在与同事讨论了c# 3中var关键字的使用后,我想知道人们对通过var适当使用类型推断的看法是什么?

例如,我很懒地在有问题的情况下使用var,例如:-

foreach(var item in someList) { // ... } // Type of 'item' not clear.
var something = someObject.SomeProperty; // Type of 'something' not clear.
var something = someMethod(); // Type of 'something' not clear.

var更合法的用法如下:-

var l = new List<string>(); // Obvious what l will be.
var s = new SomeClass(); // Obvious what s will be.

有趣的是,LINQ似乎是一个灰色区域,例如:-

var results = from r in dataContext.SomeTable
              select r; // Not *entirely clear* what results will be here.

结果很明显,因为它将是一个实现IEnumerable的类型,但它并不完全明显,与var声明一个新对象的方式相同。

当涉及到对象的LINQ时,情况就更糟了,例如:-

var results = from item in someList
              where item != 3
              select item;

这并不比等价的foreach(var item in someList){//…}相当于。

这里有一个关于类型安全的真正问题——例如,如果我们将该查询的结果放在一个接受IEnumerable<int>和IEnumerable<double>的重载方法中,调用者可能会无意中传入错误的类型。

Var确实保持强类型,但问题是,在定义中不立即显示类型是否危险,当重载意味着当你无意中将错误的类型传递给方法时,编译器可能不会发出错误,这种情况会被放大。


当前回答

没有,只是不需要写两次类型名。http://msdn.microsoft.com/en-us/library/bb383973.aspx

其他回答

我只在清楚地看到使用的类型时使用var。

例如,在这种情况下,我会使用var,因为你可以立即看到x将是“MyClass”类型:

var x = new MyClass();

我不会在这种情况下使用var,因为你必须将鼠标拖动到代码上,并查看工具提示来查看MyFunction返回的类型:

var x = MyClass.MyFunction();

特别是,在右边甚至不是一个方法,而只是一个值的情况下,我从不使用var:

var x = 5;

(因为编译器不知道我想要一个字节,短,int或其他)

这两种说法都不是绝对正确的;Var对可读性既有正面影响,也有负面影响。在我看来,var应该在以下情况之一成立时使用:

类型是匿名的(好吧,这里没有任何选择,因为在这种情况下它必须是var) 根据指定的表达式,类型是明显的(即var foo = new TypeWithAReallyLongNameTheresNoSenseRepeating())

Var没有性能影响,因为它是语法糖;一旦编译成IL,编译器就会推断出类型并定义它;没有什么动态的东西。

来自c#团队的高级软件设计工程师Eric Lippert:

为什么引入var关键字?

There are two reasons, one which exists today, one which will crop up in 3.0. The first reason is that this code is incredibly ugly because of all the redundancy: Dictionary<string, List<int>> mylists = new Dictionary<string, List<int>>(); And that's a simple example – I've written worse. Any time you're forced to type exactly the same thing twice, that's a redundancy that we can remove. Much nicer to write var mylists = new Dictionary<string,List<int>>(); and let the compiler figure out what the type is based on the assignment. Second, C# 3.0 introduces anonymous types. Since anonymous types by definition have no names, you need to be able to infer the type of the variable from the initializing expression if its type is anonymous.

我特别强调。整篇文章,c# 3.0仍然是静态类型的,真的!,以及随后的系列相当不错。

这就是var的作用。其他用途可能就没这么好用了。任何与JScript、VBScript或动态类型的比较都是胡扯。再次注意,var是为了使某些其他特性在. net中工作而必需的。

将它用于匿名类型—这就是它存在的目的。其他的都是徒劳无功。像许多使用C长大的人一样,我习惯于查看类型声明的左侧。除非迫不得已,我是不会看右边的。在任何旧的声明中使用var都让我一直这样做,我个人觉得不舒服。

Those saying 'it doesn't matter, use what you're happy with' are not seeing the whole picture. Everyone will pick up other people's code at one point or another and have to deal with whatever decisions they made at the time they wrote it. It's bad enough having to deal with radically different naming conventions, or - the classic gripe - bracing styles, without adding the whole 'var or not' thing into the mix. The worst case will be where one programmer didn't use var and then along comes a maintainer who loves it, and extends the code using it. So now you have an unholy mess.

标准是一件好事,因为它们意味着你更有可能捡起随机代码,并能够快速地理解它。不同的东西越多,就越难。而转移到“无处不在的var”风格会有很大的不同。

I don't mind dynamic typing, and I don't mind implict typing - in languages that are designed for them. I quite like Python. But C# was designed as a statically explicitly-typed language and that's how it should stay. Breaking the rules for anonymous types was bad enough; letting people take that still further and break the idioms of the language even more is something I'm not happy with. Now that the genie is out of the bottle, it'll never go back in. C# will become balkanised into camps. Not good.

在测试过程中,我发现自己的代码是这样的:

var something = myObject.SomeProperty.SomeOtherThing.CallMethod();
Console.WriteLine(something);

现在,有时,我想看看SomeOtherThing本身包含什么,SomeOtherThing不是CallMethod()返回的相同类型。因为我使用了var,我只是改变了这个:

var something = myObject.SomeProperty.SomeOtherThing.CallMethod();

:

var something = myObject.SomeProperty.SomeOtherThing;

如果没有var,我也必须不断改变左边声明的类型。我知道这是小事,但它非常方便。