How do I use the extension method Sum on a .NET list in F#?

问题

Suppose I have the following record type:

open System.Collections.Generic

type Store =
    {
        Products: List<Product>;
    }
    member this.TotalNumberOfItems =
        this.Products.Sum(fun p -> p.NumberInInventory)

I want to sum a count of total items in the store as in the method above, however the System.Collections.Generic.List extension methods don't seem to be available. Intellisense does not show them. How can I call Sum from F#?

回答1:

You just need to open the System.Linq namespace. For example:

open System.Collections.Generic
open System.Linq

type Product = { NumberInInventory : int; }

type Store =
    {
        Products: List<Product>;
    }
    member this.TotalNumberOfItems =
        this.Products.Sum(fun p -> p.NumberInInventory)

来源：https://stackoverflow.com/questions/17286953/how-do-i-use-the-extension-method-sum-on-a-net-list-in-f