CODEWITHSUNDEEP
vb.netsocketserror-handling
Aommaster
Aommaster

Reputation: 125

Catching a SocketException

I'm probably going about this the wrong way, as I have no experience with web requests, so please bear with me.

I'm trying to execute the following code:

webClient.DownloadStringAsync(New Uri("http://localhost:8115/"))

This works fine when the URI is available. However, if it is not available (ie. if the respective service is not running and is not exposing the relevant data), I get the following error message:

SocketException occurred: No connection could be made because the target machine actively refused it.

So, I tried to implement a try/catch block as follows:

If Not webClient.IsBusy Then
    Try
        webClient.DownloadStringAsync(New Uri("http://localhost:8115/"))
    Catch ex As Sockets.SocketException
        MsgBox("Error. Service is not running. No data can be extracted.")
    End Try
End If

That did not work. VB.Net still does not display the message box. So, I tried something else:

If Not webClient.IsBusy Then
    Dim req As System.Net.WebRequest
    req = System.Net.WebRequest.Create(New Uri("http://localhost:8115/"))
    Dim resp As System.Net.WebResponse
    Dim ready As Boolean = False

    Try
        resp = req.GetResponse
        resp.Close()
        ready = True
    Catch ex As Sockets.SocketException
        MsgBox("Error. Service is not running. No data can be extracted.")
    End Try

    If ready Then
        webClient.DownloadStringAsync(New Uri("http://localhost:8115/"))
        ready = False
    End If
End If

It also doesn't work. I must be approaching this problem incorrectly. Can someone show me what the correct way of doing this is? Is there a way of first checking if the data exists, before running the DownloadStringAsync function?

Thanks!

Edit: To add context to the discussion under Visual Vincent's answer, here is what my code looks like. Just a single form.

Imports System.Net

Public Class Form1
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim webClient As New System.Net.WebClient
        Try
            WebClient.DownloadStringAsync(New Uri("http://localhost:8115"))
        Catch ex As System.Net.Sockets.SocketException
            MessageBox.Show("Error")
        Catch ex As System.Net.WebException
            MessageBox.Show("Error. Service is not running. No data can be extracted.")
        Catch ex As Exception
            MessageBox.Show("An error occurred:" & Environment.NewLine & ex.Message)
        End Try
    End Sub
End Class

Upvotes: 0

Views: 1365

Answers (1)

Visual Vincent
Visual Vincent

Reputation: 18320

The WebClient.DownloadStringAsync() method doesn't throw a SocketException, but a WebException (perhaps with its inner-exception set to a SocketException).

From the documentation:

Exceptions

WebException

The URI formed by combining BaseAddress and address is invalid.

-or-

An error occurred while downloading the resource.

A SocketException is most of the time only thrown by raw sockets. Then the members of the System.Net namespace usually wrap these in a WebException.

So to fix your code:

Try
    webClient.DownloadStringAsync(New Uri("http://localhost:8115/"))
Catch ex As System.Net.WebException
    MessageBox.Show("Error. Service is not running. No data can be extracted.")
End Try

NOTE: I switched to MessageBox.Show() instead because MsgBox() is outdated, and only exists for backwards compatibility with VB6.

However, the best practice is to add another Catch statement that catches all other exceptions as well, so that you don't leave your application open to crashes.

You also ought to log the error message from the WebException, since it may be thrown for other reasons than just the endpoint not being available.

Try
    webClient.DownloadStringAsync(New Uri("http://localhost:8115/"))
Catch ex As System.Net.WebException
    MessageBox.Show("Error. Service is not running. No data can be extracted.")
Catch ex As Exception
    MessageBox.Show("An error occurred:" & Environment.NewLine & ex.Message)
End Try

Upvotes: 2

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