如何使用F＃lambda创建Linq表达式树？

Here's what can be done in C# -

以下是C＃中可以做的事情 -

var two = 2;
System.Linq.Expressions.Expression<System.Func<int, int>> expr = x => x * two;
expr.Compile().Invoke(4); // returns 8

I wish to do the precise equivalent in F#. Here's what I tried, but did not compile -

我希望在F＃中做到精确的等价物。这是我尝试过的，但没有编译 -

let two = 2
let expr = (fun x -> x * two) : System.Linq.Expressions.Expression<System.Func<int, int>>
expr.Compile().Invoke(4) // desired to return 8

Perhaps predictably, compilation fails on line 2 with the following error -

也许可以预见，编译在第2行失败，出现以下错误 -

"This function takes too many arguments, or is used in a context where a function is not expected."
let expr = (fun x -> x * two) : System.Linq.Expressions.Expression<System.Func<int, int>>
            ^^^^^^^^^^^^^^^^

1 个解决方案

#1

I'm not sure why you want to avoid using F# quotations - under the cover, they are pretty much the same thing as C# expression trees and if you want to create an expression tree in F#, the compiler will be using quotations under the cover in any case...

我不确定为什么要避免使用F＃引用 - 在封面下，它们与C＃表达式树几乎相同，如果要在F＃中创建表达式树，编译器将在封面下使用引号任何状况之下...

Anyway, you can do this without writing explicit <@ .. @> because the compiler can automatically quote a function when it is passed as an argument to a method. So you can do:

无论如何，您可以在不编写显式<@ .. @>的情况下执行此操作，因为编译器可以在将函数作为参数传递给方法时自动引用该函数。所以你可以这样做：

type Expr = 
  static member Quote(e:Expression<System.Func<int, int>>) = e

let two = 2
let expr = Expr.Quote(fun x -> x * two) 
expr.Compile().Invoke(4) // desired to return 8

EDIT: However, this really compiles to an F# quotation wrapped in a call that converts it to C# expression tree. So, in the end, you'll get the same thing as if you wrote:

编辑：但是，这实际上编译为包含在调用中的F＃引用，该调用将其转换为C＃表达式树。所以，最后，你会得到和你写的一样：

open Microsoft.FSharp.Linq.RuntimeHelpers

let two = 2
let expr = 
  <@ System.Func<_, _>(fun x -> x * two) @>
  |> LeafExpressionConverter.QuotationToExpression 
  |> unbox<Expression<Func<int, int>>>
expr.Compile().Invoke(4) // desired to return 8

#1