如何捕获 SocketException

Question

我在构造以下 F# try..with 语句的 with 部分时遇到问题。那里有很多异常处理示例，但是当主机不存在时，我找不到任何特定匹配从 Dns.GetHostEntry 返回的 SocketException 的示例。我会继续寻找，但不胜感激。

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

到目前为止，你所拥有的一切都很好；您只需要添加 as (name) 语法来为异常对象命名，然后添加 when (test) 语法来测试某个条件是否成立。例如：

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

（注意：我的原始代码针对 int 测试了 SocketErrorCode，因为我误读了 MSDN 文档。但是 SocketErrorCode is a System.Net.Sockets.SocketError enum value。）

关于您的代码的一条评论：您不需要 use tcpSocket = ... 表达式周围的括号。您可以将其与代码的其余部分内联编写，当 tcpSocket 值在函数末尾超出范围时，F# 将处理该值：

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

以下不是解决方案，而是解决方法。它解决了主机未找到错误的问题，而不必使用@rmunn 的回答中提到的 railway-oriented programming 。（顺便说一下，这是一篇很棒的文章。）

所以如果Dns.GetHostEntry抛出异常或者check_response未能从HELO得到正确的响应，那么false可以返回。

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 
        )

如何捕获 SocketException

How to catch a SocketException

sockets

f#

try-with