为什么 C# 不支持类构造函数中隐含的泛型类型?

问题描述：

C# 不要求您指定泛型类型参数，如果编译器可以推断它，例如:

C# doesn't require you to specify a generic type parameter if the compiler can infer it, for instance:

List<int> myInts = new List<int> {0,1,1,
    2,3,5,8,13,21,34,55,89,144,233,377,
    610,987,1597,2584,4181,6765};

//this statement is clunky
List<string> myStrings = myInts.
    Select<int,string>( i => i.ToString() ).
    ToList<string>();

//the type is inferred from the lambda expression
//the compiler knows that it's taking an int and 
//returning a string
List<string> myStrings = myInts.
    Select( i => i.ToString() ).
    ToList();

这对于您不知道类型参数是什么的匿名类型是必需的(在智能感知中它显示为 'a)，因为它是由编译器添加的.

This is needed for anonymous types where you don't know what the type parameter would be (in intellisense it shows up as 'a) because it's added by the compiler.

类级别的类型参数不允许您这样做:

Class-level type parameters don't let you do this:

//sample generic class
public class GenericDemo<T> 
{
    public GenericDemo ( T value ) 
    {
        GenericTypedProperty = value;
    }

    public T GenericTypedProperty {get; set;}
}

//why can't I do:
int anIntValue = 4181;
var item = new GenericDemo( anIntValue ); //type inference fails

//however I can create a wrapper like this:
public static GenericDemo<T> Create<T> ( T value )
{
    return new GenericDemo<T> ( value );
}

//then this works - type inference on the method compiles
var item = Create( anIntValue );

为什么 C# 不支持此类级别的泛型类型推断?

Why doesn't C# support this class level generic type inference?

答

其实你的问题还不错.过去几年我一直在研究泛型编程语言，虽然我从未真正开发它(可能永远不会)，但我对泛型类型推断进行了很多思考，我的首要任务之一是一直允许在不必指定泛型类型的情况下构造类.

Actually, your question isn't bad. I've been toying with a generic programming language for last few years and although I've never come around to actually develop it (and probably never will), I've thought a lot about generic type inference and one of my top priorities has always been to allow the construction of classes without having to specify the generic type.

C# 只是缺乏使这成为可能的规则集.我认为开发人员从来没有看到包含这个的必要性.实际上，以下代码将非常接近您的命题并解决问题.所有 C# 需要的是附加的语法支持.

C# simply lacks the set of rules to make this possible. I think the developers never saw the neccesity to include this. Actually, the following code would be very near to your proposition and solve the problem. All C# needs is an added syntax support.

class Foo<T> {
    public Foo(T x) { … }
}

// Notice: non-generic class overload. Possible in C#!
class Foo {
    public static Foo<T> ctor<T>(T x) { return new Foo<T>(x); }
}

var x = Foo.ctor(42);

由于这段代码确实有效，我们已经证明问题不在于语义，而在于缺乏支持.我想我必须收回我以前的帖子.;-)

Since this code actually works, we've shown that the problem is not one of semantics but simply one of lacking support. I guess I have to take back my previous posting. ;-)

为什么 C# 不支持类构造函数中隐含的泛型类型?

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