为什么不能用 VAR 返回类型声明函数? [英] Why is it not possible to declare a function with VAR return type?

问题描述

在 C# 中，我们有 var 数据类型，但我们不能将其用作函数返回类型.
为什么这是不可能的?

In C#, we have var data type but we can't use it as functions return type.
Why this is not possible?

public var myFunction()
{
    var = some operations
}

推荐答案

我相信部分原因在于编译器的设计.Eric Lippert 博客 关于为什么 fields 不能使用隐式类型，我怀疑一些相同的参数适用于方法.

I believe it's partly due to the design of the compiler. Eric Lippert blogged about why fields can't use implicit typing, and I suspect some of the same arguments hold for methods.

但无论如何，你很容易以模棱两可的方式结束.例如:

But you could easily end up with ambiguity anyway. For example:

var Method1(bool callMethod2)
{
    return callMethod2 ? Method2() : null;
}

var Method2()
{
    return Method1(false);
}

这里的类型应该是什么?

What should the type be here?

一个更简单的例子:

var Method1(bool throwException)
{
    if (!throwException)
    {
        return Method1(true);
    }
    throw new Exception("Bang!");
}

诚然，这种模棱两可的情况是完全不允许的，但我怀疑设计团队认为设计和实现增加的复杂性不值得从中受益.不要忘记它们运行时资源有限——如果在方法的 var 和 async/await 之间进行选择，我会立即选择后者.(诚​​然，我会选择其他功能而不是 dynamic，但那是另一回事...)

Admittedly this sort of ambiguity could simply be disallowed, but I suspect that the design team felt that the added complexity of both design and implementation wasn't worth the benefit. Don't forget that they're running with limited resources - given a choice between var for methods and async/await, I'd pick the latter in a heartbeat. (Admittedly there are other features I'd have picked instead of dynamic, but that's a different matter...)

请注意，返回类型推断是对 lambda 表达式执行的，所以它的想法并不疯狂.例如:

Note that return type inference is performed for lambda expressions, so the very idea of it isn't crazy. For example:

IEnumerable<string> x = new[] { "x", "y", "z" };

var result = x.Select(s => { return s.Length; }); // Long form

编译器在对 Select 执行重载解析时推断 lambda 表达式的完整类型，将其转换为 Func.将相同的想法应用于方法并非不可想象 - 只是复杂.

There the compiler infers the complete type of the lambda expression when it performs overload resolution on Select, converting it to a Func<string, int>. It's not inconceivable to apply the same ideas to methods - just complicated.

这篇关于为什么不能用 VAR 返回类型声明函数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！