使用類型填充列表

Question

我試圖用我自己的類型填充列表。

let getUsers = 
    use connection = openConnection() 
    let getString = "select * from Accounts" 
    use sqlCommand = new SqlCommand(getString, connection) 
    try 
     let usersList = [||] 
     use reader = sqlCommand.ExecuteReader() 
     while reader.Read() do 
      let floresID = reader.GetString 0 
      let exName = reader.GetString 1 
      let exPass = reader.GetString 2 
      let user = [floresID=floresID; exName=exName; exPass=exPass] 
      // what here? 
      () 
    with 
     | :? SqlException as e -> printfn "Došlo k chybě úrovni připojení:\n %s" e.Message 
     | _ -> printfn "Neznámá výjimka." 

在C＃我只想補充新的對象爲userList。我如何將新的user添加到列表中？或者更好的方法來從數據庫中獲取某種列表的數據？

Answer 1

最簡單的方法是使用類型提供程序，因此可以抽象出數據庫。對於SQLServer，您可以使用SqlDataConnection，對於所有內容（包括SQLServer），還可以使用SqlProvider，對於SQLServer也可以使用。

這裏是postgres的的dvdrental作爲（樣品）數據庫sqlProvider的一個例子：

#r @"..\packages\SQLProvider.1.0.33\lib\FSharp.Data.SqlProvider.dll" 
#r @"..\packages\Npgsql.3.1.8\lib\net451\Npgsql.dll" 

open System 
open FSharp.Data.Sql 
open Npgsql 
open NpgsqlTypes 
open System.Linq 
open System.Xml 
open System.IO 
open System.Data 

let [<Literal>] dbVendor = Common.DatabaseProviderTypes.POSTGRESQL 
let [<Literal>] connString1 = @"Server=localhost;Database=dvdrental;User Id=postgres;Password=root" 
let [<Literal>] resPath = @"C:\Users\userName\Documents\Visual Studio 2015\Projects\Postgre2\packages\Npgsql.3.1.8\lib\net451" 
let [<Literal>] indivAmount = 1000 
let [<Literal>] useOptTypes = true 

//create the type for the database, based on the connection string, etc. parameters 
type sql = SqlDataProvider<dbVendor,connString1,"",resPath,indivAmount,useOptTypes> 
//set up the datacontext, ideally you would use `use` here :-) 
let ctx = sql.GetDataContext() 
let actorTbl = ctx.Public.Actor //alias the table 

//set up the type, in this case Records: 
type ActorName = { 
    firstName:string 
    lastName:string} 

//extract the data with a query expression, this gives you type safety and intellisense over SQL (but also see the SqlClient type provider above): 
let qry = query { 
      for row in actorTbl do 
      select ({firstName=row.FirstName;lastName=row.LastName}) 
       } 
//seq is lazy so do all kinds of transformations if necessary then manifest it into a list or array: 
qry |> Seq.toArray 

的兩個重要部分所定義的演員記錄，然後在查詢中提取的字段成的演員記錄的序列。如有必要，您可以將其列入列表或數組中。

但你也可以堅持你原來的解決方案。在這種情況下，只是包裝的.Read()爲seq：

首先定義類型：

type User = { 
    floresID: string 
    exName: string 
    exPass: string 
} 

然後將解壓後的數據：

let recs = cmd.ExecuteReader() // execute the SQL Command 
//extract the users into a sequence of records: 
let users = 
    seq { 
     while recs.Read() do 
      yield {floresID=recs.[0].ToString() 
        exName=recs.[1].ToString() 
        exPass=recs.[2].ToString() 
        } 
     } |> Seq.toArray 

Answer 2

以你的代碼，你可以用列表表達：

let getUsers = 
    use connection = openConnection() 
    let getString = "select * from Accounts" 
    use sqlCommand = new SqlCommand(getString, connection) 
    try 
     [ 
      use reader = sqlCommand.ExecuteReader() 
      while reader.Read() do 
       let floresID = reader.GetString 0 
       let exName = reader.GetString 1 
       let exPass = reader.GetString 2 
       let user = [floresID=floresID; exName=exName; exPass=exPass] 
       yield user 
     ] 
    with 
     | :? SqlException as e -> failwithf "Došlo k chybě úrovni připojení:\n %s" e.Message 
     | _ -> failwithf "Neznámá výjimka." 

這就是說，我會使用FSharp.Data.SqlClient libra所以所有的鍋爐板都變成了一條單線，並增加了類型安全性的好處（如果你改變了查詢，代碼將會有編譯時錯誤，這些錯誤很明顯可以解決）。