们知道，否则，该规范为动态会同意我们的观点，P通过增加一个显式类型转换回字符串 。我们做的返回类型明确

It can't infer the return type is string, because the dynamic means it has to assume the return is dynamic, in case there is a more suitable overload of string.Format for the specific type supplied (with a different return type). Even if we know otherwise, the spec for dynamic will disagree with us ;p By adding an explicit cast back to string we make the return type clear.

就个人而言，我看不出使用了动态在这里;你还不如用对象，那么它是不是摆在首位的问题：

Personally, I see no use for dynamic here; you might as well use object, then it isn't an issue in the first place:

IEnumerable<string> strings = new[] { "a", "b", "c" };
IEnumerable<object> dynamics = strings;

IEnumerable<string> output = dynamics.Select(d => string.Format(
      "string is {0}", d));

（或实际上，保留为的IEnumerable<串> ）我假定你有一些其他原因使用动态，是不是在这个例子中可见。

(or indeed, leave as IEnumerable<string>) I assume you have some other reason to use dynamic that isn't visible in this example.

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