如何捕捉SocketException

Question

我在構建以下F＃try..with語句的with部分時遇到問題。這裏有很多異常處理的例子，但是當主機不存在時，我找不到任何與Dns.GetHostEntry返回的SocketException特別匹配的東西。我會繼續尋找，但會欣賞一個指針。

let test_smtp (smtp_server_address : string) (port : int) = 
    let expectedResultCode = 220 
    let hostEntry : IPHostEntry = 
     try 
      Dns.GetHostEntry(smtp_server_address) 
     with 
      | :? System.Net.Sockets.SocketException -> printfn "GetHostEntry threw an exception " 

    let endPoint : IPEndPoint = new IPEndPoint (hostEntry.AddressList.[0], port) 
    (use tcpSocket = new Socket(endPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) 
     tcpSocket.Connect(endPoint) 

     let temp_result = check_response tcpSocket expectedResultCode 
     temp_result 
    ) 

Answer 1

到目前爲止，您只需添加as (name)語法以給出異常對象的名稱，然後使用when (test)語法來測試某個條件是否成立。例如，：

try 
    Dns.GetHostEntry(smtp_server_address) 
with 
    | :? System.Net.Sockets.SocketException as ex when ex.SocketErrorCode = SocketError.HostNotFound -> 
    failwith "Host not found" 

（注：我的原代碼，測試SocketErrorCode對一個int，因爲我誤讀了MSDN文檔，但SocketErrorCode是System.Net.Sockets.SocketError enum value。）有關代碼

一個評論：你不需要圍繞use tcpSocket = ...表達式的括號。你可以只用代碼的其餘部分寫它內聯，而當它超出範圍在函數結束F＃將處置tcpSocket值：

let test_smtp (smtp_server_address : string) (port : int) = 
    let expectedResultCode = 220 
    let hostEntry : IPHostEntry = 
     try 
      Dns.GetHostEntry(smtp_server_address) 
     with 
      | :? System.Net.Sockets.SocketException as ex when ex.SocketErrorCode = SocketError.HostNotFound -> 
       failwith "GetHostEntry threw an exception " 

    let endPoint : IPEndPoint = new IPEndPoint (hostEntry.AddressList.[0], port) 
    use tcpSocket = new Socket(endPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) 
    tcpSocket.Connect(endPoint) 

    check_response tcpSocket expectedResultCode 

Answer 2

下面是不是一個解決方案，但解決方法。它解決了主機未發現錯誤的問題，而無需使用@rmunn的答案中提到的railway-oriented programming。 （這是一個偉大的文章，順便說一句。）

所以false可如果Dns.GetHostEntry拋出異常或返回如果check_response未能從HELO正確的響應。

module net_utilF_mod 

open System 
open System.Text 
open System.Threading 
open System.Net 
open System.Net.Sockets 
open System.IO 

type Smtp1() = 
    member this.X = "F#" 

let send_data (socket : Socket) (data : string) = 
    let dataArray : byte [] = Encoding.ASCII.GetBytes(data) 
    socket.Send(dataArray, 0, dataArray.Length, SocketFlags.None) 

let check_response (socket:Socket) expectedResultCode = 
    while socket.Available = 0 do 
     System.Threading.Thread.Sleep 100 

    let responseArray : byte [] = Array.zeroCreate 1024 
    socket.Receive(responseArray, 0, socket.Available, SocketFlags.None) |> ignore 
    let responseData : string = Encoding.ASCII.GetString(responseArray) 
    let responseCode : int = Convert.ToInt32(responseData.Substring(0,3)) 
    if responseCode = expectedResultCode then 
     true 
    else 
     false 

let test_smtp (smtp_server_address : string) (port : int) = 
    let helo_msg = String.Format("HELO {0}\r\n", Dns.GetHostName()) 
    let hostEntry : IPHostEntry = 
     try 
      Dns.GetHostEntry(smtp_server_address) 
     with 
      | :? System.Net.Sockets.SocketException as ex when ex.SocketErrorCode = SocketError.HostNotFound -> 
       let tempEntry : IPHostEntry = Dns.GetHostEntry(@"localhost") 
       tempEntry 

    let email_server_reachable = 
     match hostEntry.HostName with 
     | @"localhost" -> false 
     | _ -> true 


    if false = email_server_reachable then 
     false 
    else 
     let endPoint : IPEndPoint = new IPEndPoint (hostEntry.AddressList.[0], port) 
     (use tcpSocket = new Socket(endPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) 
      tcpSocket.Connect(endPoint) |> ignore 

      let temp_response = 
       if true = check_response tcpSocket 220 then 
       send_data tcpSocket helo_msg |> ignore 
       check_response tcpSocket 250    
       else 
       false 

      temp_response 
     )